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Title: Preliminary Discourse on the Study of Natural Philosophy

Author: John F. W. Herschel

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Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK PRELIMINARY
DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY ***

Preliminary discourse
on the Study of

NATURAL PHILOSOPHY

BY
SIR JOHN F. W. HERSCHEL, BART. K.H.
M.A.—D.C.L.—F.R.S.L&E.—M.R.I.A.—
F.R.A.S.
F.G.S.—M.C.U.P.S.—&c. &c.
NEW EDITION.
1851.

H. Corbould del. E. Finden. sculp.

NATURÆ MINISTER ET INTERPRES.

NEW EDITION.
London:
PRINTED FOR LONGMAN, BROWN, GREEN & LONGMANS, PATERNOSTER ROW

CONTENTS.
Page

PART I.
OF THE GENERAL NATURE AND ADVANTAGES OF
THE STUDY OF THE PHYSICAL SCIENCES.

CHAP. I.
Of Man regarded as a Creature of Instinct, of Reason,
and Speculation.—General Influence of Scientific
Pursuits on the Mind. 1

CHAP. II.
Of abstract Science as a Preparation for the Study of
Physics.—A profound Acquaintance with it not
indispensable for a clear Understanding of
Physical Laws.—How a Conviction of their Truth
may be obtained without it.—Instances.—Further
Division of the Subject. 18

CHAP. III.
Of the Nature and Objects, immediate and collateral,
of Physical Science, as regarded in itself, and in
its Application to the practical Purposes of Life,
and its Influence on the Well-being and Progress
of Society. 35

PART II.
OF THE PRINCIPLES ON WHICH PHYSICAL
SCIENCE RELIES FOR ITS SUCCESSFUL
PROSECUTION, AND THE RULES BY WHICH A
SYSTEMATIC EXAMINATION OF NATURE
SHOULD BE CONDUCTED, WITH
ILLUSTRATIONS OF THEIR INFLUENCE AS
EXEMPLIFIED IN THE HISTORY OF ITS
PROGRESS.

CHAP. I.
Of Experience as the Source of our Knowledge.—Of
the Dismissal of Prejudices.—Of the Evidence of
our Senses. 75

CHAP. II.
Of the Analysis of Phenomena. 85

CHAP. III.
Of the State of Physical Science in General, previous
to the Age of Galileo and Bacon. 104

CHAP. IV.
Of the Observation of Facts and the Collection of
Instances. 118

CHAP. V.
Of the Classification of Natural Objects and
Phenomena, and of Nomenclature. 135

CHAP. VI.
Of the First Stage of Induction.—The Discovery of
Proximate Causes, and Laws of the lowest
Degree of Generality, and their Verification. 144

CHAP. VII.
Of the higher Degrees of Inductive Generalization,
and of the Formation and Verification of
Theories. 190

PART III.
OF THE SUBDIVISION OF PHYSICS INTO DISTINCT
BRANCHES, AND THEIR MUTUAL RELATIONS.

CHAP. I.
Of the Phenomena of Force, and of the Constitution
of Natural Bodies. 221

CHAP. II.
Of the Communication of Motion through Bodies.—
Of Sound and Light. 246

CHAP. III.
Of Cosmical Phenomena. 265

CHAP. IV.
Of the Examination of the material Constituents of
the World. 290

CHAP. V.
Of the Imponderable Forms of Matter. 310

CHAP. VI.
Of the Causes of the actual rapid Advance of the
Physical Sciences compared with their Progress
at an earlier Period. 347

“In primis, hominis est propria VERI inquisitio
atque investigatio. Itaque cum sumus negotiis
necessariis, curisque vacui, tum avemus aliquid
videre, audire, ac dicere, cognitionemque rerum, aut
occultarum aut admirabilium, ad benè beatéque
vivendum necessariam ducimus;—ex quo
intelligitur, quod VERUM, simplex, sincerumque sit,
id esse naturæe hominis aptissimum. Huic veri
videndi cupiditati adjuncta est appetitio quædam
principatûs, ut nemini parere animus benè a naturâ

informatus velit, nisi præcipienti, aut docenti, aut
utilitatis causa justè et legitimè imperanti: ex quo
animi magnitudo existit, et humanararum rerum
contemtio.”
Cicero, de Officiis, Lib. 1. § 13.
Before all other things, man is distinguished by
his pursuit and investigation of TRUTH. And hence,
when free from needful business and cares, we
delight to see, to hear, and to communicate, and
consider a knowledge of many admirable and
abstruse things necessary to the good conduct and
happiness of our lives: whence it is clear that
whatsoever is TRUE, simple, and direct, the same is
most congenial to our nature as men. Closely allied
with this earnest longing to see and know the truth, is
a kind of dignified and princely sentiment which
forbids a mind, naturally well constituted, to submit
its faculties to any but those who announce it in
precept or in doctrine, or to yield obedience to any
orders but such as are at once just, lawful, and
founded on utility. From this source spring greatness
of mind and contempt of worldly advantages and
troubles.

PRELIMINARY DISCOURSE
ON
THE STUDY
OF
NATURAL PHILOSOPHY.

PART I.
OF THE GENERAL NATURE AND ADVANTAGES OF THE STUDY OF
THE PHYSICAL SCIENCES

CHAPTER I.
OF MAN REGARDED AS A CREATURE OF INSTINCT, OF
REASON, AND SPECULATION.—GENERAL INFLUENCE
OF SCIENTIFIC PURSUITS ON THE MIND.

(1.) The situation of man on the globe he inhabits, and over which he has
obtained the control, is in many respects exceedingly remarkable.
Compared with its other denizens, he seems, if we regard only his physical
constitution, in almost every respect their inferior, and equally unprovided
for the supply of his natural wants and his defence against the innumerable
enemies which surround him. No other animal passes so large a portion of
its existence in a state of absolute helplessness, or falls in old age into such
protracted and lamentable imbecility. To no other warm-blooded animal has
nature denied that indispensable covering without which the vicissitudes of
a temperate and the rigours of a cold climate are equally insupportable; and
to scarcely any has she been so sparing in external weapons, whether for
attack or defence. Destitute alike of speed to avoid and of arms to repel the
aggressions of his voracious foes; tenderly susceptible of atmospheric
influences; and unfitted for the coarse aliments which the earth affords
spontaneously during at least two thirds of the year, even in temperate
climates,—man, if abandoned to mere instinct, would be of all creatures the
most destitute and miserable. Distracted by terror and goaded by famine;
driven to the most abject expedients for concealment from his enemies, and
to the most cowardly devices for the seizure and destruction of his nobler
prey, his existence would be one continued subterfuge or stratagem;—his
dwelling would be in dens of the earth, in clefts of rocks, or in the hollows
of trees; his food worms, and the lower reptiles, or such few and crude
productions of the soil as his organs could be brought to assimilate, varied
with occasional relics, mangled by more powerful beasts of prey, or
contemned by their more pampered choice. Remarkable only for the

absence of those powers and qualities which obtain for other animals a
degree of security and respect, he would be disregarded by some, and
hunted down by others, till after a few generations his species would
become altogether extinct, or, at best, would be restricted to a few islands in
tropical regions, where the warmth of the climate, the paucity of enemies,
and the abundance of vegetable food, might permit it to linger.
(2.) Yet man is the undisputed lord of the creation. The strongest and
fiercest of his fellow-creatures,—the whale, the elephant, the eagle, and the
tiger,—are slaughtered by him to supply his most capricious wants, or
tamed to do him service, or imprisoned to make him sport. The spoils of all
nature are in daily requisition for his most common uses, yielded with more
or less readiness, or wrested with reluctance, from the mine, the forest, the
ocean, and the air. Such are the first fruits of reason. Were they the only or
the principal ones, were the mere acquisition of power over the materials,
and the less gifted animals which surround us, and the consequent increase
of our external comforts, and our means of preservation and sensual
enjoyment, the sum of the privileges which the possession of this faculty
conferred, we should after all have little to plume ourselves upon. But this
is so far from being the case, that every one who passes his life in tolerable
ease and comfort, or rather whose whole time is not anxiously consumed in
providing the absolute necessaries of existence, is conscious of wants and
cravings in which the senses have no part, of a series of pains and pleasures
totally distinct in kind from any which the infliction of bodily misery or the
gratification of bodily appetites has ever afforded him; and if he has
experienced these pleasures and these pains in any degree of intensity, he
will readily admit them to hold a much higher rank, and to deserve much
more attention, than the former class. Independent of the pleasures of fancy
and imagination, and social converse, man is constituted a speculative
being; he contemplates the world, and the objects around him, not with a
passive, indifferent gaze, as a set of phenomena in which he has no further
interest than as they affect his immediate situation, and can be rendered
subservient to his comfort, but as a system disposed with order and design.
He approves and feels the highest admiration for the harmony of its parts,
the skill and efficiency of its contrivances. Some of these which he can best
trace and understand he attempts to imitate, and finds that to a certain
extent, though rudely and imperfectly, he can succeed,—in others, that
although he can comprehend the nature of the contrivance, he is totally

destitute of all means of imitation;—while in others, again, and those
evidently the most important, though he sees the effect produced, yet the
means by which it is done are alike beyond his knowledge and his control.
Thus he is led to the conception of a Power and an Intelligence superior to
his own, and adequate to the production and maintenance of all that he sees
in nature,—a Power and Intelligence to which he may well apply the term
infinite, since he not only sees no actual limit to the instances in which they
are manifested, but finds, on the contrary, that the farther he enquires, and
the wider his sphere of observation extends, they continually open upon
him in increasing abundance; and that as the study of one prepares him to
understand and appreciate another, refinement follows on refinement,
wonder on wonder, till his faculties become bewildered in admiration, and
his intellect falls back on itself in utter hopelessness of arriving at an end.
(3.) When from external objects he turns his view upon himself, on his
own vital and intellectual faculties, he finds that he possesses a power of
examining and analysing his own nature to a certain extent, but no farther.
In his corporeal frame he is sensible of a power to communicate a certain
moderate amount of motion to himself and other objects; that this power
depends on his will, and that its exertion can be suspended or increased at
pleasure within certain limits; but how his will acts on his limbs he has no
consciousness: and whence he derives the power he thus exercises, there is
nothing to assure him, however he may long to know. His senses, too,
inform him of a multitude of particulars respecting the external world, and
he perceives an apparatus by which impressions from without may be
transmitted, as a sort of signals to the interior of his person, and ultimately
to his brain, wherein he is obscurely sensible that the thinking, feeling,
reasoning being he calls himself, more especially resides; but by what
means he becomes conscious of these impressions, and what is the nature of
the immediate communication between that inward sentient being, and that
machinery, his outward man, he has not the slightest conception.
(4.) Again, when he contemplates still more attentively the thoughts,
acts, and passions of this his sentient intelligent self, he finds, indeed, that
he can remember, and by the aid of memory can compare and discriminate,
can judge and resolve, and, above all, that he is irresistibly impelled, from
the perception of any phenomenon without or within him, to infer the
existence of something prior which stands to it in the relation of a cause,

without which it would not be, and that this knowledge of causes and their
consequences is what, in almost every instance, determines his choice and
will, in cases where he is nevertheless conscious of perfect freedom to act
or not to act. He finds, too, that it is in his power to acquire more or less
knowledge of causes and effects according to the degree of attention he
bestows upon them, which attention is again in great measure a voluntary
act; and often when his choice has been decided on imperfect knowledge or
insufficient attention, he finds reason to correct his judgment, though
perhaps too late to influence his decision by after consideration. A world
within him is thus opened to his intellectual view, abounding with
phenomena and relations, and of the highest immediate interest. But while
he cannot help perceiving that the insight he is enabled to obtain into this
internal sphere of thought and feeling is in reality the source of all his
power, the very fountain of his predominance over external nature, he yet
feels himself capable of entering only very imperfectly into these recesses
of his own bosom, and analysing the operations of his mind,—in this as in
all other things, in short, “a being darkly wise;” seeing that all the longest
life and most vigorous intellect can give him power to discover by his own
research, or time to know by availing himself of that of others, serves only
to place him on the very frontier of knowledge, and afford a distant glimpse
of boundless realms beyond, where no human thought has penetrated, but
which yet he is sure must be no less familiarly known to that Intelligence
which he traces throughout creation than the most obvious truths which he
himself daily applies to his most trifling purposes. Is it wonderful that a
being so constituted should first encourage a hope, and by degrees
acknowledge an assurance, that his intellectual existence will not terminate
with the dissolution of his corporeal frame, but rather that in a future state
of being, disencumbered of a thousand obstructions which his present
situation throws in his way, endowed with acuter senses, and higher
faculties, he shall drink deep at that fountain of beneficent wisdom for
which the slight taste obtained on earth has given him so keen a relish?
(5.) Nothing, then, can be more unfounded than the objection which has
been taken, in limine, by persons, well meaning perhaps, certainly narrow-
minded, against the study of natural philosophy, and indeed against all
science,—that it fosters in its cultivators an undue and overweening self-
conceit, leads them to doubt the immortality of the soul, and to scoff at
revealed religion. Its natural effect, we may confidently assert, on every

well constituted mind is and must be the direct contrary. No doubt, the
testimony of natural reason, on whatever exercised, must of necessity stop
short of those truths which it is the object of revelation to make known; but,
while it places the existence and principal attributes of a Deity on such
grounds as to render doubt impossible, it unquestionably opposes no natural
or necessary obstacle to further progress: on the contrary, by cherishing as a
vital principle an unbounded spirit of enquiry, and ardency of expectation, it
unfetters the mind from prejudices of every kind, and leaves it open and
free to every impression of a higher nature which it is susceptible of
receiving, guarding only against enthusiasm and self-deception by a habit of
strict investigation, but encouraging, rather than suppressing, every thing
that can offer a prospect or a hope beyond the present obscure and
unsatisfactory state. The character of the true philosopher is to hope all
things not impossible, and to believe all things not unreasonable. He who
has seen obscurities which appeared impenetrable in physical and
mathematical science suddenly dispelled, and the most barren and
unpromising fields of enquiry converted, as if by inspiration, into rich and
inexhaustible springs of knowledge and power on a simple change of our
point of view, or by merely bringing to bear on them some principle which
it never occurred before to try, will surely be the very last to acquiesce in
any dispiriting prospects of either the present or future destinies of
mankind; while, on the other hand, the boundless views of intellectual and
moral as well as material relations which open on him on all hands in the
course of these pursuits, the knowledge of the trivial place he occupies in
the scale of creation, and the sense continually pressed upon him of his own
weakness and incapacity to suspend or modify the slightest movement of
the machinery he sees in action around him, must effectually convince him
that humility of pretension, no less than confidence of hope, is what best
becomes his character.
(6.) But while we thus vindicate the study of natural philosophy from a
charge at one time formidable, owing to the pertinacity and acrimony with
which it was urged, and still occasionally brought forward to the distress
and disgust of every well constituted mind, we must take care that the
testimony afforded by science to religion, be its extent or value what it may,
shall be at least independent, unbiassed, and spontaneous. We do not here
allude to such reasoners as would make all nature bend to their narrow
interpretations of obscure and difficult passages in the sacred writings: such

a course might well become the persecutors of Galileo and the other bigots
of the fifteenth and sixteenth centuries, but can only be adopted by
dreamers in the present age. But, without going these lengths, it is no
uncommon thing to find persons, earnestly attached to science and anxious
for its promotion, who yet manifest a morbid sensibility on points of this
kind,—who exult and applaud when any fact starts up explanatory (as they
suppose) of some scriptural allusion and who feel pained and disappointed
when the general course of discovery in any department of science runs
wide of the notions with which particular passages in the Bible may have
impressed themselves. To persons of such a frame of mind it ought to
suffice to remark, on the one hand, that truth can never be opposed to truth,
and, on the other, that error is only to be effectually confounded by
searching deep and tracing it to its source. Nevertheless, it were much to be
wished that such persons, estimable and excellent as many of them are,
before they throw the weight of their applause or discredit into the scale of
scientific opinion on such grounds, would reflect, first, that the credit and
respectability of any evidence may be destroyed by tampering with its
honesty; and, secondly, that this very disposition of mind implies a lurking
mistrust in its own principles, since the grand and indeed only character of
truth is its capability of enduring the test of universal experience, and
coming unchanged out of every possible form of fair discussion.
(7.) But if science may be vilified by representing it as opposed to
religion, or trammelled by mistaken notions of the danger of free enquiry,
there is yet another mode by which it may be degraded from its native
dignity, and that is by placing it in the light of a mere appendage to and
caterer for our pampered appetites. The question “cui bono” to what
practical end and advantage do your researches tend? is one which the
speculative philosopher who loves knowledge for its own sake, and enjoys,
as a rational being should enjoy, the mere contemplation of harmonious and
mutually dependent truths, can seldom hear without a sense of humiliation.
He feels that there is a lofty and disinterested pleasure in his speculations
which ought to exempt them from such questioning; communicating as they
do to his own mind the purest happiness (after the exercise of the
benevolent and moral feelings) of which human nature is susceptible, and
tending to the injury of no one, he might surely allege this as a sufficient
and direct reply to those who, having themselves little capacity, and less
relish for intellectual pursuits, are constantly repeating upon him this

enquiry. But if he can bring himself to descend from this high but fair
ground, and justify himself, his pursuits, and his pleasures in the eyes of
those around him, he has only to point to the history of all science, where
speculations, apparently unprofitable, have, in innumerable instances, been
those from which great practical applications have emanated. What, for
instance, could be more so than the dry speculations of the ancient
geometers on the properties of the conic sections, or than the dreams of
Kepler (as they would naturally appear to his contemporaries) about the
numerical harmonies of the universe? Yet these are the steps by which we
have risen to a knowledge of the elliptic motions of the planets and the law
of gravitation, with all its splendid theoretical consequences, and its
inestimable practical results. The ridicule attached to “Swing-swangs” in
1
Hooke’s time did not prevent him from reviving the proposal of the
pendulum as a standard of measure, since so effectually wrought into
practice by the genius and perseverance of Captain Kater;—nor did that
which Boyle encountered in his researches on the elasticity and pressure of
the air act as any obstacle to the train of discovery which terminated in the
steam-engine. The dreams of the alchemists led them on in the path of
experiment, and drew attention to the wonders of chemistry, while they
brought their advocates (it must be admitted) to merited contempt and ruin.
But in this case it was moral dereliction which gave to ridicule a weight and
power not necessarily or naturally belonging to it: but among the alchemists
were men of superior minds, who reasoned while they worked, and who,
not content to grope always in the dark, and blunder on their object, sought
carefully in the observed nature of their agents for guides in their pursuit.
To these we owe the creation of experimental philosophy.
(8.) Not that it is meant, by any thing above said, to assert that there is
no such thing as a great or a little in speculative philosophy, or to place the
solution of an enigma on a level with the developement of a law of nature,
2
still less to adopt the homely definition of Smith , that a philosopher is a
person whose trade it is to do nothing, and speculate on every thing. The
speculations of the natural philosopher, however remote they may for a time
lead him from beaten tracks and every-day uses, being grounded in the
realities of nature, have all, of necessity, a practical application,—nay more,
such applications form the very criterions of their truth, they afford the
readiest and completest verifications of his theories;—verifications which

he will no more neglect to test them by than an arithmetician would omit to
prove his sums, or a cautious geometer to try his general theorems by
3
particular cases.
(9.) After all, however, it must be confessed, that to minds
unacquainted with science, and unused to consider the mutual dependencies
of its various branches, there is something neither unnatural nor altogether
blamable in the ready occurrence of this question of direct advantage. It
requires some habit of abstraction, some penetration of the mind with a
tincture of scientific enquiry, some conviction of the value of those
estimable and treasured principles which lie concealed in the most common
and homely facts,—some experience, in fine, of success in developing and
placing them in evidence, announcing them in precise terms, and applying
them to the explanation of other facts of a less familiar character, or to the
accomplishment of some obviously useful purpose:—to cure the mind of
this tendency to rush at once upon its object, to undervalue the means in
over-estimation of the end, and while gazing too intently at the goal which
alone it has been accustomed to desire, to lose sight of the richness and
variety of the prospects that offer themselves on either hand on the road.
(10.) We must never forget that it is principles, not phenomena,—the
interpretation, not the mere knowledge of facts,—which are the objects of
enquiry to the natural philosopher. As truth is single, and consistent with
itself, a principle may be as completely and as plainly elucidated by the
most familiar and simple fact, as by the most imposing and uncommon
phenomenon. The colours which glitter on a soap-bubble are the immediate
consequence of a principle the most important from the variety of
phenomena it explains, and the most beautiful, from its simplicity and
compendious neatness, in the whole science of optics. If the nature of
periodical colours can be made intelligible by the contemplation of such a
trivial object, from that moment it becomes a noble instrument in the eye of
correct judgment; and to blow a large, regular, and durable soap-bubble
may become the serious and praiseworthy endeavour of a sage, while
children stand round and scoff, or children of a larger growth hold up their
hands in astonishment at such waste of time and trouble. To the natural
philosopher there is no natural object unimportant or trifling. From the least
of nature’s works he may learn the greatest lessons. The fall of an apple to
the ground may raise his thoughts to the laws which govern the revolutions

of the planets in their orbits; or the situation of a pebble may afford him
evidence of the state of the globe he inhabits, myriads of ages ago, before
his species became its denizens.
(11.) And this is, in fact, one of the great sources of delight which the
study of natural science imparts to its votaries. A mind which has once
imbibed a taste for scientific enquiry, and has learnt the habit of applying its
principles readily to the cases which occur, has within itself an
inexhaustible source of pure and exciting contemplations:—one would
think that Shakspeare had such a mind in view when he describes a
contemplative man as finding all nature eloquent—the very trees, the
brooks, and the stones reading to him lessons of deep and serious import.
Accustomed to trace the operation of general causes, and the
exemplification of general laws, in circumstances where the uninformed
and unenquiring eye perceives neither novelty nor beauty, he walks in the
midst of wonders: every object which falls in his way elucidates some
principle, affords some instruction, and impresses him with a sense of
harmony and order. Nor is it a mere passive pleasure which is thus
communicated. A thousand questions are continually arising in his mind, a
thousand subjects of enquiry presenting themselves, which keep his
faculties in constant exercise, and his thoughts perpetually on the wing, so
that lassitude is excluded from his life, and that craving after artificial
excitement and dissipation of mind, which leads so many into frivolous,
unworthy, and destructive pursuits, is altogether eradicated from his bosom.
(12.) It is not one of the least advantages of these pursuits, which,
however, they possess in common with every class of intellectual pleasures,
that they are altogether independent of external circumstances, and are to be
enjoyed in every situation in which a man can be placed in life. The highest
degrees of worldly prosperity are so far from being incompatible with them,
that they supply inestimable advantages for their pursuit, and that sort of
fresh and renewed relish which arises partly from the sense of contrast,
partly from experience of the peculiar pre-eminence they possess over the
pleasures of sense in their capability of unlimited increase and continual
repetition without satiety or distaste. They may be enjoyed, too, in the
intervals of the most active business; and the calm and dispassionate
interest with which they fill the mind renders them a most delightful retreat
from the agitations and dissensions of the world, and from the conflict of

passions, prejudices, and interests in which the man of business finds
himself involved. There is something in the contemplation of general laws
which powerfully induces and persuades us to merge individual feeling, and
to commit ourselves unreservedly to their disposal; while the observation of
the calm, energetic regularity of nature, the immense scale of her
operations, and the certainty with which her ends are attained, tends,
irresistibly, to tranquillize and re-assure the mind, and render it less
accessible to repining, selfish, and turbulent emotions. And this it does, not
by debasing our nature into weak compliances and abject submission to
circumstances, but by filling us, as from an inward spring, with a sense of
nobleness and power which enables us to rise superior to them; by showing
us our strength and innate dignity, and by calling upon us for the exercise of
those powers and faculties by which we are susceptible of the
comprehension of so much greatness, and which form, as it were, a link
between ourselves and the best and noblest benefactors of our species, with
whom we hold communion in thoughts and participate in discoveries which
have raised them above their fellow-mortals, and brought them nearer to
their Creator.

CHAP. II.
OF ABSTRACT SCIENCE AS A PREPARATION FOR THE
STUDY OF PHYSICS.—A PROFOUND ACQUAINTANCE
WITH IT NOT INDISPENSABLE FOR A CLEAR
UNDERSTANDING OF PHYSICAL LAWS.—HOW A
CONVICTION OF THEIR TRUTH MAY BE OBTAINED
WITHOUT IT.—INSTANCES.—FURTHER DIVISION OF
THE SUBJECT.

(13.) Science is the knowledge of many, orderly and methodically digested
and arranged, so as to become attainable by one. The knowledge of reasons
and their conclusions constitutes abstract, that of causes and their effects,
and of the laws of nature, natural science.
(14.) Abstract science is independent of a system of nature,—of a
creation,—of every thing, in short, except memory, thought, and reason. Its
objects are, first, those primary existences and relations which we cannot
even conceive not to be, such as space, time, number, order, &c.; and,
secondly, those artificial forms, or symbols, which thought has the power of
creating for itself at pleasure, and substituting as representatives, by the aid
of memory, for combinations of those primary objects and of its own
conceptions,—either to facilitate the act of reasoning respecting them, or as
convenient deposits of its own conclusions, or for their communication to
others. Such are, first, language, oral or written; its conventional forms,
which constitute grammar, and the rules for its use in argument, in which
consists the logic of the schools; secondly, notation, which, applied to
number, is arithmetic,—and, to the more general relations of abstract
quantity or order, is algebra; and, thirdly, that higher kind of logic, which
teaches us to use our reason in the most advantageous manner for the
discovery of truth; which points out the criterions by which we may be sure

we have attained it; and which, by detecting the sources of error, and
exposing the haunts where fallacies are apt to lurk, at once warns us of their
danger, and shows us how to avoid them. This greater logic may be termed
4
rational ; while, to that inferior department which is conversant with words
5
alone, the epithet verbal may, for distinction, be applied.
(15.) A certain moderate degree of acquaintance with abstract science is
highly desirable to every one who would make any considerable progress in
physics. As the universe exists in time and place; and as motion, velocity,
quantity, number, and order, are main elements of our knowledge of
external things and their changes, an acquaintance with these, abstractedly
considered, (that is to say, independent of any consideration of the
particular things moved, measured, counted, or arranged,) must evidently be
a useful preparation for the more complex study of nature. But there is yet
another recommendation of such sciences as a preparation for the study of
natural philosophy. Their objects are so definite, and our notions of them so
distinct, that we can reason about them with an assurance, that the words
and signs used in our reasonings are full and true representatives of the
things signified; and, consequently, that when we use language or signs in
argument, we neither, by their use, introduce extraneous notions, nor
exclude any part of the case before us from consideration. For example: the
words space, square, circle, a hundred, &c., convey to the mind notions so
complete in themselves, and so distinct from every thing else, that we are
sure when we use them we know and have in view the whole of our own
meaning. It is widely different with words expressing natural objects and
mixed relations. Take, for instance, iron. Different persons attach very
different ideas to this word. One who has never heard of magnetism has a
widely different notion of iron from one in the contrary predicament. The
vulgar, who regard this metal as incombustible, and the chemist, who sees it
burn with the utmost fury, and who has other reasons for regarding it as one
of the most combustible bodies in nature;—the poet, who uses it as an
emblem of rigidity; and the smith and engineer, in whose hands it is plastic,
and moulded like wax into every form;—the jailer, who prizes it as an
obstruction, and the electrician, who sees in it only a channel of open
communication by which that most impassable of obstacles, the air, may be
traversed by his imprisoned fluid, have all different, and all imperfect,
notions of the same word. The meaning of such a term is like a rainbow—

every body sees a different one, and all maintain it to be the same. So it is
with nearly all our terms of sense. Some are indefinite, as hard or soft, light
or heavy (terms which were at one time the sources of innumerable
mistakes and controversies); some excessively complex, as man, life,
instinct. But, what is worst of all, some, nay most, have two or three
meanings; sufficiently distinct from each other to make a proposition true in
one sense and false in another, or even false altogether; yet not distinct
enough to keep us from confounding them in the process by which we
arrived at it, or to enable us immediately to recognise the fallacy when led
to it by a train of reasoning, each step of which we think we have examined
and approved. Surely those who thus attach two senses to one word, or
superadd a new meaning to an old one, act as absurdly as colonists who
distribute themselves over the world, naming every place they come to by
the names of those they have left, till all distinctions of geographical
nomenclature are confounded, and till we are unable to decide whether an
occurrence stated to have happened at Windsor took place in Europe,
6
America, or Australia.
(16.) It is, in fact, in this double or incomplete sense of words that we
must look for the origin of a very large portion of the errors into which we
fall. Now, the study of the abstract sciences, such as arithmetic, geometry,
algebra, &c., while they afford scope for the exercise of reasoning about
objects that are, or, at least, may be conceived to be, external to us; yet,
being free from these sources of error and mistake, accustom us to the strict
use of language as an instrument of reason, and by familiarizing us, in our
progress towards truth, to walk uprightly and straight-forward on firm
ground, give us that proper and dignified carriage of mind which could
never be acquired by having always to pick our steps among obstructions
and loose fragments, or to steady them in the reeling tempest of conflicting
meanings.
(17.) But there is yet another point of view under which some
acquaintance with abstract science may be regarded as highly desirable in
general education, if not indispensably necessary, to impress on us the
distinction between strict and vague reasoning, to show us what
demonstration really is, and to give us thereby a full and intimate sense of
the nature and strength of the evidence on which our knowledge of the
actual system of nature, and the laws of natural phenomena, rests. For this

purpose, however, a very moderate acquaintance with the more elementary
branches of mathematics may suffice. The chain is laid before us, and every
link is submitted to our unreserved examination, if we have patience and
inclination to enter on such detail. Hundreds have gone through it, and will
continue to do so; but, for the generality of mankind, it is enough to satisfy
themselves of the solidity and adamantine texture of its materials, and the
unreserved exposure of its weakest, as well as its strongest, parts. If,
however, we content ourselves with this general view of the matter, we
must be content also to take on trust, that is, on the authority of those who
have examined deeper, every conclusion which cannot be made apparent to
our senses. Now, among these there are many so very surprising, indeed
apparently so extravagant, that it is quite impossible for any enquiring mind
to rest contented with a mere hearsay statement of them,—we feel
irresistibly impelled to enquire further into their truth. What mere assertion
will make any man believe, that in one second of time, in one beat of the
pendulum of a clock, a ray of light travels over 192,000 miles, and would
therefore perform the tour of the world in about the same time that it
requires to wink with our eyelids, and in much less than a swift runner
occupies in taking a single stride? What mortal can be made to believe,
without demonstration, that the sun is almost a million times larger than the
earth? and that, although so remote from us, that a cannon ball shot directly
towards it, and maintaining its full speed, would be twenty years in
reaching it, it yet affects the earth by its attraction in an inappreciable
instant of time?—a closeness of union of which we can form but a feeble,
and totally inadequate, idea, by comparing it to any material connection;
since the communication of an impulse to such a distance, by any solid
intermedium we are acquainted with, would require, not moments, but
whole years. And when, with pain and difficulty we have strained our
imagination to conceive a distance so vast, a force so intense and
penetrating, if we are told that the one dwindles to an insensible point, and
the other is unfelt at the nearest of the fixed stars, from the mere effect of
their remoteness, while among those very stars are some whose actual
splendour exceeds by many hundred times that of the sun itself, although
we may not deny the truth of the assertion, we cannot but feel the keenest
curiosity to know how such things were ever made out.
(18.) The foregoing are among those results of scientific research
which, by their magnitude, seem to transcend our powers of conception.

There are others, again, which, from their minuteness, would appear to
elude the grasp of thought, much more of distinct and accurate
measurement. Who would not ask for demonstration, when told that a
gnat’s wing, in its ordinary flight, beats many hundred times in a second? or
that there exist animated and regularly organized beings, many thousands of
whose bodies laid close together would not extend an inch? But what are
these to the astonishing truths which modern optical enquiries have
disclosed, which teach us that every point of a medium through which a ray
of light passes is affected with a succession of periodical movements,
regularly recurring at equal intervals, no less than five hundred millions of
millions of times in a single second! that it is by such movements,
communicated to the nerves of our eyes, that we see:—nay more, that it is
the difference in the frequency of their recurrence which affects us with the
sense of the diversity of colour; that, for instance, in acquiring the sensation
of redness our eyes are affected four hundred and eighty-two millions of
millions of times; of yellowness, five hundred and forty-two millions of
millions of times; and of violet, seven hundred and seven millions of
7
millions of times per second. Do not such things sound more like the
ravings of madmen, than the sober conclusions of people in their waking
senses?
(19.) They are, nevertheless, conclusions to which any one may most
certainly arrive, who will only be at the trouble of examining the chain of
reasoning by which they have been deduced; but, in order to do this,
something beyond the mere elements of abstract science is required.
Waving, however, such instances as these, which, after all, are rather
calculated to surprise and astound than for any other purpose, it must be
observed that it is not possible to satisfy ourselves completely that we have
arrived at a true statement of any law of nature, until, setting out from such
statement, and making it a foundation of reasoning, we can show, by strict
argument, that the facts observed must follow from it as necessary logical
consequences, and this, not vaguely and generally, but with all possible
precision in time, place, weight, and measure.
(20.) To do this, however, as we shall presently see, requires in many
cases a degree of knowledge of mathematics and geometry altogether
unattainable by the generality of mankind, who have not the leisure, even if
they all had the capacity, to enter into such enquiries, some of which are

indeed of that degree of difficulty that they can be only successfully
prosecuted by persons who devote to them their whole attention, and make
them the serious business of their lives. But there is scarcely any person of
good ordinary understanding, however little exercised in abstract enquiries,
who may not be readily made to comprehend at least the general train of
reasoning by which any of the great truths of physics are deduced, and the
essential bearings and connections of the several parts of natural
philosophy. There are whole branches too and very extensive and important
ones, to which mathematical reasoning has never been at all applied; such
as chemistry, geology, and natural history in general, and many others, in
which it plays a very subordinate part, and of which the essential principles,
and the grounds of application to useful purposes, may be perfectly well
understood by a student who possesses no more mathematical knowledge
than the rules of arithmetic; so that no one need be deterred from the
acquisition of knowledge, or even from active original research in such
subjects, by a want of mathematical information. Even in those branches
which, like astronomy, optics, and dynamics, are almost exclusively under
the dominion of mathematics, and in which no effectual progress can be
made without some acquaintance with geometry, the principal results may
be perfectly understood without it. To one incapable of following out the
intricacies of mathematical demonstration, the conviction afforded by
verified predictions must stand in the place of that purer and more
satisfactory reliance which a verification of every step in the process of
reasoning can alone afford, since every one will acknowledge the validity of
pretensions which he is in the daily habit of seeing brought to the test of
practice.
(21.) Among the verifications of this practical kind which abound in
every department of physics, there are none more imposing than the precise
prediction of the greater phenomena of astronomy; none, certainly, which
carry a broader conviction home to every mind from their notoriety and
unequivocal character. The prediction of eclipses has accordingly from the
earliest ages excited the admiration of mankind, and been one grand
instrument by which their allegiance (so to speak) to natural science, and
their respect for its professors, has been maintained; and though strangely
abused in unenlightened ages by the supernatural pretensions of astrologers,
the credence given even to their absurdities shows the force of this kind of
evidence on men’s minds. The predictions of astronomers are, however,

now far too familiar to endanger the just equipoise of our judgment, since
even the return of comets, true to their paths and exact to the hour of their
appointment, has ceased to amaze, though it must ever delight all who have
souls capable of being penetrated by such beautiful instances of accordance
between theory and facts. But the age of mere wonder in such things is past,
and men prefer being guided and enlightened, to being astonished and
dazzled. Eclipses, comets, and the like, afford but rare and transient
displays of the powers of calculation, and of the certainty of the principles
on which it is grounded. A page of “lunar distances” from the Nautical
Almanack is worth all the eclipses that have ever happened for inspiring
this necessary confidence in the conclusions of science. That a man, by
merely measuring the moon’s apparent distance from a star with a little
portable instrument held in his hand, and applied to his eye, even with so
unstable a footing as the deck of a ship, shall say positively, within five
miles, where he is, on a boundless ocean, cannot but appear to persons
ignorant of physical astronomy an approach to the miraculous. Yet, the
alternatives of life and death, wealth and ruin, are daily and hourly staked
with perfect confidence on these marvellous computations, which might
almost seem to have been devised on purpose to show how closely the
extremes of speculative refinement and practical utility can be brought to
approximate. We have before us an anecdote communicated to us by a naval
8
officer , distinguished for the extent and variety of his attainments, which
shows how impressive such results may become in practice. He sailed from
San Blas on the west coast of Mexico, and after a voyage of 8000 miles,
occupying 89 days, arrived off Rio de Janeiro, having, in this interval,
passed through the Pacific Ocean, rounded Cape Horn, and crossed the
South Atlantic, without making any land, or even seeing a single sail, with
the exception of an American whaler off Cape Horn. Arrived within a
week’s sail of Rio, he set seriously about determining, by lunar
observations, the precise line of the ship’s course and its situation in it at a
determinate moment, and having ascertained this within from five to ten
miles, ran the rest of the way by those more ready and compendious
methods, known to navigators, which can be safely employed for short trips
between one known point and another, but which cannot be trusted in long
voyages, where the moon is the only sure guide. The rest of the tale we are
enabled by his kindness to state in his own words:—“We steered towards
Rio de Janeiro for some days after taking the lunars above described, and

having arrived within fifteen or twenty miles of the coast, I hove to at four
in the morning till the day should break, and then bore up; for although it
was very hazy, we could see before us a couple of miles or so. About eight
o’clock it became so foggy that I did not like to stand in farther, and was
just bringing the ship to the wind again before sending the people to
breakfast, when it suddenly cleared off, and I had the satisfaction of seeing
the great Sugar Loaf Rock, which stands on one side of the harbour’s
mouth, so nearly right ahead that we had not to alter our course above a
point in order to hit the entrance of Rio. This was the first land we had seen
for three months, after crossing so many seas and being set backwards and
forwards by innumerable currents and foul winds.” The effect on all on
board might well be conceived to have been electric; and it is needless to
remark how essentially the authority of a commanding officer over his crew
may be strengthened by the occurrence of such incidents, indicative of a
degree of knowledge and consequent power beyond their reach.
(22.) But even such results as these, striking as they are, yet fall short
of the force with which conviction is urged upon us when, through the
medium of reasoning too abstract for common apprehension, we arrive at
conclusions which outrun experience, and describe beforehand what will
happen under new combinations, or even correct imperfect experiments,
and lead us to a knowledge of facts contrary to received analogies drawn
from an experience wrongly interpreted or overhastily generalised. To give
an example:—every body knows that objects viewed through a transparent
medium, such as water or glass, appear distorted or displaced. Thus, a stick
in water appears bent, and an object seen through a prism or wedge of glass
seems to be thrown aside from its true place. This effect is owing to what is
called the refraction of light; and a simple rule discovered by Willebrod
Snell enables any one to say exactly how much the stick will be bent, and
how far, and in what direction, the apparent situation of an object seen
through the glass will deviate from the real one. If a shilling be laid at the
bottom of a basin of water and viewed obliquely, it will appear to be raised
by the water; if instead of water spirits of wine be used it will appear more
raised; if oil, still more:—but in none of these cases will it appear to be
thrown aside to the right or left of its true place, however the eye be
situated. The plane, in which are contained the eye, the object, and the point
in the surface of the liquid at which the object is seen, is an upright or
vertical plane; and this is one of the principal characters in the ordinary

refraction of light, viz. that the ray by which we see an object through a
refracting surface, although it undergoes a bending, and is, as it were,
broken at the surface, yet, in pursuing its course to the eye, does not quit a
plane perpendicular to the refracting surface. But there are again other
substances, such as rock-crystal, and especially Iceland spar, which possess
the singular property of doubling the image or appearance of an object seen
through them in certain directions; so that instead of seeing one object we
see two, side by side, when such a crystal or spar is interposed between the
object and the eye; and if a ray or small sunbeam be thrown upon a surface
of either of these substances, it will be split into two, making an angle with
each other, and each pursuing its own separate course,—this is called
double refraction. Now, of these images or doubly refracted rays, one
always follows the same rule as if the substance were glass or water: its
deviation can be correctly calculated by Snell’s law above mentioned, and it
does not quit the plane perpendicular to the refracting surface. The other
ray, on the contrary, (which is therefore said to have undergone
extraordinary refraction) does quit that plane, and the amount of its
deviation from its former course requires for its determination a much more
complicated rule, which cannot be understood or even stated without a
pretty intimate knowledge of geometry. Now, rock-crystal and Iceland spar
differ from glass in a very remarkable circumstance. They affect naturally
certain regular figures, not being found in shapeless lumps, but in
determinate geometrical forms; and they are susceptible of being cleft or
split much easier in certain directions than in others—they have a grain
which glass has not. When other substances having this peculiarity (and
which are called crystallized substances) were examined, they were all, or
by far the greater part, found to possess this singular property of double
refraction; and it was very natural to conclude, therefore, that the same
thing took place in all of them, viz. that of the two rays, into which any
beam of light falling on the surface of such a substance was split, or of the
two images of an object seen through it, one only was turned aside out of its
plane and extraordinarily refracted, while the other followed the ordinary
rule. Accordingly this was supposed to be the case; and not only so, but
from some trials and measurements purposely made by a philosopher of
great eminence, it was considered to be a fact sufficiently established by
experiment.

(23.) Perhaps we might have remained long under this impression, for
the measurements are delicate, and the subject very difficult. But it has
lately been demonstrated by an eminent French philosopher and
mathematician, M. Fresnel, that, granting certain principles or postulates,
all the phenomena of double refraction, including perhaps the greatest
variety of facts that have ever yet been arranged under one general head,
may be satisfactorily explained and deduced from them by strict
mathematical calculation; and that, when applied to the cases first
mentioned, these principles give a satisfactory account of the want of the
extraordinary image; that when applied to such cases as those of rock-
crystal or Iceland spar, they also give a correct account of both the images,
and agree in their conclusions with the rules before ascertained for them:
but so far from coinciding with that part of the previous statement, which
would make these conclusions extend to all crystallised substances, M.
Fresnel’s principles lead to a conclusion quite opposite, and point to a fact
which had never been observed, viz. that in by far the greater number of
crystallized substances which possess the property of double refraction,
neither of the images follows the ordinary law, but both undergo a deviation
from their original plane. Now this had never been observed to be the case
in any previous trial, and all opinion was against it. But when put to the test
of experiment in a great variety of new and ingenious methods, it was found
to be fully verified; and to complete the evidence, the substances on whose
imperfect examination the first erroneous conclusion was founded, having
been lately subjected to a fresh and more scrupulous examination, the result
has shown the insufficiency of the former measurements, and proved in
perfect accordance with the newly discovered laws. Now it will be observed
in this case, first, that, so far from the principles assumed by M. Fresnel
being at all obvious, they are extremely remote from ordinary observation;
and, secondly, that the chain of reasoning by which they are brought to the
test is one of such length and complexity, and the purely mathematical
difficulty of their application so great, that no mere good common sense, no
general tact or ordinary practical reasoning, would afford the slightest
chance of threading their mazes. Cases like this are the triumph of theories.
They show at once how large a part pure reason has to perform in our
examination of nature, and how implicit our reliance ought to be on that
powerful and methodical system of rules and processes which constitute the

modern mathematical analysis, in all the more difficult applications of exact
calculation to her phenomena.
(24.) To take an instance more within ordinary apprehension. An
eminent living geometer had proved by calculations, founded on strict
optical principles, that in the centre of the shadow of a small circular plate
of metal, exposed in a dark room to a beam of light emanating from a very
small brilliant point, there ought to be no darkness,—in fact, no shadow at
that place; but, on the contrary, a degree of illumination precisely as bright
as if the metal plate were away. Strange and even impossible as this
conclusion may seem, it has been put to the trial, and found perfectly
9
correct.
(25.) We shall now proceed to consider more particularly, and in
detail,—

I. The nature and objects immediate and collateral of physical
science, as regarded in itself, and in its application to the
practical purposes of life, and its influence on the well-being
and progress of society.
II. The principles on which it relies for its successful prosecution,
and the rules by which a systematic examination of nature
should be conducted, with examples illustrative of their
influence.
III. The subdivision of physical science into distinct branches, and
their mutual relations.

CHAP. III.
OF THE NATURE AND OBJECTS, IMMEDIATE AND
COLLATERAL, OF PHYSICAL SCIENCE, AS REGARDED
IN ITSELF, AND IN ITS APPLICATION TO THE
PRACTICAL PURPOSES OF LIFE, AND ITS INFLUENCE
ON THE WELL-BEING AND PROGRESS OF SOCIETY.

(26.) The first thing impressed on us from our earliest infancy is, that
events do not succeed one another at random, but with a certain degree of
order, regularity, and connection;—some constantly, and, as we are apt to
think, immutably,—as the alternation of day and night, summer and winter,
—others contingently, as the motion of a body from its place, if pushed, or
the burning of a stick if thrust into the fire. The knowledge that the former
class of events has gone on, uninterruptedly, for ages beyond all memory,
impresses us with a strong expectation that it will continue to do so in the
same manner; and thus our notion of an order of nature is originated and
confirmed. If every thing were equally regular and periodical, and the
succession of events liable to no change depending on our own will, it may
be doubted whether we should ever think of looking for causes. No one
regards the night as the cause of the day, or the day of night. They are
alternate effects of a common cause, which their regular succession alone
gives us no sufficient clue for determining. It is chiefly, perhaps entirely,
from the other or contingent class of events that we gain our notions of
cause and effect. From them alone we gather that there are such things as
laws of nature. The very idea of a law includes that of contingency. “Si quis
mala carmina condidisset, fuste ferito;” if such a case arise, such a course
shall be followed,—if the match be applied to the gunpowder, it will
explode. Every law is a provision for cases which may occur, and has
relation to an infinite number of cases that never have occurred, and never
will. Now, it is this provision, à priori, for contingencies, this

contemplation of possible occurrences, and predisposal of what shall
happen, that impresses us with the notion of a law and a cause. Among all
the possible combinations of the fifty or sixty elements which chemistry
shows to exist on the earth, it is likely, nay almost certain, that some have
never been formed; that some elements, in some proportions, and under
some circumstances, have never yet been placed in relation with one
another. Yet no chemist can doubt that it is already fixed what they will do
when the case does occur. They will obey certain laws, of which we know
nothing at present, but which must be already fixed, or they could not be
laws. It is not by habit, or by trial and failure, that they will learn what to
do. When the contingency occurs, there will be no hesitation, no
consultation;—their course will at once be decided, and will always be the
same if it occur ever so often in succession, or in ever so many places at one
and the same instant. This is the perfection of a law, that it includes all
possible contingencies, and ensures implicit obedience,—and of this kind
are the laws of nature.
(27.) This use of the word law, however, our readers will of course
perceive has relation to us as understanding, rather than to the materials of
which the universe consists as obeying, certain rules. To obey a law, to act
in compliance with a rule, supposes an understanding and a will, a power of
complying or not, in the being who obeys and complies, which we do not
admit as belonging to mere matter. The Divine Author of the universe
cannot be supposed to have laid down particular laws, enumerating all
individual contingencies, which his materials have understood and obey,—
this would be to attribute to him the imperfections of human legislation;—
but rather, by creating them, endued with certain fixed qualities and powers,
he has impressed them in their origin with the spirit, not the letter, of his
law, and made all their subsequent combinations and relations inevitable
consequences of this first impression, by which, however, we would no way
be understood to deny the constant exercise of his direct power in
maintaining the system of nature, or the ultimate emanation of every energy
which material agents exert from his immediate will, acting in conformity
with his own laws.
(28.) The discoveries of modern chemistry have gone far to establish
the truth of an opinion entertained by some of the ancients, that the universe
consists of distinct, separate, indivisible atoms, or individual beings so

minute as to escape our senses, except when united by millions, and by this
aggregation making up bodies of even the smallest visible bulk; and we
have the strongest evidence that, although there exist great and essential
differences in individuals among these atoms, they may yet all be arranged
in a very limited number of groups or classes, all the individuals of each of
which are, to all intents and purposes, exactly alike in all their properties.
Now, when we see a great number of things precisely alike, we do not
believe this similarity to have originated except from a common principle
independent of them; and that we recognise this likeness, chiefly by the
identity of their deportment under similar circumstances, strengthens rather
10
than weakens the conclusion. A line of spinning-jennies , or a regiment of
soldiers dressed exactly alike, and going through precisely the same
evolutions, gives us no idea of independent existence: we must see them act
out of concert before we can believe them to have independent wills and
properties, not impressed on them from without. And this conclusion, which
would be strong even were there only two individuals precisely alike in all
respects and for ever, acquires irresistible force when their number is
multiplied beyond the power of imagination to conceive. If we mistake not,
then, the discoveries alluded to effectually destroy the idea of an eternal
self-existent matter, by giving to each of its atoms the essential characters,
at once, of a manufactured article, and a subordinate agent.
(29.) But to ascend to the origin of things, and speculate on the
creation, is not the business of the natural philosopher. An humbler field is
sufficient for him in the endeavour to discover, as far as our faculties will
permit, what are these primary qualities originally and unalterably
impressed on matter, and to discover the spirit of the laws of nature, which
includes groups and classes of relations and facts from the letter which, as
before observed, is presented to us by single phenomena: or if, after all, this
should prove impossible; if such a step be beyond our faculties; and the
essential qualities of material agents be really occult, or incapable of being
expressed in any form intelligible to our understandings, at least to
approach as near to their comprehension as the nature of the case will
allow; and devise such forms of words as shall include and represent the
greatest possible multitude and variety of phenomena.
(30.) Now, in this research there would seem one great question to be
disposed of before our enquiries can even be commenced with any thing

like a prospect of success, which is, whether the laws of nature themselves
have that degree of permanence and fixity which can render them subjects
of systematic discussion; or whether, on the other hand, the qualities of
natural agents are subject to mutation from the lapse of time. To the
ancients, who lived in the infancy of the world, or rather, in the infancy of
man’s experience, this was a very rational subject of question, and hence
their distinctions between corruptible and incorruptible matter. Thus,
according to some among them, the matter only of the celestial spaces is
pure, immutable, and incorruptible, while all sublunary things are in a
constant state of lapse and change; the world becoming paralysed and effete
with age, and man himself deteriorating in character, and diminishing at
once in intellectual and bodily stature. But to us, who have the experience
of some additional thousands of years, the question of permanence is
already, in a great measure, decided in the affirmative. The refined
speculations of modern astronomy, grounding their conclusions on
observations made at very remote periods, have proved to demonstration,
that one at least of the great powers of nature, the force of gravitation, the
main bond and support of the material universe, has undergone no change
in intensity from a high antiquity. The stature of mankind is just what it was
three thousand years ago, as the specimens of mummies which have been
examined at various times sufficiently show. The intellect of Newton,
Laplace, or Lagrange, may stand in fair competition with that of
Archimedes, Aristotle, or Plato; and the virtues and patriotism of
Washington with the brightest examples of ancient history.
(31.) Again, the researches of chemists have shown that what the vulgar
call corruption, destruction, &c., is nothing but a change of arrangement of
the same ingredient elements, the disposition of the same materials into
other forms, without the loss or actual destruction of a single atom; and thus
any doubts of the permanence of natural laws are discountenanced, and the
whole weight of appearances thrown into the opposite scale. One of the
most obvious cases of apparent destruction is, when any thing is ground to
dust and scattered to the winds. But it is one thing to grind a fabric to
powder, and another to annihilate its materials: scattered as they may be,
they must fall somewhere, and continue, if only as ingredients of the soil, to
perform their humble but useful part in the economy of nature. The
destruction produced by fire is more striking: in many cases, as in the
burning of a piece of charcoal or a taper, there is no smoke, nothing visibly

dissipated and carried away; the burning body wastes and disappears, while
nothing seems to be produced but warmth and light, which we are not in the
habit of considering as substances; and when all has disappeared, except
perhaps some trifling ashes, we naturally enough suppose it is gone, lost,
destroyed. But when the question is examined more exactly, we detect, in
the invisible stream of heated air which ascends from the glowing coal or
flaming wax, the whole ponderable matter, only united in a new
combination with the air, and dissolved in it. Yet, so far from being thereby
destroyed, it is only become again what it was before it existed in the form
of charcoal or wax, an active agent in the business of the world, and a main
support of vegetable and animal life, and is still susceptible of running
again and again the same round, as circumstances may determine; so that,
for aught we can see to the contrary, the same identical atom may lie
concealed for thousands of centuries in a limestone rock; may at length be
quarried, set free in the limekiln, mix with the air, be absorbed from it by
plants, and, in succession, become a part of the frames of myriads of living
beings, till some concurrence of events consigns it once more to a long
repose, which, however, no way unfits it from again resuming its former
activity.
(32.) Now, this absolute indestructibility of the ultimate materials of the
world, in periods commensurate to our experience, and their obstinate
retention of the same properties, under whatever variety of circumstances
we choose to place them, however violent and seemingly contradictory to
their natures, is, of itself, enough to render it highly improbable that time
alone should have any influence over them. All that age or decay can do
seems to be included in a wasting of parts which are only dissipated, not
destroyed, or in a change of sensible properties, which chemistry
demonstrates to arise only from new combinations of the same ingredients.
But, after all, the question is one entirely of experience: we cannot be sure,
à priori, that the laws of nature are immutable; but we can ascertain, by
enquiry, whether they change or not; and to this enquiry all experience
answers in the negative. It is not, of course, intended here to deny that great
operations, productive of extensive changes in the visible state of nature,—
such as, for instance, those contemplated by the geologists, and embracing
for their completion vast periods of time,—are constantly going on; but
these are consequences and fulfilments of the laws of nature, not
contradictions or exceptions to them. No theorist regards such changes as

alterations in the fundamental principles of nature; he only endeavours to
reconcile them, and show how they result from laws already known, and
judges of the correctness of his theory by their ultimate agreement.
(33.) But the laws of nature are not only permanent, but consistent,
intelligible, and discoverable with such a moderate degree of research, as is
calculated rather to stimulate than to weary curiosity. If we were set down,
as creatures of another world, in any existing society of mankind, and began
to speculate on their actions, we should find it difficult at first to ascertain
whether they were subject to any laws at all: but when, by degrees, we had
found out that they did consider themselves to be so; and would then
proceed to ascertain, from their conduct and its consequences, what these
laws were, and in what spirit conceived; though we might not perhaps have
much difficulty in discovering single rules applicable to particular cases,
yet, the moment we came to generalize, and endeavour from these to
ascend, step by step, and discover any steady pervading principle, the mass
of incongruities, absurdities, and contradictions, we should encounter,
would either dishearten us from further enquiry or satisfy us that what we
were in search of did not exist. It is quite the contrary in nature; there we
find no contradictions, no incongruities, but all is harmony. What once is
learnt we never have to unlearn. As rules advance in generality, apparent
exceptions become regular; and equivoque, in her sublime legislation, is as
unheard of as maladministration.
(34.) Living, then, in a world where such laws obtain, and under their
immediate dominion, it is manifestly of the utmost importance to know
them, were it for no other reason than to be sure, in all we undertake, to
have, at least, the law on our side, so as not to struggle in vain against some
insuperable difficulty opposed to us by natural causes. What pains and
expense would not the alchemists, for instance, have been spared by a
knowledge of those simple laws of composition and decomposition, which
now preclude all idea of the attainment of their declared object! what an
amount of ingenuity, thrown away on the pursuit of the perpetual motion,
might have been turned to better use, if the simplest laws of mechanics had
been known and attended to by the inventors of innumerable contrivances
destined to that end! What tortures, inflicted on patients by imaginary cures
of incurable diseases, might have been dispensed with, had a few simple
principles of physiology been earlier recognised!

(35.) But if the laws of nature, on the one hand, are invincible
opponents, on the other, they are irresistible auxiliaries; and it will not be
amiss if we regard them in each of those characters, and consider the great
importance of a knowledge of them to mankind,—

I. In showing us how to avoid attempting impossibilities.
II. In securing us from important mistakes in attempting what is, in
itself, possible, by means either inadequate, or actually
opposed, to the end in view.
III. In enabling us to accomplish our ends in the easiest, shortest,
most economical, and most effectual manner.
IV. In inducing us to attempt, and enabling us to accomplish,
objects which, but for such knowledge, we should never have
thought of undertaking.

We shall therefore proceed to illustrate by examples the effect of physical
knowledge under each of these heads:—
(36.) Ex. 1. (35.) I. It is not many years since an attempt was made to
establish a colliery at Bexhill, in Sussex. The appearance of thin seams and
sheets of fossil-wood and wood-coal, with some other indications similar to
what occur in the neighbourhood of the great coal-beds in the north of
England, having led to the sinking of a shaft, and the erection of machinery
on a scale of vast expense, not less than eighty thousand pounds are said to
have been laid out on this project, which, it is almost needless to add,
proved completely abortive, as every geologist would have at once declared
it must, the whole assemblage of geological facts being adverse to the
existence of a regular coal-bed in the Hastings’ sand; while this, on which
Bexhill is situated, is separated from the coal-strata by a series of
interposed beds of such enormous thickness as to render all idea of
penetrating through them absurd. The history of mining operations is full of
similar cases, where a very moderate acquaintance with the usual order of
nature, to say nothing of theoretical views, would have saved many a
sanguine adventurer from utter ruin.
(37.) Ex. 2. (35.) II. The smelting of iron requires the application of the
most violent heat that can be raised, and is commonly performed in tall

furnaces, urged by great iron bellows driven by steam-engines. Instead of
employing this power to force air into the furnace through the intervention
of bellows, it was, on one occasion, attempted to employ the steam itself in,
apparently, a much less circuitous manner; viz. by directing the current of
steam in a violent blast, from the boiler at once into the fire. From one of
the known ingredients of steam being a highly inflammable body, and the
other that essential part of the air which supports combustion, it was
imagined that this would have the effect of increasing the fire to tenfold
fury, whereas it simply blew it out; a result which a slight consideration of
the laws of chemical combination, and the state in which the ingredient
elements exist in steam, would have enabled any one to predict without a
trial.
(38.) Ex. 3. (35.) II. After the invention of the diving-bell, and its
success in subaqueous processes, it was considered highly desirable to
devise some means of remaining for any length of time under water, and
rising at pleasure without assistance, so as either to examine, at leisure, the
bottom, or perform, at ease, any work that might be required. Some years
ago, an ingenious individual proposed a project by which this end was to be
accomplished. It consisted in sinking the hull of a ship made quite water-
tight, with the decks and sides strongly supported by shores, and the only
entry secured by a stout trap-door, in such a manner, that by disengaging,
from within, the weights employed to sink it, it might rise of itself to the
surface. To render the trial more satisfactory, and the result more striking,
the projector himself made the first essay. It was agreed that he should sink
in twenty fathoms water, and rise again without assistance at the expiration
of twenty-four hours. Accordingly, making all secure, fastening down his
trap-door, and provided with all necessaries, as well as with the means of
making signals to indicate his situation, this unhappy victim of his own
ingenuity entered and was sunk. No signal was made, and the time
appointed elapsed. An immense concourse of people had assembled to
witness his rising, but in vain; for the vessel was never seen more. The
pressure of the water at so great a depth had, no doubt, been completely
under-estimated, and the sides of the vessel being at once crushed in, the
unfortunate projector perished before he could even make the signal
concerted to indicate his distress.

(39.) Ex. 4. (35.) III. In the granite quarries near Seringapatam the most
enormous blocks are separated from the solid rock by the following neat
and simple process. The workman having found a portion of the rock
sufficiently extensive, and situated near the edge of the part already
quarried, lays bare the upper surface, and marks on it a line in the direction
of the intended separation, along which a groove is cut with a chisel about a
couple of inches in depth. Above this groove a narrow line of fire is then
kindled, and maintained till the rock below is thoroughly heated,
immediately on which a line of men and women, each provided with a pot
full of cold water, suddenly sweep off the ashes, and pour the water into the
heated groove, when the rock at once splits with a clean fracture. Square
blocks of six feet in the side, and upwards of eighty feet in length, are
sometimes detached by this method, or by another equally simple and
efficacious, but not easily explained without entering into particulars of
11
mineralogical detail.
(40.) Ex. 5. (35.) III. Hardly less simple and efficacious is the process
used in some parts of France, where mill-stones are made. When a mass of
stone sufficiently large is found, it is cut into a cylinder several feet high,
and the question then arises how to subdivide this into horizontal pieces so
as to make as many mill-stones. For this purpose horizontal indentations or
grooves are chiselled out quite round the cylinder, at distances
corresponding to the thickness intended to be given to the mill-stones, into
which wedges of dried wood are driven. These are then wetted, or exposed
to the night dew, and next morning the different pieces are found separated
from each other by the expansion of the wood, consequent on its absorption
of moisture; an irresistible natural power thus accomplishing, almost
without any trouble, and at no expense, an operation which, from the
peculiar hardness and texture of the stone, would otherwise be
impracticable but by the most powerful machinery or the most persevering
labour.
(41.) Ex. 6. (35.) III. To accomplish our ends quickly is often of, at
least, as much importance as to accomplish them with little labour and
expense. There are innumerable processes which, if left to themselves, i. e.
to the ordinary operation of natural causes, are done, and well done, but
with extreme slowness, and in such cases it is often of the highest practical
importance to accelerate them. The bleaching of linen, for instance,

performed in the natural way by exposure to sun, rain, and wind, requires
many weeks or even months for its completion; whereas, by the simple
immersion of the cloth in a liquid, chemically prepared, the same effect is
produced in a few hours. The whole circle of the arts, indeed, is nothing but
one continued comment upon this head of our subject. The instances above
given are selected, not on account of their superior importance, but for the
simplicity and directness of application of the principles on which they
depend, to the objects intended to be attained.
(42.) But so constituted is the mind of man, that his views enlarge, and
his desires and wants increase, in the full proportion of the facilities
afforded to their gratification, and, indeed, with augmented rapidity, so that
no sooner has the successful exercise of his powers accomplished any
considerable simplification or improvement of processes subservient to his
use or comfort, than his faculties are again on the stretch to extend the
limits of his newly acquired power; and having once experienced the
advantages which are to be gathered by availing himself of some of the
powers of nature to accomplish his ends, he is led thenceforward to regard
them all as a treasure placed at his disposal, if he have only the art, the
industry, or the good fortune, to penetrate those recesses which conceal
them from immediate view. Having once learned to look on knowledge as
power, and to avail himself of it as such, he is no longer content to limit his
enterprises to the beaten track of former usage, but is constantly led
onwards to contemplate objects which, in a previous stage of his progress,
he would have regarded as unattainable and visionary, had he even thought
of them at all. It is here that the investigation of the hidden powers of nature
becomes a mine, every vein of which is pregnant with inexhaustible wealth,
and whose ramifications appear to extend in all directions wherever human
wants or curiosity may lead us to explore.
(43.) Between the physical sciences and the arts of life there subsists a
constant mutual interchange of good offices, and no considerable progress
can be made in the one without of necessity giving rise to corresponding
steps in the other. On the one hand, every art is in some measure, and many
entirely, dependent on those very powers and qualities of the material world
which it is the object of physical enquiry to investigate and explain; and,
accordingly, abundant examples might be cited of cases where the remarks
of experienced artists, or even ordinary workmen, have led to the discovery

of natural qualities, elements, or combinations which have proved of the
highest importance in physics. Thus (to give an instance), a soap-
manufacturer remarks that the residuum of his ley, when exhausted of the
alkali for which he employs it, produces a corrosion of his copper boiler for
which he cannot account. He puts it into the hands of a scientific chemist
for analysis, and the result is the discovery of one of the most singular and
important chemical elements, iodine. The properties of this, being studied,
are found to occur most appositely in illustration and support of a variety of
new, curious, and instructive views then gaining ground in chemistry, and
thus exercise a marked influence over the whole body of that science.
Curiosity is excited: the origin of the new substance is traced to the sea-
plants from whose ashes the principal ingredient of soap is obtained, and
ultimately to the sea-water itself. It is thence hunted through nature,
discovered in salt mines and springs, and pursued into all bodies which
12
have a marine origin; among the rest, into sponge. A medical practitioner
then calls to mind a reputed remedy for the cure of one of the most grievous
and unsightly disorders to which the human species is subject—the goître—
which infests the inhabitants of mountainous districts to an extent that in
this favoured land we have happily no experience of, and which was said to
have been originally cured by the ashes of burnt sponge. Led by this
indication he tries the effect of iodine on that complaint, and the result
establishes the extraordinary fact that this singular substance, taken as a
medicine, acts with the utmost promptitude and energy on goître,
dissipating the largest and most inveterate in a short time, and acting (of
course, like all medicines, even the most approved, with occasional
failures,) as a specific, or natural antagonist, against that odious deformity.
It is thus that any accession to our knowledge of nature is sure, sooner or
later, to make itself felt in some practical application, and that a benefit
conferred on science by the casual observation or shrewd remark of even an
unscientific or illiterate person infallibly repays itself with interest, though
often in a way that could never have been at first contemplated.
(44.) It is to such observation, reflected upon, however, and matured
into a rational and scientific form by a mind deeply imbued with the best
principles of sound philosophy, that we owe the practice of vaccination; a
practice which has effectually subdued, in every country where it has been
introduced, one of the most frightful scourges of the human race, and in

some extirpated it altogether. Happily for us we know only by tradition the
ravages of the small-pox, as it existed among us hardly more than a century
ago, and as it would in a few years infallibly exist again, were the barriers
which this practice, and that of inoculation, oppose to its progress
abandoned. Hardly inferior to this terrible scourge on land was, within the
last seventy or eighty years, the scurvy at sea. The sufferings and
destruction produced by this horrid disorder on board our ships when, as a
matter of course, it broke out after a few months’ voyage, seem now almost
incredible. Deaths to the amount of eight or ten a day in a moderate ship’s
company; bodies sewn up in hammocks and washing about the decks for
want of strength and spirits on the part of the miserable survivors to cast
them overboard; and every form of loathsome and excruciating misery of
which the human frame is susceptible:—such are the pictures which the
13
narratives of nautical adventure in those days continually offer. At present
the scurvy is almost completely eradicated in the navy, partly, no doubt,
from increased and increasing attention to general cleanliness, comfort, and
diet; but mainly from the constant use of a simple and palatable preventive,
the acid of the lemon, served out in daily rations. If the gratitude of
mankind be allowed on all hands to be the just meed of the philosophic
physician, to whose discernment in seizing, and perseverance in forcing it
on public notice we owe the great safeguard of infant life, it ought not to be
14
denied to those whose skill and discrimination have thus strengthened the
sinews of our most powerful arm, and obliterated one of the darkest features
in the most glorious of all professions.
(45.) These last, however, are instances of simple observation, limited
to the point immediately in view, and assuming only so far the character of
science as a systematic adoption of good and rejection of evil, when
grounded on experience carefully weighed, justly entitle it to do. They are
not on that account less appositely cited as instances of the importance of a
knowledge of nature and its laws to our well-being; though, like the great
inventions of the mariner’s compass and of gunpowder, they may have
stood, in their origin, unconnected with more general views. They are rather
to be looked upon as the spontaneous produce of a territory essentially
fertile, than as forming part of the succession of harvests which the same
bountiful soil, diligently cultivated, is capable of yielding. The history of
iodine above related affords, however, a perfect specimen of the manner in

which a knowledge of natural properties and laws, collected from facts
having no reference to the object to which they have been subsequently
applied, enables us to set in array the resources of nature against herself;
and deliberately, of afore-thought, to devise remedies against the dangers
and inconveniences which beset us. In this view we might instance, too, the
conductor, which, in countries where thunder-storms are more frequent and
violent than in our own, and at sea (where they are attended with peculiar
danger, both from the greater probability of accident, and its more terrible
consequences when it does occur,) forms a most real and efficient
15
preservative against the effects of lightning :—the safety-lamp, which
enables us to walk with light and security while surrounded with an
atmosphere more explosive than gunpowder:—the life-boat, which cannot
be sunk, and which offers relief in circumstances of all others the most
distressing to humanity, and of which a recent invention promises to extend
the principle to ships of the largest class:—the lighthouse, with the capital
improvements which the lenses of Brewster and Fresnel, and the elegant
lamp of lieutenant Drummond, have conferred, and promise yet to confer
by their wonderful powers, the one of producing the most intense light yet
known, the others of conveying it undispersed to great distances:—the
discovery of the disinfectant powers of chlorine, and its application to the
destruction of miasma and contagion:—that of quinine, the essential
principle in which reside the febrifuge qualities of the Peruvian bark, a
discovery by which posterity is yet to benefit in its full extent, but which
has already begun to diffuse comparative comfort and health through
16
regions almost desolated by pestiferous exhalations ;—and, if we desist, it
is not because the list is exhausted, but because a sample, not a catalogue, is
intended.
(46.) One instance more, however, we will add, to illustrate the manner
in which a most familiar effect, which seemed destined only to amuse
children, or, at best, to furnish a philosophic toy, may become a safeguard
of human life, and a remedy for a most serious and distressing evil. In
needle manufactories the workmen who point the needles are constantly
exposed to excessively minute particles of steel which fly from the
grindstones, and mix, though imperceptible to the eye, as the finest dust in
the air, and are inhaled with their breath. The effect, though imperceptible
on a short exposure, yet, being constantly repeated from day to day,

produces a constitutional irritation dependent on the tonic properties of the
steel, which is sure to terminate in pulmonary consumption; insomuch, that
persons employed in this kind of work used scarcely ever to attain the age
17
of forty years. In vain was it attempted to purify the air before its entry
into the lungs by gauzes or linen guards; the dust was too fine and
penetrating to be obstructed by such coarse expedients, till some ingenious
person bethought him of that wonderful power which every child who
searches for its mother’s needle with a magnet, or admires the motions and
arrangement of a few steel filings on a sheet of paper held above it, sees in
exercise. Masks of magnetized steel wire are now constructed and adapted
to the faces of the workmen. By these the air is not merely strained but
searched in its passage through them, and each obnoxious atom arrested
and removed.
(47.) Perhaps there is no result which places in a stronger light the
advantages which are to be derived from a mere knowledge of the usual
order of nature, without any attempt on our part to modify it, and apart from
all consideration of its causes, than the institution of life-assurances.
Nothing is more uncertain than the life of a single individual; and it is the
sense of this insecurity which has given rise to such institutions. They are,
in their nature and objects, the precise reverse of gambling speculations,
their object being to equalize vicissitude, and to place the pecuniary
relations of numerous masses of mankind, in so far as they extend, on a
footing independent of individual casualty. To do this with the greatest
possible advantage, or indeed with any advantage at all, it is necessary to
know the laws of mortality, or the average numbers of individuals, out of a
great multitude, who die at every period of life from infancy to extreme old
age. At first sight this would seem a hopeless enquiry; to some, perhaps, a
presumptuous one. But it has been made; and the result is, that, abating
extraordinary causes, such as wars, pestilence, and the like, a remarkable
regularity does obtain, quite sufficient to afford grounds not only for
general estimations, but for nice calculations of risk and adventure, such as
infallibly to insure the success of any such institution founded on good
computations; and thus to confer such stability on the fortunes of families
dependent on the exertions of one individual as to constitute an important
feature in modern civilization. The only thing to be feared in such
institutions is their too great multiplication and consequent competition, by

which a spirit of gambling and underbidding is liable to be generated
among their conductors, and the very mischief may be produced, on a scale
of frightful extent, which they are especially intended to prevent.
(48.) We have hitherto considered only cases in which a knowledge of
natural laws enables us to improve our condition, by counteracting evils of
which, but for its possession, we must have remained forever the helpless
victims. Let us now take a similar view of those in which we are enabled to
call in nature as an auxiliary to augment our actual power, and capacitate us
for undertakings, which without such aid might seem to be hopeless. Now,
to this end, it is necessary that we should form a just conception of what
those hidden powers of nature are, which we can at pleasure call into
action;—how far they transcend the measure of human force, and set at
naught the efforts not only of individuals but of whole nations of men.
(49.) It is well known to modern engineers, that there is virtue in a
bushel of coals properly consumed, to raise seventy millions of pounds
weight a foot high. This is actually the average effect of an engine at this
18
moment working in Cornwall. Let us pause a moment, and consider what
this is equivalent to in matters of practice.
(50.) The ascent of Mont Blanc from the valley of Chamouni is
considered, and with justice, as the most toilsome feat that a strong man can
execute in two days. The combustion of two pounds of coal would place
19
him on the summit.
(51.) The Menai Bridge, one of the most stupendous works of art that
has been raised by man in modern ages, consists of a mass of iron, not less
than four millions of pounds in weight, suspended at a medium height of
about 120 feet above the sea. The consumption of seven bushels of coal
would suffice to raise it to the place where it hangs.
(52.) The great pyramid of Egypt is composed of granite. It is 700 feet
in the side of its base, and 500 in perpendicular height, and stands on eleven
acres of ground. Its weight is, therefore, 12,760 millions of pounds, at a
medium height of 125 feet; consequently it would be raised by the effort of
about 630 chaldrons of coal, a quantity consumed in some founderies in a
week.

(53.) The annual consumption of coal in London is estimated at
1,500,000 chaldrons. The effort of this quantity would suffice to raise a
cubical block of marble, 2200 feet in the side, through a space equal to its
own height, or to pile one such mountain upon another. The Monte Nuovo,
near Pozzuoli, (which was erupted in a single night by volcanic fire,) might
have been raised by such an effort, from a depth of 40,000 feet, or about
eight miles.
(54.) It will be observed, that, in the above statement, the inherent
power of fuel is, of necessity, greatly under-rated. It is not pretended by
engineers that the economy of fuel is yet pushed to its utmost limit, or that
the whole effective power is obtained in any application of fire yet devised;
so that were we to say 100 millions instead of 70, we should probably be
nearer the truth.
(55.) The powers of wind and water, which we are constantly
impressing into our service, can scarcely be called latent or hidden, yet it is
not fully considered, in general, what they do effect for us. Those who
would judge of what advantage may be taken of the wind, for example,
even on land (not to speak of navigation), may turn their eyes on Holland. A
great portion of the most valuable and populous tract of this country lies
much below the level of the sea, and is only preserved from inundation by
the maintenance of embankments. Though these suffice to keep out the
abrupt influx of the ocean, they cannot oppose that law of nature, by which
fluids, in seeking their level, insinuate themselves through the pores and
subterraneous channels of a loose sandy soil, and keep the country in a
constant state of infiltration from below upwards. To counteract this
tendency, as well as to get rid of the rain water, which has no natural outlet,
pumps worked by windmills are established in great numbers, on the dams
and embankments, which pour out the water, as from a leaky ship, and in
effect preserve the country from submersion, by taking advantage of every
20
wind that blows. To drain the Haarlem lake would seem a hopeless project
to any speculators but those who had the steam-engine at their command, or
had learnt in Holland what might be accomplished by the constant agency
of the desultory but unwearied powers of wind. But the Dutch engineer
measures his surface, calculates the number of his pumps, and, trusting to
time and his experience of the operation of the winds for the success of his

undertaking, boldly forms his plans to lay dry the bed of an inland sea, of
21
which those who stand on one shore cannot see the other.
(56.) To gunpowder, as a source of mechanical power, it seems hardly
necessary to call attention; yet it is only when we endeavour to confine it,
that we get a full conception of the immense energy of that astonishing
agent. In count Rumford’s experiments, twenty-eight grains of powder
confined in a cylindrical space, which it just filled, tore asunder a piece of
22
iron which would have resisted a strain of 400,000 lbs. , applied at no
greater mechanical disadvantage.
(57.) But chemistry furnishes us with means of calling into sudden
action forces of a character infinitely more tremendous than that of
gunpowder. The terrific violence of the different fulminating compositions
is such, that they can only be compared to those untameable animals, whose
ferocious strength has hitherto defied all useful management, or rather to
spirits evoked by the spells of a magician, manifesting a destructive and
unapproachable power, which makes him but too happy to close his book,
and break his wand, as the price of escaping: unhurt from the storm he has
raised. Such powers are not yet subdued to our purposes, whatever they
may hereafter be; but, in the expansive force of gases, liberated slowly and
manageably from chemical mixtures, we have a host of inferior, yet still
most powerful, energies, capable of being employed in a variety of useful
23
ways, according to emergencies.
(58.) Such are the forces which nature lends us for the accomplishment
of our purposes, and which it is the province of practical Mechanics to
teach us to combine and apply in the most advantageous manner; without
which the mere command of power would amount to nothing. Practical
Mechanics is, in the most pre-eminent sense, a scientific art; and it may be
truly asserted, that almost all the great combinations of modern mechanism,
and many of its refinements and nicer improvements, are creations of pure
intellect, grounding its exertion upon a moderate number of very
elementary propositions in theoretical mechanics and geometry. On this
head we might dwell long, and find ample matter, both for reflection and
wonder; but it would require not volumes merely, but libraries, to
enumerate and describe the prodigies of ingenuity which have been

lavished on every thing connected with machinery and engineering. By
these it is that we are enabled to diffuse over the whole earth the
productions of any part of it; to fill every corner of it with miracles of art
and labour, in exchange for its peculiar commodities; and to concentrate
around us, in our dwellings, apparel and utensils, the skill and workmanship
not of a few expert individuals, but of all who, in the present and past
generations, have contributed their improvements to the processes of our
manufactures.
(59.) The transformations of chemistry, by which we are enabled to
convert the most apparently useless materials into important objects in the
arts, are opening up to us every day sources of wealth and convenience of
which former ages had no idea, and which have been pure gifts of science
to man. Every department of art has felt their influence, and new instances
are continually starting forth of the unlimited resources which this
wonderful science developes in the most sterile parts of nature. Not to
mention the impulse which its progress has given to a host of other
sciences, which will come more particularly under consideration in another
part of this discourse, what strange and unexpected results has it not
brought to light in its application to some of the most common objects!
Who, for instance, would have conceived that linen rags were capable of
producing more than their own weight of sugar, by the simple agency of one
24
of the cheapest and most abundant acids? —that dry bones could be a
magazine of nutriment, capable of preservation for years, and ready to yield
up their sustenance in the form best adapted to the support of life, on the
application of that powerful agent, steam, which enters so largely into all
25
our processes, or of an acid at once cheap and durable? —that sawdust
itself is susceptible of conversion into a substance bearing no remote
analogy to bread; and though certainly less palatable than that of flour, yet
no way disagreeable, and both wholesome and digestible as well as highly
26
nutritive? What economy, in all processes where chemical agents are
employed, is introduced by the exact knowledge of the proportions in which
natural elements unite, and their mutual powers of displacing each other!
What perfection in all the arts where fire is employed, either in its more
violent applications, (as, for instance, in the smelting of metals by the
introduction of well adapted fluxes, whereby we obtain the whole produce

of the ore in its purest state,) or in its milder forms, as in sugar-refining (the
whole modern practice of which depends on a curious and delicate remark
of a late eminent scientific chemist on the nice adjustment of temperature at
which the crystallization of syrup takes place); and a thousand other arts
which it would be tedious to enumerate!
(60.) Armed with such powers and resources, it is no wonder if the
enterprise of man should lead him to form and execute projects which, to
one uninformed of their grounds, would seem altogether disproportionate.
Were they to have been proposed at once, we should, no doubt, have
rejected them as such: but developed, as they have been, in the slow
succession of ages, they have only taught us that things regarded impossible
in one generation may become easy in the next; and that the power of man
over nature is limited only by the one condition, that it must be exercised in
conformity with the laws of nature. He must study those laws as he would
the disposition of a horse he would ride, or the character of a nation he
would govern; and the moment he presumes either to thwart her
fundamental rules, or ventures to measure his strength with hers, he is at
once rendered severely sensible of his imbecility, and meets the deserved
punishment of his rashness and folly. But if, on the other hand, he will
consent to use, without abusing, the resources thus abundantly placed at his
disposal, and obey that he may command, there seems scarcely any
conceivable limit to the degree in which the average physical condition of
great masses of mankind may be improved, their wants supplied, and their
conveniences and comforts increased. Without adopting such an
exaggerated view, as to assert that the meanest inhabitant of a civilized
society is superior in physical condition to the lordly savage, whose energy
and uncultivated ability gives him a natural predominance over his fellow
denizens of the forest,—at least, if we compare like with like, and consider
the multitude of human beings who are enabled, in an advanced state of
society, to subsist in a degree of comfort and abundance, which at best only
a few of the most fortunate in a less civilized state could command, we shall
not be at a loss to perceive the principle on which we ought to rest our
estimate of the advantages of civilization; and which applies with hardly
less force to every degree of it, when contrasted with that next inferior, than
to the broad distinction between civilized and barbarous life in general.

(61.) The difference of the degrees in which the individuals of a great
community enjoy the good things of life has been a theme of declamation
and discontent in all ages; and it is doubtless our paramount duty, in every
state of society, to alleviate the pressure of the purely evil part of this
distribution as much as possible, and, by all the means we can devise,
secure the lower links in the chain of society from dragging in dishonour
and wretchedness: but there is a point of view in which the picture is at least
materially altered in its expression. In comparing society on its present
immense scale, with its infant or less developed state, we must at least take
care to enlarge every feature in the same proportion. If, on comparing the
very lowest states in civilized and savage life, we admit a difficulty in
deciding to which the preference is due, at least in every superior grade we
cannot hesitate a moment; and if we institute a similar comparison in every
different stage of its progress, we cannot fail to be struck with the rapid rate
of dilatation which every degree upward of the scale, so to speak, exhibits,
and which, in an estimate of averages, gives an immense preponderance to
the present over every former condition of mankind, and, for aught we can
see to the contrary, will place succeeding generations in the same degree of
superior relation to the present that this holds to those passed away. Or we
may put the same proposition in other words, and, admitting the existence
of every inferior grade of advantage in a higher state of civilization which
subsisted in the preceding, we shall find, first, that, taking state for state, the
proportional numbers of those who enjoy the higher degrees of advantage
increases with a constantly accelerated rapidity as society advances; and,
secondly, that the superior extremity of the scale is constantly enlarging by
the addition of new degrees. The condition of a European prince is now as
far superior, in the command of real comforts and conveniences, to that of
one in the middle ages, as that to the condition of one of his own
dependants.
(62.) The advantages conferred by the augmentation of our physical
resources through the medium of increased knowledge and improved art
have this peculiar and remarkable property,—that they are in their nature
diffusive, and cannot be enjoyed in any exclusive manner by a few. An
eastern despot may extort the riches and monopolize the art of his subjects
for his own personal use; he may spread around him an unnatural splendour
and luxury, and stand in strange and preposterous contrast with the general
penury and discomfort of his people; he may glitter in jewels of gold and

raiment of needlework; but the wonders of well contrived and executed
manufacture which we use daily, and the comforts which have been
invented, tried, and improved upon by thousands, in every form of domestic
convenience, and for every ordinary purpose of life, can never be enjoyed
by him. To produce a state of things in which the physical advantages of
civilized life can exist in a high degree, the stimulus of increasing comforts
and constantly elevated desires, must have been felt by millions; since it is
not in the power of a few individuals to create that wide demand for useful
and ingenious applications, which alone can lead to great and rapid
improvements, unless backed by that arising from the speedy diffusion of
the same advantages among the mass of mankind.
(63.) If this be true of physical advantages, it applies with still greater
force to intellectual. Knowledge can neither be adequately cultivated nor
adequately enjoyed by a few; and although the conditions of our existence
on earth may be such as to preclude an abundant supply of the physical
necessities of all who may be born, there is no such law of nature in force
against that of our intellectual and moral wants. Knowledge is not, like
food, destroyed by use, but rather augmented and perfected. It acquires not,
perhaps, a greater certainty, but at least a confirmed authority and a
probable duration, by universal assent; and there is no body of knowledge
so complete, but that it may acquire accession, or so free from error but that
it may receive correction in passing through the minds of millions. Those
who admire and love knowledge for its own sake ought to wish to see its
elements made accessible to all, were it only that they may be the more
thoroughly examined into, and more effectually developed in their
consequences, and receive that ductility and plastic quality which the
pressure of minds of all descriptions, constantly moulding them to their
purposes, can alone bestow. But to this end it is necessary that it should be
divested, as far as possible, of artificial difficulties, and stripped of all such
technicalities as tend to place it in the light of a craft and a mystery,
inaccessible without a kind of apprenticeship. Science, of course, like every
thing else, has its own peculiar terms, and, so to speak, its idioms of
language; and these it would be unwise, were it even possible, to relinquish:
but every thing that tends to clothe it in a strange and repulsive garb, and
especially every thing that, to keep up an appearance of superiority in its
professors over the rest of mankind, assumes an unnecessary guise of
profundity and obscurity, should be sacrificed without mercy. Not to do this,

is to deliberately reject the light which the natural unencumbered good
sense of mankind is capable of throwing on every subject, even in the
elucidation of principles: but where principles are to be applied to practical
uses it becomes absolutely necessary; as all mankind have then an interest
in their being so familiarly understood, that no mistakes shall arise in their
application.
(64.) The same remark applies to arts. They cannot be perfected till
their whole processes are laid open, and their language simplified and
rendered universally intelligible. Art is the application of knowledge to a
practical end. If the knowledge be merely accumulated experience, the art is
empirical; but if it be experience reasoned upon and brought under general
principles, it assumes a higher character, and becomes a scientific art. In the
progress of mankind from barbarism to civilised life, the arts necessarily
precede science. The wants and cravings of our animal constitution must be
satisfied; the comforts, and some of the luxuries, of life must exist.
Something must be given to the vanity of show, and more to the pride of
power: the round of baser pleasures must have been tried and found
insufficient, before intellectual ones can gain a footing; and when they have
obtained it, the delights of poetry and its sister arts still take precedence of
contemplative enjoyments, and the severer pursuits of thought; and when
these in time begin to charm from their novelty, and sciences begin to arise,
they will at first be those of pure speculation. The mind delights to escape
from the trammels which had bound it to earth, and luxuriates in its newly
found powers. Hence, the abstractions of geometry—the properties of
numbers—the movements of the celestial spheres—whatever is abstruse,
remote, and extramundane—become the first objects of infant science.
Applications come late: the arts continue slowly progressive, but their realm
remains separated from that of science by a wide gulf which can only be
passed by a powerful spring. They form their own language and their own
conventions, which none but artists can understand. The whole tendency of
empirical art, is to bury itself in technicalities, and to place its pride in
particular short cuts and mysteries known only to adepts; to surprise and
astonish by results, but conceal processes. The character of science is the
direct contrary. It delights to lay itself open to enquiry, and is not satisfied
with its conclusions, till it can make the road to them broad and beaten: and
in its applications it preserves the same character; its whole aim being to
strip away all technical mystery, to illuminate every dark recess, and to gain

free access to all processes, with a view to improve them on rational
principles. It would seem that a union of two qualities almost opposite to
each other—a going forth of the thoughts in two directions, and a sudden
transfer of ideas from a remote station in one to an equally distant one in the
other—is required to start the first idea of applying science. Among the
Greeks, this point was attained by Archimedes, but attained too late, on the
eve of that great eclipse of science which was destined to continue for
nearly eighteen centuries, till Galileo in Italy, and Bacon in England, at
once dispelled the darkness: the one, by his inventions and discoveries; the
other, by the irresistible force of his arguments and eloquence.
(65.) Finally, the improvement effected in the condition of mankind by
advances in physical science as applied to the useful purposes of life, is
very far from being limited to their direct consequences in the more
abundant supply of our physical wants, and the increase of our comforts.
Great as these benefits are, they are yet but steps to others of a still higher
kind. The successful results of our experiments and reasonings in natural
philosophy, and the incalculable advantages which experience,
systematically consulted and dispassionately reasoned on, has conferred in
matters purely physical, tend of necessity to impress something of the well
weighed and progressive character of science on the more complicated
conduct of our social and moral relations. It is thus that legislation and
politics become gradually regarded as experimental sciences; and history,
not, as formerly, the mere record of tyrannies and slaughters, which, by
immortalizing the execrable actions of one age, perpetuates the ambition of
committing them in every succeeding one, but as the archive of
experiments, successful and unsuccessful, gradually accumulating towards
the solution of the grand problem—how the advantages of government are
to be secured with the least possible inconvenience to the governed. The
celebrated apophthegm, that nations never profit by experience, becomes
yearly more and more untrue. Political economy, at least, is found to have
sound principles, founded in the moral and physical nature of man, which,
however lost sight of in particular measures—however even temporarily
controverted and borne down by clamour—have yet a stronger and stronger
testimony borne to them in each succeeding generation, by which they
must, sooner or later, prevail. The idea once conceived and verified, that
great and noble ends are to be achieved, by which the condition of the
whole human species shall be permanently bettered, by bringing into

exercise a sufficient quantity of sober thought, and by a proper adaptation
of means, is of itself sufficient to set us earnestly on reflecting what ends
are truly great and noble, either in themselves, or as conducive to others of
a still loftier character; because we are not now, as heretofore, hopeless of
attaining them. It is not now equally harmless and insignificant, whether we
are right or wrong; since we are no longer supinely and helplessly carried
down the stream of events, but feel ourselves capable of buffetting at least
with its waves, and perhaps of riding triumphantly over them: for why
should we despair that the reason which has enabled us to subdue all nature
to our purposes, should (if permitted and assisted by the providence of God)
achieve a far more difficult conquest; and ultimately find some means of
enabling the collective wisdom of mankind to bear down those obstacles
which individual short-sightedness, selfishness, and passion, oppose to all
improvements, and by which the highest hopes are continually blighted, and
the fairest prospects marred.

PART II.
OF THE PRINCIPLES ON WHICH PHYSICAL SCIENCE
RELIES FOR ITS SUCCESSFUL PROSECUTION, AND THE
RULES BY WHICH A SYSTEMATIC EXAMINATION OF
NATURE SHOULD BE CONDUCTED, WITH
ILLUSTRATIONS OF THEIR INFLUENCE AS
EXEMPLIFIED IN THE HISTORY OF ITS PROGRESS.

CHAPTER I.
OF EXPERIENCE AS THE SOURCE OF OUR KNOWLEDGE.—
OF THE DISMISSAL OF PREJUDICES.—OF THE EVIDENCE
OF OUR SENSES.

(66.) Into abstract science, as we have before observed, the notion of cause
does not enter. The truths it is conversant with are necessary ones, and exist
independent of cause. There may be no such real thing as a right-lined
triangle marked out in space; but the moment we conceive one in our
minds, we cannot refuse to admit the sum of its three angles to be equal to
two right angles; and if in addition we conceive one of its angles to be a
right angle, we cannot thenceforth refuse to admit that the sum of the
squares on the two sides, including the right angle, is equal to the square on
the side subtending it. To maintain the contrary, would be, in effect, to deny
its being right angled. No one causes or makes all the diameters of an
ellipse to be bisected in its centre. To assert the contrary, would not be to
rebel against a power, but to deny our own words. But in natural science
cause and effect are the ultimate relations we contemplate; and laws,
whether imposed or maintained, which, for aught we can perceive, might
have been other than they are. This distinction is very important. A clever
man, shut up alone and allowed unlimited time, might reason out for
himself all the truths of mathematics, by proceeding from those simple
notions of space and number of which he cannot divest himself without
ceasing to think. But he could never tell, by any effort of reasoning, what
would become of a lump of sugar if immersed in water, or what impression
would be produced on his eye by mixing the colours yellow and blue.
(67.) We have thus pointed out to us, as the great, and indeed only
ultimate source of our knowledge of nature and its laws, EXPERIENCE; by
which we mean, not the experience of one man only, or of one generation,

but the accumulated experience of all mankind in all ages, registered in
books or recorded by tradition. But experience may be acquired in two
ways: either, first, by noticing facts as they occur, without any attempt to
influence the frequency of their occurrence, or to vary the circumstances
under which they occur; this is OBSERVATION: or, secondly, by putting in
action causes and agents over which we have control, and purposely
varying their combinations, and noticing what effects take place; this is
EXPERIMENT. To these two sources we must look as the fountains of all
natural science. It is not intended, however, by thus distinguishing
observation from experiment, to place them in any kind of contrast.
Essentially they are much alike, and differ rather in degree than in kind; so
that, perhaps, the terms passive and active observation might better express
their distinction; but it is, nevertheless, highly important to mark the
different states of mind in inquiries carried on by their respective aids, as
well as their different effects in promoting the progress of science. In the
former, we sit still and listen to a tale, told us, perhaps obscurely, piecemeal,
and at long intervals of time, with our attention more or less awake. It is
only by after-rumination that we gather its full import; and often, when the
opportunity is gone by, we have to regret that our attention was not more
particularly directed to some point which, at the time, appeared of little
moment, but of which we at length appretiate the importance. In the latter,
on the other hand, we cross-examine our witness, and by comparing one
part of his evidence with the other, while he is yet before us, and reasoning
upon it in his presence, are enabled to put pointed and searching questions,
the answer to which may at once enable us to make up our minds.
Accordingly it has been found invariably, that in those departments of
physics where the phenomena are beyond our control, or into which
experimental enquiry, from other causes, has not been carried, the progress
of knowledge has been slow, uncertain, and irregular; while in such as
admit of experiment, and in which mankind have agreed to its adoption, it
has been rapid, sure, and steady. For example, in our knowledge of the
nature and causes of volcanoes, earthquakes, the fall of stones from the sky,
the appearance of new stars and disappearance of old ones, and other of
those great phenomena of nature which are altogether beyond our
command, and at the same time are of too rare occurrence to permit any one
to repeat and rectify his impressions respecting them, we know little more
now than in the earliest times. Here our tale is told us slowly, and in broken

sentences. In astronomy, again, we have at least an uninterrupted narrative;
the opportunity of observation is constantly present, and makes up in some
measure for the impossibility of varying our point of view, and calling for
information at the precise moment it is wanted. Accordingly, astronomy,
regarded as a science of mere observation, arrived, though by very slow
degrees, to a state of considerable maturity. But the moment that it became
a branch of mechanics, a science essentially experimental, (that is to say,
one in which any principle laid down can be subjected to immediate and
decisive trial, and where experience does not require to be waited for,) its
progress suddenly acquired a tenfold acceleration; nay, to such a degree,
that it has been asserted, and we believe with truth, that were the records of
all observations from the earliest ages annihilated, leaving only those made
27 28
in a single observatory , during a single lifetime , the whole of this most
perfect of sciences might, from those data, and as to the objects included in
them, be at once reconstructed, and appear precisely as it stood at their
conclusion. To take another instance: mineralogy, till modern times, could
hardly be said to exist. The description of even the precious stones in
Theophrastus and Pliny are, in most cases, hardly sufficient to identify
them, and in many fall short even of that humble object; more recent
observers, by attending more carefully to the obvious characters of
minerals, had formed a pretty extensive catalogue of them, and made
various attempts to arrange and methodize the knowledge thus acquired,
and even to deduce some general conclusions respecting the forms they
habitually assume: but from the moment that chemical analysis was applied
to resolve them into their constituent elements, and that, led by a happy
accident, the genius of Bergmann discovered the general fact, that they
could be cloven or split in such directions as to lay bare their peculiar
primitive or fundamental forms, (which lay concealed within them, as the
statue might be conceived encrusted in its marble envelope,)—from that
moment, mineralogy ceased to be an unmeaning list of names, a mere
laborious cataloguing of stones and rubbish, and became, what it now is, a
regular, methodical, and most important science, in which every year is
bringing to light new relations, new laws, and new practical applications.
(68.) Experience once recognized as the fountain of all our knowledge
of nature, it follows that, in the study of nature and its laws, we ought at
once to make up our minds to dismiss as idle prejudices, or at least suspend

as premature, any preconceived notion of what might or what ought to be
the order of nature in any proposed case, and content ourselves with
observing, as a plain matter of fact, what is. To experience we refer, as the
only ground of all physical enquiry. But before experience itself can be used
with advantage, there is one preliminary step to make, which depends
wholly on ourselves: it is the absolute dismissal and clearing the mind of all
prejudice, from whatever source arising, and the determination to stand and
fall by the result of a direct appeal to facts in the first instance, and of strict
logical deduction from them afterwards. Now, it is necessary to distinguish
between two kinds of prejudices, which exercise very different dominion
over the mind, and, moreover, differ extremely in the difficulty of
dispossessing them, and the process to be gone through for that purpose.
These are,—
1. Prejudices of opinion.
2. Prejudices of sense.
(69.) By prejudices of opinion, we mean opinions hastily taken up,
either from the assertion of others, from our own superficial views, or from
vulgar observation, and which, from being constantly admitted without
dispute, have obtained the strong hold of habit on our minds. Such were the
opinions once maintained that the earth is the greatest body in the universe,
and placed immovable in its centre, and all the rest of the universe created
for its sole use; that it is the nature of fire and of sounds to ascend; that the
moonlight is cold; that dews fall from the air, &c.
(70.) To combat and destroy such prejudices we may proceed in two
ways, either by demonstrating the falsehood of the facts alleged in their
support, or by showing how the appearances, which seem to countenance
them, are more satisfactorily accounted for without their admission. But it is
unfortunately the nature of prejudices of opinion to adhere, in a certain
degree, to every mind, and to some with pertinacious obstinacy, pigris
radicibus, after all ground for their reasonable entertainment is destroyed.
Against such a disposition the student of natural science must contend with
all his power. Not that we are so unreasonable as to demand of him an
instant and peremptory dismission of all his former opinions and
judgments; all we require is, that he will hold them without bigotry, retain
till he shall see reason to question them, and be ready to resign them when

fairly proved untenable, and to doubt them when the weight of probability
is shown to lie against them. If he refuse this, he is incapable of science.
(71.) Our resistance against the destruction of the other class of
prejudices, those of sense, is commonly more violent at first, but less
persistent, than in the case of those of opinion. Not to trust the evidence of
our senses, seems, indeed, a hard condition, and one which, if proposed,
none would comply with. But it is not the direct evidence of our senses that
we are in any case called upon to reject, but only the erroneous judgments
we unconsciously form from them, and this only when they can be shown
to be so by counter evidence of the same sort; when one sense is brought to
testify against another, for instance; or the same sense against itself, and the
obvious conclusions in the two cases disagree, so as to compel us to
acknowledge that one or other must be wrong. For example, nothing at first
can seem a more rational, obvious, and incontrovertible conclusion, than
that the colour of an object is an inherent quality, like its weight, hardness,
&c. and that to see the object, and see it of its own colour, when nothing
intervenes between our eyes and it, are one and the same thing. Yet this is
only a prejudice; and that it is so, is shown by bringing forward the same
sense of vision which led to its adoption, as evidence on the other side; for,
when the differently coloured prismatic rays are thrown, in a dark room, in
succession upon any object, whatever be the colour we are in the habit of
calling its own, it will appear of the particular hue of the light which falls
upon it: a yellow paper, for instance, will appear scarlet when illuminated
by red rays, yellow when by yellow, green by green, and blue by blue rays;
its own (so called) proper colour not in the least degree mixing with that it
so exhibits.
(72.) To give one or two more examples of the kind of illusion which
the senses practise on us, or rather which we practise on ourselves, by a
misinterpretation of their evidence: the moon at its rising and setting
appears much larger than when high up in the sky. This is, however, a mere
erroneous judgment; for when we come to measure its diameter, so far from
finding our conclusion borne out by fact, we actually find it to measure
materially less. Here is eyesight opposed to eyesight, with the advantage of
deliberate measurement. In ventriloquism we have the hearing at variance
with all the other senses, and especially with the sight, which is sometimes
contradicted by it in a very extraordinary and surprising manner, as when

the voice is made to seem to issue from an inanimate and motionless object.
If we plunge our hands, one into ice-cold water, and the other into water as
hot as can be borne, and, after letting them stay awhile, suddenly transfer
them both to a vessel full of water at a blood heat, the one will feel a
sensation of heat, the other of cold. And if we cross the two first fingers of
one hand, and place a pea in the fork between them, moving and rolling it
about on a table, we shall (especially if we close our eyes) be fully
persuaded we have two peas. If the nose be held while we are eating
cinnamon, we shall perceive no difference between its flavour and that of a
deal shaving.
(73.) These, and innumerable instances we might cite, will convince us,
that though we are never deceived in the sensible impression made by
external objects on us, yet in forming our judgments of them we are greatly
at the mercy of circumstances, which either modify the impressions actually
received, or combine them with adjuncts which have become habitually
associated with different judgments; and, therefore, that, in estimating the
degree of confidence we are to place in our conclusions, we must, of
necessity, take into account these modifying or accompanying
circumstances, whatever they may be. We do not, of course, here speak of
deranged organization; such as, for instance, a distortion of the eye,
producing double vision, and still less of mental delusion, which absolutely
perverts the meaning of sensible impressions.
(74.) As the mind exists not in the place of sensible objects, and is not
brought into immediate relation with them, we can only regard sensible
impressions as signals conveyed from them by a wonderful, and, to us,
inexplicable mechanism, to our minds, which receives and reviews them,
and, by habit and association, connects them with corresponding qualities
or affections in the objects; just as a person writing down and comparing
the signals of a telegraph might interpret their meaning. As, for instance, if
he had constantly observed that the exhibition of a certain signal was sure to
be followed next day by the announcement of the arrival of a ship at
Portsmouth, he would connect the two facts by a link of the very same
nature with that which connects the notion of a large wooden building,
filled with sailors, with the impression of her outline on the retina of a
spectator on the beach.

(75.) In captain Head’s amusing and vivid description of his journey
across the Pampas of South America occurs an anecdote quite in point. His
guide one day suddenly stopped him, and, pointing high into the air, cried
out, “A lion!” Surprised at such an exclamation, accompanied with such an
act, he turned up his eyes, and with difficulty perceived, at an immeasurable
height, a flight of condors soaring in circles in a particular spot. Beneath
that spot, far out of sight of himself or guide, lay the carcass of a horse, and
over that carcass stood (as the guide well knew) the lion, whom the condors
were eyeing with envy from their airy height. The signal of the birds was to
him what the sight of the lion alone could have been to the traveller, a full
assurance of its existence.

CHAP. II.
OF THE ANALYSIS OF PHENOMENA

(76.) Phenomena, then, or appearances, as the word is literally rendered, are
the sensible results of processes and operations carried on among external
objects, or their constituent principles, of which they are only signals,
conveyed to our minds as aforesaid. Now, these processes themselves may
be in many instances rendered sensible; that is to say, analysed, and shown
to consist in the motions or other affections of sensible objects themselves.
For instance, the phenomenon of the sound produced by a musical string, or
a bell, when struck, may be shown to be the result of a process consisting in
the rapid vibratory motion of its parts communicated to the air, and thence
to our ears; though the immediate effect on our organs of hearing does not
excite the least idea of such a motion. On the other hand, there are
innumerable instances of sensible impressions which (at least at present) we
are incapable of tracing beyond the mere sensation; for example, in the
sensations of bitterness, sweetness, &c. These, accordingly, if we were
inclined to form hasty decisions, might be regarded as ultimate qualities;
but the instance of sounds, just adduced, alone would teach us caution in
such decisions, and incline us to believe them mere results of some secret
process going on in our organs of taste, which is too subtle for us to trace. A
simple experiment will serve to set this in a clearer light. A solution of the
salt called by chemists nitrate of silver, and another of the hyposulphite of
soda, have each of them separately, when taken into the mouth, a
disgustingly bitter taste; but if they be mixed, or if one be tasted before the
mouth is thoroughly cleared of the other, the sensible impression is that of
intense sweetness. Again, the salt called tungstate of soda when first tasted
29
is sweet, but speedily changes to an intense and pure bitter, like quassia.

(77.) How far we may ever be enabled to attain a knowledge of the
ultimate and inward processes of nature in the production of phenomena,
we have no means of knowing; but, to judge from the degree of obscurity
which hangs about the only case in which we feel within ourselves a direct
power to produce any one, there seems no great hope of penetrating so far.
The case alluded to is the production of motion by the exertion of force. We
are conscious of a power to move our limbs, and by their intervention other
bodies; and that this effect is the result of a certain inexplicable process
which we are aware of, but can no way describe in words, by which we
exert force. And even when such exertion produces no visible effect, (as
when we press our two hands violently together, so as just to oppose each
other’s effort,) we still perceive, by the fatigue and exhaustion, and by the
impossibility of maintaining the effort long, that something is going on
within us, of which the mind is the agent, and the will the determining
cause. This impression which we receive of the nature of force, from our
own effort and our sense of fatigue, is quite different from that which we
obtain of it from seeing the effect of force exerted by others in producing
motion. Were there no such thing as motion, had we been from infancy shut
up in a dark dungeon, and every limb encrusted with plaster, this internal
consciousness would give us a complete idea of force; but when set at
liberty, habit alone would enable us to recognize its exertion by its signal,
motion, and that only by finding that the same action of the mind which in
our confined state enables us to fatigue and exhaust ourselves by the tension
of our muscles, puts it in our power, when at liberty, to move ourselves and
other bodies. But how obscure is our knowledge of the process going on
within us in the exercise of this important privilege, in virtue of which alone
we act as direct causes, we may judge from this, that when we put any limb
in motion, the seat of the exertion seems to us to be in the limb, whereas it
is demonstrably no such thing, but either in the brain or in the spinal
marrow; the proof of which is, that if a little fibre, called a nerve, which
forms a communication between the limb and the brain, or spine, be divided
in any part of its course, however we may make the effort, the limb will not
move.
(78.) This one instance of the obscurity which hangs about the only act
of direct causation of which we have an immediate consciousness, will
suffice to show how little prospect there is that, in our investigation of
nature, we shall ever be able to arrive at a knowledge of ultimate causes,

and will teach us to limit our views to that of laws, and to the analysis of
complex phenomena by which they are resolved into simpler ones, which,
appearing to us incapable of further analysis, we must consent to regard as
causes. Nor let any one complain of this as a limitation of his faculties. We
have here “ample room and verge enough” for the full exercise of all the
powers we possess; and, besides, it does so happen, that we are actually
able to trace up a very large portion of the phenomena of the universe to
this one cause, viz. the exertion of mechanical force; indeed, so large a
portion, that it has been made a matter of speculation whether this is not the
only one that is capable of acting on material beings.
(79.) What we mean by the analysis of complex phenomena into
simpler ones, will best be understood by an instance. Let us, therefore, take
the phenomenon of sound, and, by considering the various cases in which
sounds of all kinds are produced, we shall find that they all agree in these
points:—1st, The excitement of a motion in the sounding body. 2dly, The
communication of this motion to the air or other intermedium which is
interposed between the sounding body and our ears. 3dly, The propagation
of such motion from particle to particle of such intermedium in due
succession. 4thly, Its communication, from the particles of the intermedium
adjacent to the ear, to the ear itself. 5thly, Its conveyance in the ear, by a
certain mechanism, to the auditory nerves. 6thly, The excitement of
sensation. Now, in this analysis, we perceive that two principal matters
must be understood, before we can have a true and complete knowledge of
sound:—1st, The excitement and propagation of motion. 2dly, The
production of sensation. These, then, are two other phenomena, of a
simpler, or, it would be more correct to say, of a more general or elementary
order, into which the complex phenomenon of sound resolves itself. But
again, if we consider the communication of motion from body to body, or
from one part to another of the same, we shall perceive that it is again
resolvable into several other phenomena. 1st, The original setting in motion
of a material body, or any part of one. 2dly, The behaviour of a particle set
in motion, when it meets another lying in its way, or is otherwise impeded
or influenced by its connection with surrounding particles. 3dly, The
behaviour of the particles so impeding or influencing it under such
circumstances; besides which, the last two point out another phenomenon,
which it is necessary also to consider, viz. the phenomenon of the

connection of the parts of material bodies in masses, by which they form
aggregates, and are enabled to influence each other’s motions.
(80.) Thus, then, we see that an analysis of the phenomenon of sound
leads to the enquiry, 1st, of two causes, viz. the cause of motion, and the
cause of sensation, these being phenomena which (at least as human
knowledge stands at present) we are unable to analyse further; and,
therefore, we set them down as simple, elementary, and referable, for any
thing we can see to the contrary, to the immediate action of their causes.
2dly, Of several questions relating to the connection between the motion of
material bodies and its cause, such as, What will happen when a moving
body is surrounded on all sides by others not in motion? What will happen
when a body not in motion is advanced upon by a moving one? It is evident
that the answers to such questions as these can be no other than laws of
motion, in the sense we have above attributed to laws of nature, viz. a
statement in words of what will happen in such and such proposed general
contingencies. Lastly, we are led, by pursuing the analysis, and considering
the phenomenon of the aggregation of the parts of material bodies, and the
way in which they influence each other, to two other general phenomena,
viz., the cohesion and elasticity of matter; and these we have no means of
analysing further, and must, therefore, regard them (till we see reasons to
the contrary) as ultimate phenomena, and referable to the direct action of
causes, viz. an attractive and a repulsive force.
(81.) Of force, as counterbalanced by opposing force, we have, as
already said, an internal consciousness; and though it may seem strange to
us that matter should be capable of exerting on matter the same kind of
effort, which, judging alone from this consciousness, we might be led to
regard as a mental one; yet we cannot refuse the direct evidence of our
senses, which shows us that when we keep a spring stretched with one
hand, we feel our effort opposed exactly in the same way as if we had
ourselves opposed it with the other hand, or as it would be by that of
another person. The enquiry, therefore, into the aggregation of matter
resolves itself into the general question, What will be the behaviour of
material particles under the mutual action of opposing forces capable of
counterbalancing each other? and the answer to this question can be no
other than the announcement of the law of equilibrium, whatever law that
may be.

(82.) With regard to the cause of sensation, it must be regarded as much
more obscure than that of motion, inasmuch as we have no conscious
knowledge of it, i. e. we have no power, by any act of our minds and will, to
call up a sensation. It is true, we are not destitute of an approach to it, since,
by an effort of memory and imagination, we can produce in our minds an
impression, or idea, of a sensation which, in peculiar cases, may even
approach in vividness to actual reality. In dreams, too, and, in some cases of
disordered nerves, we have sensations without objects. But if force, as a
cause of motion, is obscure to us, even while we are in the act of exercising
it, how much more so is this other cause, whose exercise we can only
imitate imperfectly by any voluntary act, and of whose purely internal
action we are only fully conscious when in a state that incapacitates us from
reasoning, and almost from observation!
(83.) Dismissing, then, as beyond our reach, the enquiry into causes, we
must be content at present to concentrate our attention on the laws which
prevail among phenomena, and which seem to be their immediate results.
From the instance we have just given, we may perceive that every enquiry
into the intimate nature of a complex phenomenon branches out into as
many different and distinct enquiries as there are simple or elementary
phenomena into which it may be analysed; and that, therefore, it would
greatly assist us in our study of nature, if we could, by any means, ascertain
what are the ultimate phenomena into which all the composite ones
presented by it may be resolved. There is, however, clearly no way by
which this can be ascertained à priori. We must go to nature itself, and be
guided by the same kind of rule as the chemist in his analysis, who accounts
every ingredient an element till it can be decompounded and resolved into
others. So, in natural philosophy, we must account every phenomenon an
elementary or simple one till we can analyse it, and show that it is the result
of others, which in their turn become elementary. Thus, in a modified and
relative sense, we may still continue to speak of causes, not intending
thereby those ultimate principles of action on whose exertion the whole
frame of nature depends, but of those proximate links which connect
phenomena with others of a simpler, higher, and more general or
elementary kind. For example: we may regard the vibration of a musical
string as the proximate cause of the sound it yields, receiving it, so far, as an
ultimate fact, and waving or deferring enquiry into the cause of vibrations,
which is of a higher and more general nature.

(84.) Moreover, as in chemistry we are sometimes compelled to
acknowledge the existence of elements different from those already
identified and known, though we cannot insulate them, and to perceive that
substances have the characters of compounds, and must therefore be
susceptible of analysis, though we do not see how it is to be set about; so, in
physics, we may perceive the complexity of a phenomenon, without being
able to perform its analysis. For example: in magnetism, the agency of
electricity is clearly made out, and they are shown to stand to one another in
the relation of effect and cause. But the analysis of magnetism, in its
relation to particular metals, is not yet quite satisfactorily performed; and
we are compelled to admit the existence of some cause, whether proximate
or ultimate, whose presence in different metals, or in different states of the
same metal, determines that peculiar electric condition which constitutes
permanent magnetism. Cases like these, of all which science presents, offer
the highest interest. They excite enquiry, like the near approach to the
solution of an enigma; they show us that there is light, could only a certain
veil be drawn aside.
(85.) In pursuing the analysis of any phenomenon, the moment we find
ourselves stopped by one of which we perceive no analysis, and which,
therefore, we are forced to refer (at least provisionally) to the class of
ultimate facts, and to regard as elementary, the study of that phenomenon
and of its laws becomes a separate branch of science. If we encounter the
same elementary phenomenon in the analysis of several composite ones, it
becomes still more interesting, and assumes additional importance; while at
the same time we acquire information respecting the phenomenon itself, by
observing those with which it is habitually associated, that may help us at
length to its analysis. It is thus that sciences increase, and acquire a mutual
relation and dependency. It is thus, too, that we are at length enabled to
trace parallels and analogies between great branches of science themselves,
which at length terminate in a perception of their dependence on some
common phenomenon of a more general and elementary nature than that
which form the subject of either separately. It was thus, for example, that,
previous to Oërsted’s great discovery of electro-magnetism, a general
resemblance between the two sciences of electricity and magnetism was
recognised, and many of the chief phenomena in each were ascertained to
have their parallels, mutatis mutandis, in the other. It was thus, too, that an
analogy subsisting between sound and light has been gradually traced into a

closeness of agreement, which can hardly leave any reasonable doubt of
their ultimate coincidence in one common phenomenon, the vibratory
motion of an elastic medium. If it be allowed to pursue our illustration from
chemistry, and to ground its application not on what has been, but on what
may one day be, done, it is thus that the general family resemblance
between certain groups of bodies, now regarded as elementary, (as nickel
and cobalt, for instance, chlorine, iode, and brome,) will, perhaps, lead us
hereafter to perceive relations between them of a more intimate kind than
we can at present trace.
(86.) On those phenomena which are most frequently encountered in an
analysis of nature and which most decidedly resist further decomposition, it
is evident that the greatest pains and attention ought to be bestowed, not
only because they furnish a key to the greatest number of enquiries, and
serve to group and classify together the greatest range of phenomena, but by
reason of their higher nature, and because it is in these that we must look
for the direct action of causes, and the most extensive and general
enunciation of the laws of nature. These, once discovered, place in our
power the explanation of all particular facts, and become grounds of
reasoning, independent of particular trial: thus playing the same part in
natural philosophy that axioms do in geometry; containing, in a refined and
condensed state, and as it were in a quintessence, all that our reason has
occasion to draw from experience to enable it to follow out the truths of
physics by the mere application of logical argument. Indeed, the axioms of
geometry themselves may be regarded as in some sort an appeal to
experience, not corporeal, but mental. When we say, the whole is greater
than its part, we announce a general fact, which rests, it is true, on our ideas
of whole and part; but, in abstracting these notions, we begin by
considering them as subsisting in space, and time, and body, and again, in
linear, and superficial, and solid space. Again, when we say, the equals of
equals are equal, we mentally make comparisons, in equal spaces, equal
times, &c.; so that these axioms, however self-evident, are still general
propositions so far of the inductive kind, that, independently of experience,
they would not present themselves to the mind.
The only difference between these and axioms obtained from extensive
induction is this, that, in raising the axioms of geometry, the instances offer
themselves spontaneously, and without the trouble of search, and are few

and simple; in raising those of nature, they are infinitely numerous,
complicated, and remote; so that the most diligent research and the utmost
acuteness are required to unravel their web, and place their meaning in
evidence.
(87.) By far the most general phenomenon with which we are
acquainted, and that which occurs most constantly, in every enquiry into
which we enter, is motion, and its communication. Dynamics, then, or the
science of force and motion, is thus placed at the head of all the sciences;
and, happily for human knowledge, it is one in which the highest certainty
is attainable, a certainty no way inferior to mathematical demonstration. As
its axioms are few, simple, and in the highest degree distinct and definite, so
they have at the same time an immediate relation to geometrical quantity,
space, time, and direction, and thus accommodate themselves with
remarkable facility to geometrical reasoning. Accordingly, their
consequences may be pursued, by arguments purely mathematical, to any
extent, insomuch that the limit of our knowledge of dynamics is determined
only by that of pure mathematics, which is the case in no other branch of
physical science.
(88.) But, it will now be asked, how we are to proceed to analyse a
composite phenomenon into simpler ones, and whether any general rules
can be given for this important process? We answer, None; any more than
(to pursue the illustration we have already had recourse to) general rules
can be laid down by the chemist for the analysis of substances of which all
the ingredients are unknown. Such rules, could they be discovered, would
include the whole of natural science; but we are very far, indeed, from being
able to propound them. However, we are to recollect that the analysis of
phenomena, philosophically speaking, is principally useful, as it enables us
to recognize, and mark for special investigation, those which appear to us
simple; to set methodically about determining their laws, and thus to
facilitate the work of raising up general axioms, or forms of words, which
shall include the whole of them; which shall, as it were, transplant them out
of the external into the intellectual world, render them creatures of pure
thought, and enable us to reason them out à priori. And what renders the
power of doing this so eminently desirable is, that, in thus reasoning back
from generals to particulars, the propositions at which we arrive apply to an
immense multitude of combinations and cases, which were never

individually contemplated in the mental process by which our axioms were
first discovered; and that, consequently, when our reasonings are pushed to
the utmost limit of particularity, their results appear in the form of
individual facts, of which we might have had no knowledge from
immediate experience; and thus we are not only furnished with the
explanation of all known facts, but with the actual discovery of such as
were before unknown. A remarkable example of this has already been
mentioned in Fresnel’s à priori discovery of the extraordinary refraction of
both rays in a doubly refracting medium. To give another example:—The
law of gravitation is a physical axiom of a very high and universal kind, and
has been raised by a succession of inductions and abstractions drawn from
the observation of numerous facts and subordinate laws in the planetary
system. When this law is taken for granted, and laid down as a basis of
reasoning, and applied to the actual condition of our own planet, one of the
consequences to which it leads is, that the earth, instead of being an exact
sphere, must be compressed or flattened in the direction of its polar
diameter, the one diameter being about thirty miles shorter than the other;
and this conclusion, deduced at first by mere reasoning, has been since
found to be true in fact. All astronomical predictions are examples of the
same thing.
(89.) In the important business of raising these axioms of nature, we are
not, as in the analysis of phenomena, left wholly without a guide. The
nature of abstract or general reasoning points out in a great measure the
course we must pursue. A law of nature, being the statement of what will
happen in certain general contingencies, may be regarded as the
announcement, in the same words, of a whole group or class of phenomena.
Whenever, therefore, we perceive that two or more phenomena agree in so
many or so remarkable points, as to lead us to regard them as forming a
class or group, if we lay out of consideration, or abstract, all the
circumstances in which they disagree, and retain in our minds those only in
which they agree, and then, under this kind of mental convention, frame a
definition or statement of one of them, in such words that it shall apply
equally to them all, such statement will appear in the form of a general
proposition, having so far at least the character of a law of nature.
(90.) For example: a great number of transparent substances, when
exposed, in a certain particular manner, to a beam of light which has been

prepared by undergoing certain reflexions or refractions, (and has thereby
acquired peculiar properties, and is said to be “polarized,”) exhibit very
vivid and beautiful colours, disposed in streaks, bands, &c. of great
regularity, which seem to arise within the substance, and which, from a
certain regular succession observed in their appearance, are called
“periodical colours.” Among the substances which exhibit these periodical
colours occur a great variety of transparent solids, but no fluids and no
opake solids. Here, then, there seems to be sufficient community of nature
to enable us to use a general term, and to state the proposition as a law, viz.
transparent solids exhibit periodical colours by exposure to polarized light.
However, this, though true of many, does not apply to all transparent solids,
and therefore we cannot state it as a general truth or law of nature in this
form; although the reverse proposition, that all solids which exhibit such
colours in such circumstances are transparent, would be correct and
general. It becomes necessary, then, to make a list of those to which it does
apply; and thus a great number of substances of all kinds become grouped
together, in a class linked by this common property. If we examine the
individuals of this group, we find among them the utmost variety of colour,
texture, weight, hardness, form and composition; so that, in these respects,
we seem to have fallen upon an assemblage of contraries. But when we
come to examine them closely, in all their properties, we find they have all
one point of agreement, in the property of double refraction, (see page 30.)
and therefore we may describe them all truly as doubly refracting
substances. We may, therefore, state the fact in the form, “Doubly refracting
substances exhibit periodical colours by exposure to polarized light;” and in
this form it is found, on further examination, to be true, not only for those
particular instances which we had in view when we first propounded it, but
in all cases which have since occurred on further enquiry, without a single
exception; so that the proposition is general, and entitled to be regarded as a
law of nature.
(91.) We may therefore regard a law of nature either, 1st, as a general
proposition, announcing, in abstract terms, a whole group of particular facts
relating to the behaviour of natural agents in proposed circumstances; or,
2dly, as a proposition announcing that a whole class of individuals agreeing
in one character agree also in another. For example: in the case before us,
the law arrived at includes, in its general announcement, among others, the
particular facts, that rock crystal and saltpetre exhibit periodical colours; for

these are both of them doubly refracting substances. Or, it may be regarded
as announcing a relation between the two phenomena of double refraction,
and the exhibition of periodical colours; which in the actual case is one of
the most important, viz. the relation of constant association, inasmuch as it
asserts that in whatever individual the one character is found, the other will
invariably be found also.
(92.) These two lights, in which the announcement of a general law
may be regarded, though at bottom they come to the same thing, yet differ
widely in their influence on our minds. The former exhibits a law as little
more than a kind of artificial memory; but in the latter it becomes a step in
philosophical investigation, leading directly to the consideration of a
proximate, if not an ultimate, cause; inasmuch as, whenever two
phenomena are observed to be invariably connected together, we conclude
them to be related to each other, either as cause and effect, or as common
effects of a single cause.
(93.) There is still another light in which we may regard a law of the
kind in question, viz. as a proposition asserting the mutual connection, or in
some cases the entire identity, of two classes of individuals (whether
individual objects or individual facts); and this is, perhaps, the simplest and
most instructive way in which it can be conceived, and that which furnishes
the readiest handle to further generalization in the raising of yet higher
axioms. For example: in the case above mentioned, if observation had
enabled us to establish the existence of a class of bodies possessing the
property of double refraction, and observations of another kind had,
independently of the former, led as to recognize a class possessing that of
the exhibition of periodical colours in polarized light, a mere comparison of
lists would at once demonstrate the identity of the two classes, or enable us
to ascertain whether one was or was not included in the other.
(94.) It is thus we perceive the high importance in physical science of
just and accurate classifications of particular facts, or individual objects,
under general well considered heads or points of agreement (for which there
are none better adapted than the simple phenomena themselves into which
they can be analysed in the first instance); for by so doing each of such
phenomena, or heads of classification, becomes not a particular but a
general fact; and when we have amassed a great store of such general facts,
they become the objects of another and higher species of classification, and

are themselves included in laws which, as they dispose of groups, not
individuals, have a far superior degree of generality, till at length, by
continuing the process, we arrive at axioms of the highest degree of
generality of which science is capable.
(95.) This process is what we mean by induction; and, from what has
been said, it appears that induction may be carried on in two different ways,
—either by the simple juxta-position and comparison of ascertained classes,
and marking their agreements and disagreements; or by considering the
individuals of a class, and casting about, as it were to find in what particular
they all agree, besides that which serves as their principle of classification.
Either of these methods may be put in practice as one or the other may
afford facilities in any case; but it will naturally happen that, where facts are
numerous, well observed, and methodically arranged, the former will be
more applicable than in the contrary case: the one is better adapted to the
maturity, the other to the infancy, of science: the one employs, as an engine,
the division of labour; the other mainly relies on individual penetration, and
requires a union of many branches of knowledge in one person.

CHAP. III.
OF THE STATE OF PHYSICAL SCIENCE IN GENERAL, PREVIOUS
TO THE AGE OF GALILEO AND BACON.

(96.) It is to our immortal countryman Bacon that we owe the broad
announcement of this grand and fertile principle; and the developement of
the idea, that the whole of natural philosophy consists entirely of a series of
inductive generalizations, commencing with the most circumstantially
stated particulars, and carried up to universal laws, or axioms, which
comprehend in their statements every subordinate degree of generality, and
of a corresponding series of inverted reasoning from generals to particulars,
by which these axioms are traced back into their remotest consequences,
and all particular propositions deduced from them; as well those by whose
immediate consideration we rose to their discovery, as those of which we
had no previous knowledge. In the course of this descent to particulars, we
must of necessity encounter all those facts on which the arts and works that
tend to the accommodation of human life depend, and acquire thereby the
command of an unlimited practice, and a disposal of the powers of nature
co-extensive with those powers themselves. A noble promise, indeed, and
one which ought, surely, to animate us to the highest exertion of our
faculties; especially since we have already such convincing proof that it is
neither vain nor rash, but, on the contrary, has been, and continues to be,
fulfilled, with a promptness and liberality which even its illustrious author
in his most sanguine mood would have hardly ventured to anticipate.
(97.) Previous to the publication of the Novum Organum of Bacon,
natural philosophy, in any legitimate and extensive sense of the word, could
hardly be said to exist. Among the Greek philosophers, of whose
attainments in science alone, in the earlier ages of the world, we have any
positive knowledge, and that but a very limited one, we are struck with the

remarkable contrast between their powers of acute and subtle disputation,
their extraordinary success in abstract reasoning, and their intimate
familiarity with subjects purely intellectual, on the one hand; and, on the
other, with their loose and careless consideration of external nature, their
grossly illogical deductions of principles of sweeping generality from few
and ill-observed facts, in some cases; and their reckless assumption of
abstract principles having no foundation but in their own imaginations, in
others; mere forms of words, with nothing corresponding to them in nature,
from which, as from mathematical definitions, postulates, and axioms, they
imagined that all phenomena could be derived, all the laws of nature
deduced. Thus, for instance, having settled it in their own minds, that a
circle is the most perfect of figures, they concluded, of course, that the
movements of the heavenly bodies must all be performed in exact circles,
and with uniform motions; and when the plainest observation demonstrated
the contrary, instead of doubting the principle, they saw no better way of
getting out of the difficulty than by having recourse to endless combinations
of circular motions to preserve their ideal perfection.
(98.) Undoubtedly among the Greek philosophers were many men of
transcendent talents and virtues, the ornaments of their species, and justly
entitled to the veneration of all posterity; but regarded as a body they can
hardly be considered otherwise than as a knot of disputatious candidates for
popular favour, too busy in maintaining their ascendency over their
followers and admirers, by an ostentatious display of superior knowledge,
to have the leisure (had they always the inclination) to base their
pretensions on a deep and sure foundation, and yet too sensible of the
disgrace and inconvenience of failure, not to defend their dogmas, however
shallow, when once promulgated, against their keen and sagacious
opponents, by every art of sophism or appeal to passion. Hence the crudities
and chimerical views with which their systems of philosophy, both natural
and moral, were overloaded; their endless disputes about verbal subtleties,
and, last and worst, the proud assumption with which they sheltered
ignorance and indolence under the screen of unintelligible jargon or
dogmatical assertion. Perhaps, however, this character applies rather to the
later than to the earlier of the Greek philosophers. The spirit of rational
enquiry into nature seems, if we can judge from the uncertain and often
contradictory notices handed down to us of their tenets, to have been far
more alive, and less warped by this vain and arrogant turn, then than at a

later period. We know not now what was the precise meaning attached by
Thales to his opinion, that water was the origin of all things; but modern
geologists will not be at a loss to conceive how an observant traveller might
become impressed with this notion, without having recourse to the mystic
records of Egypt or Chaldea. His ideas of eclipses and of the nature of the
moon were sound; and his prediction of an eclipse of the sun, in particular,
was attended with circumstances so remarkable as to have made it a matter
of important investigation to modern astronomers. Anaxagoras, among a
number of crude and imperfectly explained notions, speculated rationally
enough on the cause of the winds and of the rainbow, and less absurdly on
earthquakes than many modern geologists have done, and appears generally
to have had his attention alive to nature, and his mind open to just reasoning
on its phenomena; while Pythagoras, whether he reasoned it out for himself,
or borrowed the notion from Egypt or India, had attained a just conception
of the general disposition of the parts of the solar system, and the place held
by the earth in it; nay, according to some accounts, had even raised his
views so far as to speculate on the attraction of the sun as the bond of its
union.
(99.) But the successors of these bonâ fide enquirers into nature
debased the standard of truth; and, taking advantage of the credit justly
attached to their discoveries, renounced the modest character of learners,
and erected themselves into teachers, and, to maintain their pretensions to
this character, adopted the tone of men who had nothing further to learn.
Unfortunately for true science, the national character gave every
encouragement to pretensions of this kind. That restless craving after
novelty, which distinguished the Greeks in their civil and political relations,
pursued them into their philosophy. Whatever speculations were only
ingenious and new had irresistible charms; and the teacher who could
embody a clever thought in elegant language, or at once save his followers
and himself the trouble of thinking or reasoning, by bold assertion, was too
often induced to acquire cheaply the reputation of superior knowledge,
snatch a few superficial notions from the most ordinary and obvious facts,
envelope them in a parade of abstruse words, declare them the primary and
ultimate principles of all things, and denounce as absurd and impious all
opinions opposed to his own.

(100.) In this war of words the study of nature was neglected, and an
humble and patient enquiry after facts altogether despised, as unworthy of
the high priori ground a true philosopher ought to take. It was the radical
error of the Greek philosophy to imagine that the same method which
proved so eminently successful in mathematical, would be equally so in
physical, enquiries, and that, by setting out from a few simple and almost
self-evident notions, or axioms, every thing could be reasoned out.
Accordingly, we find them constantly straining their invention to discover
these principles, which were to prove so pregnant. One makes fire the
essential matter and origin of the universe; another, air; a third, discovers
the key to every difficulty, and the explanation of all phenomena, in the “το
απειρον” or infinitude of things; a fourth, in the το ὁν and the το μη ὁν, that
is to say, in entity and nonentity;—till at length an authority, which was
destined to command opinions for nearly two thousand years, settled this
important point, by deciding, that matter, form, and privation, were to be
considered the principles of all things.
(101.) It were to do injustice to Aristotle, however, to judge of him by
such a sample of his philosophy. He, at least, saw the necessity of having
recourse to nature for something like principles of physical science; and, as
an observer, a collector and recorder of facts and phenomena, stood without
an equal in his age. It was the fault of that age, and of the perverse and
flimsy style of verbal disputation which had infected all learning, rather
than his own, that he allowed himself to be contented with vague and loose
notions drawn from general and vulgar observation, in place of seeking
carefully, in well arranged and thoroughly considered instances, for the true
laws of nature. His voluminous works, on every department of human
knowledge existing in his time, have nearly all perished. From his work on
animals, which has descended to us, we are, however, enabled to appreciate
his powers of observation; and a parallel drawn by an eminent Oxford
professor between his classifications and those of the most illustrious of
living naturalists, shows him to have attained a view of animated nature in a
remarkable degree comprehensive, and which contrasts strikingly with the
confusion, vagueness, and assumption of his physical opinions and dogmas.
In these it is easy to recognize a mind not at home, and an impression of the
necessity of saying something learned and systematic, without knowing
what to say. Thus he divides motions into natural and unnatural; the natural
motion of fire and light bodies being upwards, those of heavy downwards,

each seeking its kindred nature in the heavens and the earth. Thus, too, the
immediate impressions made on us by external objects, such as hardness,
colour, heat, &c. are referred at once, in the Aristotelian philosophy, to
occult qualities, in virtue of which they are as they are, and beyond which it
30
is useless to enquire. Of course there will occur a limit beyond which it is
useless for merely human faculties to enquire; but where that limit is
placed, experience alone can teach us; and at least to assert that we have
attained it, is now universally recognized as the sure criterion of
dogmatism.
(102.) In the early ages of the church the writings of Aristotle were
condemned, as allowing too much to reason and sense; and even so late as
the twelfth century they were sought out and burned, and their readers
excommunicated. By degrees, however, the extreme injustice of this
impeachment of their character was acknowledged: they became the
favourite study of the schoolmen, and furnished the keenest weapons of
their controversy, being appealed to in all disputes as of sovereign
authority; so that the slightest dissent from any opinion of the “great
master,” however absurd or unintelligible, was at once drowned by clamour,
or silenced by the still more effectual argument of bitter persecution. If the
logic of that gloomy period could be justly described as “the art of talking
unintelligibly on matters of which we are ignorant,” its physics might, with
equal truth, be summed up in a deliberate preference of ignorance to
knowledge, in matters of every day’s experience and use.
(103.) In “this opake of nature and of soul,” the perverse activity of the
31
alchemists from time to time struck out a doubtful spark ; and our
illustrious countryman, Roger Bacon, shone out at the obscurest moment,
like an early star predicting dawn. It was not, however, till the sixteenth
century that the light of nature began to break forth with a regular and
progressive increase. The vaunts of Paracelsus of the power of his chemical
remedies and elixirs, and his open condemnation of the ancient pharmacy,
backed as they were by many surprising cures, convinced all rational
physicians that chemistry could furnish many excellent remedies, unknown
32
till that time , and a number of valuable experiments began to be made by
physicians and chemists, desirous of discovering and describing new
chemical remedies. The chemical and metallurgic arts, exercised by persons

empirically acquainted with their secrets, began to be seriously studied with
a view to the acquisition of rational and useful knowledge, and regular
treatises on branches of natural science at length to appear. George
Agricola, in particular, devoted himself with ardour to the study of
mineralogy and metallurgy in the mining districts of Bohemia and
Schemnitz, and published copious and methodical accounts of all the facts
within his knowledge: and our countryman, Dr. Gilbert of Colchester, in
1590, published a treatise on magnetism, full of valuable facts and
experiments, ingeniously reasoned on; and he likewise extended his
enquiries to a variety of other subjects, in particular to electricity.
(104.) But, as the decisive mark of a great commencing change in the
direction of the human faculties, astronomy, the only science in which the
ancients had made any real progress, and ascended to any thing like large
and general conceptions, began once more to be studied in the best spirit of
a candid philosophy; and the Copernican or Pythagorean system arose or
revived, and rapidly gained advocates. Galileo at length appeared, and
openly attacked and refuted the Aristotelian dogmas respecting motion, by
direct appeal to the evidence of sense, and by experiments of the most
convincing kind. The persecutions which such a step drew upon him, the
record of his perseverance and sufferings, and the ultimate triumph of his
33
opinions and reasonings, have been too lately and too well related to
require repetition here.
(105.) By the discoveries of Copernicus, Kepler, and Galileo, the errors
of the Aristotelian philosophy were effectually overturned on a plain appeal
to the facts of nature; but it remained to show on broad and general
principles, how and why Aristotle was in the wrong; to set in evidence the
peculiar weakness of his method of philosophizing, and to substitute in its
place a stronger and better. This important task was executed by Francis
Bacon, Lord Verulam, who will, therefore, justly be looked upon in all
future ages as the great reformer of philosophy, though his own actual
contributions to the stock of physical truths were small, and his ideas of
particular points strongly tinctured with mistakes and errors, which were the
fault rather of the general want of physical information of the age than of
any narrowness of view on his own part; and of this he was fully aware. It
has been attempted by some to lessen the merit of this great achievement,
by showing that the inductive method had been practised in many instances,

both ancient and modern, by the mere instinct of mankind; but it is not the
introduction of inductive reasoning, as a new and hitherto untried process,
which characterizes the Baconian philosophy, but his keen perception, and
his broad and spirit-stirring, almost enthusiastic, announcement of its
paramount importance, as the alpha and omega of science, as the grand and
only chain for the linking together of physical truths, and the eventual key
to every discovery and every application. Those who would deny him his
just glory on such grounds would refuse to Jenner or to Howard their civic
crowns, because a few farmers in a remote province had, time out of mind,
been acquainted with vaccination, or philanthropists, in all ages, had
occasionally visited the prisoner in his dungeon.
(106.) An immense impulse was now given to science, and it seemed as
if the genius of mankind, long pent up, had at length rushed eagerly upon
Nature, and commenced, with one accord, the great work of turning up her
hitherto unbroken soil, and exposing the treasures so long concealed. A
general sense now prevailed of the poverty and insufficiency of existing
knowledge in matters of fact; and, as information flowed fast in, an era of
excitement and wonder commenced, to which the annals of mankind had
furnished nothing similar. It seemed, too, as if Nature herself seconded the
impulse; and, while she supplied new and extraordinary aids to those senses
which were henceforth to be exercised in her investigation,—while the
telescope and the microscope laid open the infinite in both directions,—as if
to call attention to her wonders, and signalize the epoch, she displayed the
rarest, the most splendid and mysterious, of all astronomical phenomena,
the appearance and subsequent total extinction of a new and brilliant fixed
34
star twice within the lifetime of Galileo himself.
(107.) The immediate followers of Bacon and Galileo ransacked all
nature for new and surprising facts, with something of that craving for the
marvellous, which might be regarded as a remnant of the age of alchemy
and natural magic, but which, under proper regulation, is a most powerful
and useful stimulus to experimental enquiry. Boyle, in particular, seemed
animated by an enthusiasm of ardour, which hurried him from subject to
subject, and from experiment to experiment, without a moment’s
intermission, and with a sort of undistinguishing appetite; while Hooke (the
great contemporary, and almost the worthy rival, of Newton) carried a
keener eye of scrutinizing reason into a range of research even yet more

extensive. As facts multiplied, leading phenomena became prominent, laws
began to emerge, and generalizations to commence; and so rapid was the
career of discovery, so signal the triumph of the inductive philosophy, that a
single generation and the efforts of a single mind sufficed for the
establishment of the system of the universe, on a basis never after to be
shaken.
(108.) We shall now endeavour to enumerate and explain in detail the
principal steps by which legitimate and extensive inductions are arrived at,
and the processes by which the mind, in the investigation of natural laws,
purges itself by successive degrees of the superfluities and incumbrances
which hang about particulars, and obscure the perception of their points of
resemblance and connection. We shall state the helps which may be
afforded us, in a work of so much thought and labour, by a methodical
course of proceeding, and by a careful notice of those means which have at
any time been found successful, with a view to their better understanding
and adaptation to other cases: a species of mental induction of no mean
utility and extent in itself; inasmuch as by pursuing it alone we can attain a
more intimate knowledge than we actually possess of the laws which
regulate our discovery of truth, and of the rules, so far as they extend, to
which invention is reducible. In doing this, we shall commence at the
beginning, with experience itself, considered as the accumulation of the
knowledge of individual objects and facts.

CHAP. IV.
OF THE OBSERVATION OF FACTS AND THE COLLECTION OF
INSTANCES.

(109.) Nature offers us two sorts of subjects of contemplation in the
external world,—objects, and their mutual actions. But, after what has been
said on the subject of sensation, the reader will be at no loss to perceive that
we know nothing of the objects themselves which compose the universe,
except through the medium of the impressions they excite in us, which
impressions are the results of certain actions and processes in which
sensible objects and the material parts of ourselves are directly concerned.
Thus, our observation of external nature is limited to the mutual action of
material objects on one another; and to facts, that is, the associations of
phenomena or appearances. We gain no information by perceiving merely
that an object is black; but if we also perceive it to be fluid, we at least
acquire the knowledge that blackness is not incompatible with fluidity, and
have thus made a step, however trifling, to a knowledge of the more
intimate nature of these two qualities. Whenever, therefore, we would either
analyse a phenomenon into simpler ones, or ascertain what is the course or
law of nature under any proposed general contingency, the first step is to
accumulate a sufficient quantity of well ascertained facts or recorded
instances, bearing on the point in question. Common sense dictates this, as
affording us the means of examining the same subject in several points of
view; and it would also dictate, that the more different these collected facts
are in all other circumstances but that which forms the subject of enquiry,
the better; because they are then in some sort brought into contrast with one
another in their points of disagreement, and thus tend to render those in
which they agree more prominent and striking.

(110.) The only facts which can ever become useful as grounds of
physical enquiry are those which happen uniformly and invariably under
the same circumstances. This is evident: for if they have not this character
they cannot be included in laws; they want that universality which fits them
to enter as elementary particles into the constitution of those universal
axioms which we aim at discovering. If one and the same result does not
constantly happen under a given combination of circumstances, apparently
the same, one of two things must be supposed,—caprice (i. e. the arbitrary
intervention of mental agency), or differences in the circumstances
themselves, really existing, but unobserved by us. In either case, though we
may record such facts as curiosities, or as awaiting explanation when the
difference of circumstances shall be understood, we can make no use of
them in scientific enquiry. Hence, whenever we notice a remarkable effect
of any kind, our first question ought to be, Can it be reproduced? What are
the circumstances under which it has happened? And will it always happen
again if those circumstances, so far as we have been able to collect them,
co-exist?
(111.) The circumstances, then, which accompany any observed fact,
are main features in its observation, at least until it is ascertained by
sufficient experience what circumstances have nothing to do with it, and
might therefore have been left unobserved without sacrificing the fact. In
observing and recording a fact, therefore, altogether new, we ought not to
omit any circumstance capable of being noted, lest some one of the omitted
circumstances should be essentially connected with the fact, and its
omission should, therefore, reduce the implied statement of a law of nature
to the mere record of an historical event. For instance, in the fall of
meteoric stones, flashes of fire are seen proceeding from a cloud, and a loud
rattling noise like thunder is heard. These circumstances, and the sudden
stroke and destruction ensuing, long caused them to be confounded with an
effect of lightning, and called thunderbolts. But one circumstance is enough
to mark the difference: the flash and sound have been perceived
occasionally to emanate from a very small cloud insulated in a clear sky; a
combination of circumstances which never happens in a thunder storm, but
which is undoubtedly intimately connected with their real origin.
(112.) Recorded observation consists of two distinct parts: 1st, an exact
notice of the thing observed, and of all the particulars which may be

supposed to have any natural connection with it; and, 2dly, a true and
faithful record of them. As our senses are the only inlets by which we
receive impressions of facts, we must take care, in observing, to have them
all in activity, and to let nothing escape notice which affects any one of
them. Thus, if lightning were to strike the house we inhabit, we ought to
notice what kind of light we saw—whether a sheet of flame, a darting
spark, or a broken zig-zag; in what direction moving, to what objects
adhering, its colour, its duration, &c.; what sounds were heard—explosive,
crashing, rattling, momentary, or gradually increasing and fading, &c.;
whether any smell of fire was perceptible, and if sulphureous, metallic, or
such as would arise merely from substances scorched by the flash, &c.;
whether we felt any shock, stroke, or peculiar sensation, or experienced any
strange taste in our mouths. Then, besides detailing the effects of the stroke,
all the circumstances which might in any degree seem likely to attract,
produce, or modify it, such as the presence of conductors, neighbouring
objects, the state of the atmosphere, the barometer, thermometer, &c., and
the disposition of the clouds, should be noted; and after all this particularity,
the question how the house came to be struck? might ultimately depend on
the fact that a flash of lightning twenty miles off passed at that particular
moment from the ground to the clouds, by an effect of what has been
termed the returning stroke.
35
(113.) A writer in the Edinburgh Philosophical Journal states himself
to have been led into a series of investigations on the chemical nature of a
peculiar acid, by noticing, accidentally, a bitter taste in a liquid about to be
thrown away. Chemistry is full of such incidents.
(114.) In transient phenomena, if the number of particulars be great,
and the time to observe them short, we must consult our memory before
they have had time to fade, or refresh it by placing ourselves as nearly as
possible in the same circumstances again; go back to the spot, for instance,
and try the words of our statement by appeal to all remaining indications,
&c. This is most especially necessary where we have not observed
ourselves, but only collect and record the observations of others,
particularly of illiterate or prejudiced persons, on any rare phenomenon,
such as the passing of a great meteor,—the fall of a stone from the sky,—
the shock of an earthquake,—an extraordinary hailstorm, &c.

(115.) In all cases which admit of numeration or measurement, it is of
the utmost consequence to obtain precise numerical statements, whether in
the measure of time, space, or quantity of any kind. To omit this, is, in the
first place, to expose ourselves to illusions of sense which may lead to the
grossest errors. Thus, in alpine countries, we are constantly deceived in
heights and distances; and when we have overcome the first impression
which leads us to under-estimate them, we are then hardly less apt to run
into the opposite extreme. But it is not merely in preserving us from
exaggerated impressions that numerical precision is desirable. It is the very
soul of science; and its attainment affords the only criterion, or at least the
best, of the truth of theories, and the correctness of experiments. Thus, it
was entirely to the omission of exact numerical determinations of quantity
that the mistakes and confusion of the Stahlian chemistry were attributable,
—a confusion which dissipated like a morning mist as soon as precision, in
this respect, came to be regarded as essential. Chemistry is in the most pre-
eminent degree a science of quantity; and to enumerate the discoveries
which have arisen in it, from the mere determination of weights and
measures, would be nearly to give a synopsis of this branch of knowledge.
We need only mention the law of definite proportions, which fixes the
composition of every body in nature in determinate proportional weights of
its ingredients.
(116.) Indeed, it is a character of all the higher laws of nature to assume
the form of precise quantitative statement. Thus, the law of gravitation, the
most universal truth at which human reason has yet arrived, expresses not
merely the general fact of the mutual attraction of all matter; not merely the
vague statement that its influence decreases as the distance increases, but
the exact numerical rate at which that decrease takes place; so that when its
amount is known at any one distance it may be calculated exactly for any
other. Thus, too, the laws of crystallography, which limit the forms assumed
by natural substances, when left to their own inherent powers of
aggregation, to precise geometrical figures, with fixed angles and
proportions, have the same essential character of strict mathematical
expression, without which no exact particular conclusions could ever be
drawn from them.
(117.) But, to arrive at laws of this description, it is evident that every
step of our enquiry must be perfectly free from the slightest degree of

looseness and indecision, and carry with it the full force of strict numerical
announcement; and that, therefore, the observations themselves on which
all laws ultimately rest ought to have the same property. None of our senses,
however, gives us direct information for the exact comparison of quantity.
Number, indeed, that is to say, integer number, is an object of sense,
because we can count; but we can neither weigh, measure, nor form any
precise estimate of fractional parts by the unassisted senses. Scarcely any
man could tell the difference between twenty pounds and the same weight
increased or diminished by a few ounces; still less could he judge of the
proportion between an ounce of gold and a hundred grains of cotton by
balancing them in his hands. To take another instance: the eye is no judge of
the proportion of different degrees of illumination, even when seen side by
side; and if an interval elapses, and circumstances change, nothing can be
more vague than its judgments. When we gaze with admiration at the
gorgeous spectacle of the golden clouds at sunset, which seem drenched in
light and glowing like flames of real fire, it is hardly by any effort we can
persuade ourselves to regard them as the very same objects which at
noonday pass unnoticed as mere white clouds basking in the sun, only
participating, from their great horizontal distance, in the ruddy tint which
luminaries acquire by shining through a great extent of the vapours of the
atmosphere, and thereby even losing something of their light. So it is with
our estimates of time, velocity, and all other matters of quantity; they are
absolutely vague, and inadequate to form a foundation for any exact
conclusion.
(118.) In this emergency we are obliged to have recourse to
instrumental aids, that is, to contrivances which shall substitute for the
vague impressions of sense the precise one of number, and reduce all
measurement to counting. As a first preliminary towards effecting this, we
fix on convenient standards of weight, dimension, time, &c., and invent
contrivances for readily and correctly repeating them as often as we please,
and counting how often such a standard unit is contained in the thing, be it
weight, space, time, or angle, we wish to measure; and if there be a
fractional part over, we measure this as a new quantity by aliquot parts of
the former standard.
(119.) If every scientific enquirer observed only for his own
satisfaction, and reasoned only on his own observations, it would be of little

importance what standards he used, or what contrivances (if only just ones)
he employed for this purpose; but if it be intended (as it is most important
they should) that observations once made should remain as records to all
mankind, and to all posterity, it is evidently of the highest consequence that
all enquirers should agree on the use of a common standard, and that this
should be one not liable to change by lapse of time. The selection and
verification of such standards, however, will easily be understood to be a
matter of extreme difficulty, if only from the mere circumstance that, to
verify the permanence of one standard, we must compare it with others,
which it is possible may be themselves inaccurate, or, at least, stand in need
of verification.
(120.) Here we can only call to our assistance the presumed
permanence of the great laws of Nature, with all experience in its favour,
and the strong impression we have of the general composure and steadiness
of every thing relating to the gigantic mass we inhabit—“the great globe
itself.” In its uniform rotation on its axis, accordingly, we find a standard of
time, which nothing has ever given us reason to regard as subject to change,
and which, compared with other periods which the revolutions of the
planets about the sun afford, has demonstrably undergone none since the
earliest history. In the dimensions of the earth we find a natural unit of the
measure of space, which possesses in perfection every quality that can be
desired; and in its attraction combined with its rotation the researches of
dynamical science have enabled us, through the medium of the pendulum,
to obtain another invariable standard, more refined and less obvious, it is
true, in its origin, but possessing a great advantage in its capability of ready
verification, and therefore easily made to serve as a check on the other. The
former, viz. direct measurement of the dimensions of the earth, is the origin
of the mètre, the French unit of linear measure; the latter, of the British
yard. Theoretically speaking, they are equally eligible; but when we
consider that the quantity directly measured, in the case of the mètre, is a
length a great many thousand times the final unit, and in the pendulum or
yard very nearly the unit itself, there can be no hesitation in giving the
preference as an original measure to the former, because any error
committed in the process by which that is determined becomes subdivided
in the final result; while, on the other hand, any minute error committed in
determining the length of the pendulum becomes multiplied by the

repetition of the unit in all measurements of considerable lengths performed
in yards.
(121.) The same admirable invention of the pendulum affords a means
of subdividing time to an almost unlimited nicety. A clock is nothing more
than a piece of mechanism for counting the oscillations of a pendulum; and
by that peculiar property of the pendulum, that one vibration commences
exactly where the last terminates, no part of time is lost or gained in the
juxta-position of the units so counted, so that the precise fractional part of a
day can be ascertained which each such unit measures.
(122.) It is owing to this peculiar property by which the juxta-position
of units of time and weight can be performed without error, that the whole
of the accuracy with which time and weight can be multiplied and
36
subdivided is owing. The same thing cannot be accomplished in space, by
any method we are yet acquainted with, so that our means of subdividing
space are much inferior in precision. The beautiful principle of repetition,
invented by Borda, offers the nearest approach to it, but cannot be said to be
absolutely free from the source of error in question. The method of “double
weighing,” which we owe to the same distinguished observer, affords an
instance of the direct comparison of two equal weights independent of
almost every source of error which can affect the comparison of one object
with another. It has been remarked by Biot, that previous to the invention of
this elegant method, instruments afforded no perfect means of ascertaining
the weight of a body.
(123.) But it is not enough to possess a standard of this abstract kind: a
real material measure must be constructed, and exact copies of it taken.
This, however, is not very difficult; the great difficulty is to preserve it
unaltered from age to age; for unless we transmit to posterity the units of
our measurements, such as we have ourselves used them, we, in fact, only
half bequeath to them our observations. This is a point too much lost sight
of, and it were much to be wished that some direct provision for so
37
important an object were made.
(124.) But, it may be asked, if our measurement of quantity is thus
unavoidably liable to error, how is it possible that our observations can
possess that quality of numerical veracity which is requisite to render them

the foundation of laws, whose distinguishing perfection consists in their
strict mathematical expression? To this the reply is twofold. 1st, that though
we admit the necessary existence of numerical error in every observation,
we can always assign a limit which such error cannot possibly exceed; and
the extent of this latitude of error of observation is less in proportion to the
perfection of the instrumental means we possess, and the care bestowed on
their employment. In the greater part of modern measurements it is, in point
of fact, extremely minute, and may be still further diminished, almost to any
required extent, by repeating the measurements a great number of times,
and under a great variety of circumstances, and taking a mean of the results,
when errors of opposite kinds will, at length, compensate each other. But,
2dly, there exists a much more fundamental reply to this objection. In
reasoning upon our observations, the existence and possible amount of
quantitative error is always to be allowed for; and the extent to which
theories may be affected by it is never to be lost sight of. In reasoning
upwards, from observations confessedly imperfect to general laws, we must
take care always to regard our conclusions as conditional, so far as they
may be affected by such unavoidable imperfections; and when at length we
shall have arrived at our highest point, and attained to axioms which admit
of general and deductive reasoning, the question, whether they are vitiated
by the errors of observation or not, will still remain to be decided, and must
become the object of subsequent verification. This point will be made the
subject of more distinct consideration hereafter, when we come to speak of
the verification of theories and the laws of probability.
(125.) With respect to our record of observations, it should be not only
circumstantial but faithful; by which we mean, that it should contain all we
did observe, and nothing else. Without any intention of falsifying our
record, we may do so unperceived by ourselves, owing to a mixture of the
views and language of an erroneous theory with that of simple fact. Thus,
for example, if, in describing the effect of lightning, we should say, “The
thunderbolt struck with violence against the side of the house, and beat in
the wall,” a fact would be stated which we did not see, and would lead our
hearers to believe that a solid or ponderable projectile was concerned. The
“strong smell of sulphur,” which is sometimes said to accompany lightning,
is a remnant of the theory which made thunder and lightning the explosion
of a kind of aërial gunpowder, composed of sulphureous and nitrous
exhalations. There are some subjects particularly infested with this mixture

of theory in the statement of observed fact. The older chemistry was so
overborne by this mischief, as quite to confound and nullify the descriptions
of innumerable curious and laborious experiments. And in geology, till a
very recent period, it was often extremely difficult, from this circumstance,
to know what were the facts observed. Thus, Faujas de St. Fond, in his work
on the volcanoes of central France, describes with every appearance of
minute precision craters existing no where but in his own imagination.
There is no greater fault (direct falsification of fact excepted) which can be
committed by an observer.
(126.) When particular branches of science have acquired that degree of
consistency and generality, which admits of an abstract statement of laws,
and legitimate deductive reasoning, the principle of the division of labour
tends to separate the province of the observer from that of the theorist.
There is no accounting for the difference of minds or inclinations, which
leads one man to observe with interest the developements of phenomena,
another to speculate on their causes; but were it not for this happy
disagreement, it may be doubted whether the higher sciences could ever
have attained even their present degree of perfection. As laws acquire
generality, the influence of individual observations becomes less, and a
higher and higher degree of refinement in their performance, as well as a
great multiplication in their number, becomes necessary to give them
importance. In astronomy, for instance, the superior departments of theory
are completely disjoined from the routine of practical observation.
(127.) To make a perfect observer, however, either in astronomy or in
any other department of science, an extensive acquaintance is requisite, not
only with the particular science to which his observations relate, but with
every branch of knowledge which may enable him to appretiate and
neutralize the effect of extraneous disturbing causes. Thus furnished, he will
be prepared to seize on any of those minute indications, which (such is the
subtlety of nature) often connect phenomena which seem quite remote from
each other. He will have his eyes as it were opened, that they may be struck
at once with any occurrence which, according to received theories, ought
not to happen; for these are the facts which serve as clews to new
discoveries. The deviation of the magnetic needle, by the influence of an
electrified wire, must have happened a thousand times to a perceptible
amount, under the eyes of persons engaged in galvanic experiments, with

philosophical apparatus of all kinds standing around them; but it required
the eye of a philosopher such as Oërsted to seize the indication, refer it to
its origin, and thereby connect two great branches of science. The grand
discovery of Malus of the polarization of light by reflection originated in
his casual remark of the disappearance of one of the images of a window in
the Luxembourg palace, one evening, when strongly illuminated by the
setting sun, viewed through a doubly refracting prism.
(128.) To avail ourselves as far as possible of the advantages which a
division of labour may afford for the collection of facts, by the industry and
activity which the general diffusion of information, in the present age,
brings into exercise, is an object of great importance. There is scarcely any
well-informed person, who, if he has but the will, has not also the power to
add something essential to the general stock of knowledge, if he will only
observe regularly and methodically some particular class of facts which
may most excite his attention, or which his situation may best enable him to
study with effect. To instance one or two subjects, which can only be
effectually improved by the united observations of great numbers widely
dispersed:—Meteorology, one of the most complicated but important
branches of science, is at the same time one in which any person who will
attend to plain rules, and bestow the necessary degree of attention, may do
effectual service. What benefits has not Geology reaped from the activity of
industrious individuals, who, setting aside all theoretical views, have been
content to exercise the useful and highly entertaining occupation of
collecting specimens from the countries which they visit? In short, there is
no branch of science whatever in which, at least, if useful and sensible
queries were distinctly proposed, an immense mass of valuable information
might not be collected from those who, in their various lines of life, at home
or abroad, stationary or in travel, would gladly avail themselves of
opportunities of being useful. Nothing would tend better to attain this end
than the circulation of printed skeleton forms, on various subjects, which
should be so formed as, 1st, to ask distinct and pertinent questions,
admitting of short and definite answers; 2dly, To call for exact numerical
statement on all principal points; 3dly, To point out the attendant
circumstances most likely to prove influential, and which ought to be
observed; 4thly, To call for their transmission to a common centre.

CHAP. V.
OF THE CLASSIFICATION OF NATURAL OBJECTS AND
PHENOMENA, AND OF NOMENCLATURE.

(129.) The number and variety of objects and relations which the
observation of nature brings before us are so great as to distract the
attention, unless assisted and methodized by such judicious distribution of
them in classes as shall limit our view to a few at a time, or to groups so
bound together by general resemblances that, for the immediate purpose for
which we consider them, they may be regarded as individuals. Before we
can enter into any thing which deserves to be called a general and
systematic view of nature, it is necessary that we should possess an
enumeration, if not complete, at least of considerable extent, of her
materials and combinations; and that those which appear in any degree
important should be distinguished by names which may not only tend to fix
them in our recollection, but may constitute, as it were, nuclei or centres,
about which information may collect into masses. The imposition of a name
on any subject of contemplation, be it a material object, a phenomenon of
nature, or a group of facts and relations, looked upon in a peculiar point of
view, is an epoch in its history of great importance. It not only enables us
readily to refer to it in conversation or writing, without circumlocution, but,
what is of more consequence, it gives it a recognized existence in our own
minds, as a matter for separate and peculiar consideration; places it on a list
for examination; and renders it a head or title, under which information of
various descriptions may be arranged; and, in consequence, fits it to
perform the office of a connecting link between all the subjects to which
such information may refer.
(130.) For these purposes, however, a temporary or provisional name,
or one adapted for common parlance, may suffice. But when a very great

multitude of objects come to be referred to one class, especially of such as
do not offer very obvious and remarkable distinctions, a more systematic
and regular nomenclature becomes necessary, in which the names shall
recall the differences as well as the resemblances between the individuals of
a class, and in which the direct relation between the name and the object
shall materially assist the solution of the problem, “given the one, to
determine the other.” How necessary this may become, will be at once seen,
when we consider the immense number of individual objects, or rather
species, presented by almost every branch of science of any extent; which
absolutely require to be distinguished by names. Thus, the botanist is
conversant with from 80,000 to 100,000 species of plants; the entomologist
with, perhaps, as many, of insects: the chemist has to register the properties
of combinations, by twos, threes, fours, and upwards, in various doses of
upwards of fifty different elements, all distinguished from each other by
essential differences; and of which though a great many thousands are
known, by far the greater part have never yet been formed, although
hundreds of new ones are coming to light, in perpetual succession, as the
science advances; all of which are to be named as they arise. The objects of
astronomy are, literally, as numerous as the stars of heaven; and although
not more than one or two thousand require to be expressed by distinct
names, yet the number, respecting which particular information is required,
is not less than a hundred times that amount; and all these must be
registered in lists, (so as to be at once referred to, and so that none shall
escape,) if not by actual names, at least by some equivalent means.
(131.) Nomenclature, then, is, in itself, undoubtedly an important part
of science, as it prevents our being lost in a wilderness of particulars, and
involved in inextricable confusion. Happily, in those great branches of
science where the objects of classification are most numerous, and the
necessity for a clear and convenient nomenclature most pressing, no very
great difficulty in its establishment is felt. The very multitude of the objects
themselves affords the power of grouping them in subordinate classes,
sufficiently well defined to admit of names, and these again into others,
whose names may become attached to, or compounded with, the former, till
at length the particular species is identified. The facility with which the
botanist, the entomologist, or the chemist, refers by name to any individual
object in his science shows what may be accomplished in this way when
characters are themselves distinct. In other branches, however, considerable

difficulty is experienced. This arises mostly where the species to be
distinguished are separated from each other chiefly by difference in degree,
of certain qualities common to all, and where the degrees shade into each
other insensibly. Perhaps such subjects can hardly be considered ripe for
systematic nomenclature; and that the attempt to apply it ought only to be
partial, embracing such groups and parcels of individuals as agree in
characters evidently natural and generic, and leaving the remainder under
trivial or provisional denominations, till they shall be better known, and
capable of being scientifically grouped.
(132.) Indeed, nomenclature, in a systematic point of view, is as much,
perhaps more, a consequence than a cause of extended knowledge. Any one
may give an arbitrary name to a thing, merely to be able to talk of it; but, to
give a name which shall at once refer it to a place in a system, we must
know its properties; and we must have a system, large enough, and regular
enough, to receive it in a place which belongs to it, and to no other. It
appears, therefore, doubtful whether it is desirable, for the essential
purposes of science, that extreme refinement in systematic nomenclature
should be insisted on. Were science perfect, indeed, systems of
classification might be agreed on, which should assign to every object in
nature a place in some class, to which it more remarkably and pre-
eminently belonged than to any other, and under which it might acquire a
name, never afterwards subject to change. But, so long as this is not the
case, and new relations are daily discovered, we must be very cautious how
we insist strongly on the establishment and extension of classes which have
in them any thing artificial, as a basis of a rigid nomenclature; and
especially how we mistake the means for the end, and sacrifice convenience
and distinctness to a rage for arrangement. Every nomenclature dependent
on artificial classifications is necessarily subject to fluctuations; and hardly
any thing can counterbalance the evil of disturbing well-established names,
which have once acquired a general circulation. In nature, one and the same
object makes a part of an infinite number of different systems,—an
individual in an infinite number of groups, some of greater, some of less
importance, according to the different points of view in which they may be
considered. Hence, as many different systems of nomenclature may be
imagined as there can be discovered different heads of classification, while
yet it is highly desirable that each object should be universally spoken of
under one name, if possible. Consequently, in all subjects where

comprehensive heads of classification do not prominently offer themselves,
all nomenclature must be a balance of difficulties, and a good, short,
unmeaning name, which has once obtained a footing in usage, is preferable
to almost any other.
(133.) There is no science in which the evils resulting from a rage for
nomenclature have been felt to such an extent as in mineralogy. The number
of simple minerals actually recognised by mineralogists does not exceed a
few hundreds, yet there is scarcely one which has not four or five names in
different books. The consequence is most unhappy. No name is suffered to
endure long enough to take root; and every new writer on this interesting
science begins, as a matter of course, by making a tabula rasa of all former
nomenclature, and proposing a new one in its place. The climax has at
length been put to this most inconvenient and bewildering state of things by
the appearance of a system supported by extraordinary merit in other
respects, and therefore carrying the highest authority, in which names which
had acquired universal circulation, and had hitherto maintained their ground
in the midst of the general confusion, and even worked their way into
common language, as denotive of species too definite to admit of mistake,
are actually rendered generic, and extended to whole groups, comprising
objects agreeing in nothing but the arbitrary heads of a classification from
which the most important natural relations are professedly and purposely
38
rejected.
(134.) The classifications by which science is advanced, however, are
widely different from those which serve as bases for artificial systems of
nomenclature. They cross and intersect one another, as it were, in every
possible way, and have for their very aim to interweave all the objects of
nature in a close and compact web of mutual relations and dependence. As
soon, then, as any resemblance or analogy, any point of agreement
whatever, is perceived between any two or more things,—be they what they
will, whether objects, or phenomena, or laws,—they immediately and ipso
facto constitute themselves into a group or class, which may become
enlarged to any extent by the accession of such new objects, phenomena, or
laws, agreeing in the same point, as may come to be subsequently
ascertained. It is thus that the materials of the world become grouped in
natural families, such as chemistry furnishes examples of, in its various
groups of acids, alkalies, sulphurets, &c.; or botany, in its euphorbiaceæ,

umbelliferæ, &c. It is thus, too, that phenomena assume their places under
general points of resemblance; as, in optics, those which refer themselves to
the class of periodic colours, double refraction, &c.; and that resemblances
themselves become traced, which it is the business of induction to
generalize and include in abstract propositions.
(135.) But every class formed on a positive resemblance of characters,
or on a distinct analogy, draws with it the consideration of a negative class,
in which that resemblance either does not subsist at all, or the contrary takes
place; and again, there are classes in which a given quality is possessed by
the different individuals in a descending scale of intensity. Now, it is of
consequence to distinguish between cases in which there is a real opposition
of quality, or a mere diminution of intensity, in some quality susceptible of
degrees, till it becomes imperceptible. For example, between transparency
and opacity there would at first sight appear a direct opposition; but, on
nearer consideration, when we consider the gradations by which
transparency diminishes in natural substances, we shall see reason to admit
that the latter quality, instead of being the opposite of the former, is only its
extreme lowest degree. Again, in the arrangement of natural objects under
the head of weight or specific gravity, the scale extends through all nature,
and we know of no natural body in which the opposite of gravity, or
positive levity, subsists. On the other hand, the opposite electricities; the
north and south magnetic polarities; the alkaline and acid qualities of
chemical agents; the positive and negative rotations impressed by plates of
rock crystal on the planes of polarization of the rays of light, and many
other cases, exemplify not merely a negation, but an active opposition of
quality. Both these modes of classification have their peculiar importance in
the inductive process: the one, as affording an opportunity of tracing a
relation between phenomena by the observation of a correspondence in
their scales of intensity; the other, by that of contrast, as we shall show
more at large in the next section.
(136.) There is a very wide distinction, too, to be taken between such
classes as turn upon a single head of resemblance among individuals
otherwise very different, and such as bind together in natural groups, by a
great variety of analogies, objects which yet differ in many remarkable
particulars. For example: if we make colourless transparency a head of
classification, the list of the class will comprise objects differing most

widely in their nature, such as water, air, diamond, spirit of wine, glass, &c.
On the other hand, the chemical families of alkalies, metals, &c. are
instances of groups of the other kind; which, with properties in many
respects different, still agree in a general resemblance of several others,
which at once decides us in considering them as having a natural relation.
In the former cases, our ingenuity is exercised to determine what can be the
cause of their resemblance, in the latter, of their difference; the former
belong to the province of inductive generalization, and afford the most
instructive cases for the investigation of causes; the latter appertain to the
more secret recesses of nature; the very existence of such families being in
itself one of the great and complicated phenomena of the universe, which
we cannot hope to unriddle without an intimate and extensive acquaintance
39
with the highest laws.

CHAP. VI.
OF THE FIRST STAGE OF INDUCTION.—THE DISCOVERY
OF PROXIMATE CAUSES, AND LAWS OF THE LOWEST
DEGREE OF GENERALITY, AND THEIR VERIFICATION.

(137.) The first thing that a philosophic mind considers, when any new
phenomenon presents itself, is its explanation, or reference to an immediate
producing cause. If that cannot be ascertained, the next is to generalize the
phenomenon, and include it, with others analogous to it, in the expression
of some law, in the hope that its consideration, in a more advanced state of
knowledge, may lead to the discovery of an adequate proximate cause.
(138.) Experience having shown us the manner in which one
phenomenon depends on another in a great variety of cases, we find
ourselves provided, as science extends, with a continually increasing stock
of such antecedent phenomena, or causes (meaning at present merely
proximate causes), competent, under different modifications, to the
production of a great multitude of effects, besides those which originally led
to a knowledge of them. To such causes Newton has applied the term veræ
causæ; that is, causes recognized as having a real existence in nature, and
not being mere hypotheses or figments of the mind. To exemplify the
distinction:—The phenomenon of shells found in rocks, at a great height
above the sea, has been attributed to several causes. By some it has been
ascribed to a plastic virtue in the soil; by some, to fermentation; by some, to
the influence of the celestial bodies; by some, to the casual passage of
pilgrims with their scallops; by some, to birds feeding on shell-fish; and by
all modern geologists, with one consent, to the life and death of real
mollusca at the bottom of the sea, and a subsequent alteration of the relative
level of the land and sea. Of these, the plastic virtue and celestial influence
belong to the class of figments of fancy. Casual transport by pilgrims is a

real cause, and might account for a few shells here and there dropped on
frequented passes, but is not extensive enough for the purpose of
explanation. Fermentation, generally, is a real cause, so far as that there is
such a thing; but it is not a real cause of the production of a shell in a rock,
since no such thing was ever witnessed as one of its effects, and rocks and
stones do not ferment. On the other hand, for a shell-fish dying at the
bottom of the sea to leave his shell in the mud, where it becomes silted over
and imbedded, happens daily; and the elevation of the bottom of the sea to
become dry land has really been witnessed so often, and on such a scale, as
to qualify it for a vera causa available in sound philosophy.
(139.) To take another instance, likewise drawn from the same
deservedly popular science:—The fact of a great change in the general
climate of large tracts of the globe, if not of the whole earth, and of a
diminution of general temperature, having been recognised by geologists,
from their examination of the remains of animals and vegetables of former
ages enclosed in the strata, various causes for such diminution of
temperature have been assigned. Some consider the whole globe as having
gradually cooled from absolute fusion; some regard the immensely superior
activity of former volcanoes, and consequent more copious communication
of internal heat to the surface, in former ages, as the cause. Neither of these
can be regarded as real causes in the sense here intended; for we do not
know that the globe has so cooled from fusion, nor are we sure that such
supposed greater activity of former than of present volcanoes really did
exist. A cause, possessing the essential requisites of a vera causa, has,
40
however, been brought forward in the varying influence of the distribution
of land and sea over the surface of the globe: a change of such distribution,
in the lapse of ages, by the degradation of the old continents, and the
elevation of new, being a demonstrated fact; and the influence of such a
change on the climates of particular regions, if not of the whole globe,
being a perfectly fair conclusion, from what we know of continental,
insular, and oceanic climates by actual observation. Here, then, we have, at
least, a cause on which a philosopher may consent to reason; though,
whether the changes actually going on are such as to warrant the whole
extent of the conclusion, or are even taking place in the right direction, may
be considered as undecided till the matter has been more thoroughly
examined.

(140.) To this we may add another, which has likewise the essential
characters of a vera causa, in the astronomical fact of the actual slow
diminution of the eccentricity of the earth’s orbit round the sun; and which,
as a general one, affecting the mean temperature of the whole globe, and as
one of which the effect is both inevitable, and susceptible, to a certain
degree, of exact estimation, deserves consideration. It is evident that the
mean temperature of the whole surface of the globe, in so far as it is
maintained by the action of the sun at a higher degree than it would have
were the sun extinguished, must depend on the mean quantity of the sun’s
rays which it receives, or, which comes to the same thing, on the total
quantity received in a given invariable time: and the length of the year
being unchangeable in all the fluctuations of the planetary system, it
follows, that the total annual amount of solar radiation will determine,
cæteris paribus, the general climate of the earth. Now, it is not difficult to
show that this amount is inversely proportional to the minor axis of the
ellipse described by the earth about the sun, regarded as slowly variable;
and that, therefore, the major axis remaining, as we know it to be, constant,
and the orbit being actually in a state of approach to a circle, and,
consequently, the minor axis being on the increase, the mean annual amount
of solar radiation received by the whole earth must be actually on the
decrease. We have here, therefore, an evident real cause, of sufficient
universality, and acting in the right direction, to account for the
41
phenomenon. Its adequacy is another consideration.
(141.) Whenever, therefore, any phenomenon presents itself for
explanation, we naturally seek, in the first instance, to refer it to some one
or other of those real causes which experience has shown to exist, and to be
efficacious in producing similar phenomena. In this attempt our probability
of success will, of course, mainly depend, 1st, On the number and variety of
causes experience has placed at our disposal; 2dly, On our habit of applying
them to the explanation of natural phenomena; and, 3dly, On the number of
analogous phenomena we can collect, which have either been explained, or
which admit of explanation by some one or other of those causes, and the
closeness of their analogy with that in question.
(142.) Here, then, we see the great importance of possessing a stock of
analogous instances or phenomena which class themselves with that under
consideration, the explanation of one among which may naturally be

expected to lead to that of all the rest. If the analogy of two phenomena be
very close and striking, while, at the same time, the cause of one is very
obvious, it becomes scarcely possible to refuse to admit the action of an
analogous cause in the other, though not so obvious in itself. For instance,
when we see a stone whirled round in a sling, describing a circular orbit
round the hand, keeping the string stretched, and flying away the moment it
breaks, we never hesitate to regard it as retained in its orbit by the tension
of the string, that is, by a force directed to the centre; for we feel that we do
really exert such a force. We have here the direct perception of the cause.
When, therefore, we see a great body like the moon circulating round the
earth and not flying off, we cannot help believing it to be prevented from so
doing, not indeed by a material tie, but by that which operates in the other
case through the intermedium of the string,—a force directed constantly to
the centre. It is thus that we are continually acquiring a knowledge of the
existence of causes acting under circumstances of such concealment as
effectually to prevent their direct discovery.
(143.) In general we must observe that motion, wherever produced or
changed, invariably points out the existence of force as its cause; and thus
the forces of nature become known and measured by the motions they
produce. Thus, the force of magnetism becomes known by the deviation
produced by iron in a compass needle, or by a needle leaping up to a
magnet held over it, as certainly as by that adhesion to it, when in contact
and at rest, which requires force to break the connection; and thus the
currents produced in the surface of a quantity of quicksilver, electrified
under a conducting fluid, have pointed out the existence and direction of
forces of enormous intensity developed by the electric circuit, of which we
42
should not otherwise have had the least suspicion.
(144.) But when the cause of a phenomenon neither presents itself
obviously on the consideration of the phenomenon itself, nor is as it were
forced on our attention by a case of strong analogy, such as above
described, we have then no resource but in a deliberate assemblage of all
the parallel instances we can muster; that is, to the formation of a class of
facts, having the phenomenon in question for a head of classification; and to
a search among the individuals of this class for some other common points
of agreement, among which the cause will of necessity be found. But if
more than one cause should appear, we must then endeavour to find, or, if

we cannot find, to produce, new facts, in which each of these in succession
shall be wanting, while yet they agree in the general point in question. Here
we find the use of what Bacon terms “crucial instances,” which are
phenomena brought forward to decide between two causes, each having the
same analogies in its favour. And here, too, we perceive the utility of
experiment as distinguished from mere passive observation. We make an
experiment of the crucial kind when we form combinations, and put in
action causes from which some particular one shall be deliberately
excluded, and some other purposely admitted; and by the agreement or
disagreement of the resulting phenomena with those of the class under
examination, we decide our judgment.
(145.) When we would lay down general rules for guiding and
facilitating our search, among a great mass of assembled facts, for their
common cause, we must have regard to the characters of that relation which
we intend by cause and effect. Now, these are,—

1st, Invariable connection, and, in particular, invariable antecedence
of the cause and consequence of the effect, unless prevented by
some counteracting cause. But it must be observed, that, in a
great number of natural phenomena, the effect is produced
gradually, while the cause often goes on increasing in intensity;
so that the antecedence of the one and consequence of the other
becomes difficult to trace, though it really exists. On the other
hand, the effect often follows the cause so instantaneously, that
the interval cannot be perceived. In consequence of this, it is
sometimes difficult to decide, of two phenomena constantly
accompanying one another, which is cause or which effect.

2d, Invariable negation of the effect with absence of the cause,
unless some other cause be capable of producing the same
effect.

3d, Increase or diminution of the effect, with the increased or
diminished intensity of the cause, in cases which admit of
increase and diminution.

4th, Proportionality of the effect to its cause in all cases of direct
unimpeded action.

5th, Reversal of the effect with that of the cause.

(146.) From these characters we are led to the following observations,
which may be considered as so many propositions readily applicable to
particular cases, or rules of philosophizing: we conclude, 1st, That if in our
group of facts there be one in which any assigned peculiarity, or attendant
circumstance, is wanting or opposite, such peculiarity cannot be the cause
we seek.
(147.) 2d, That any circumstance in which all the facts without
exception agree, may be the cause in question, or, if not, at least a collateral
effect of the same cause: if there be but one such point of agreement, this
possibility becomes a certainty; and, on the other hand, if there be more
than one, they may be concurrent causes.
(148.) 3d, That we are not to deny the existence of a cause in favour of
which we have a unanimous agreement of strong analogies, though it may
not be apparent how such a cause can produce the effect, or even though it
may be difficult to conceive its existence under the circumstances of the
case; in such cases we should rather appeal to experience when possible,
than decide à priori against the cause, and try whether it cannot be made
apparent.
(149.) For instance: seeing the sun vividly luminous, every analogy
leads us to conclude it intensely hot. How heat can produce light, we know
not; and how such a heat can be maintained, we can form no conception.
Yet we are not, therefore, entitled to deny the inference.
(150.) 4th, That contrary or opposing facts are equally instructive for
the discovery of causes with favourable ones.
(151.) For instance: when air is confined with moistened iron filings in
a close vessel over water, its bulk is diminished, by a certain portion of it
being abstracted and combining with the iron, producing rust. And, if the
remainder be examined, it is found that it will not support flame or animal
life. This contrary fact shows that the cause of the support of flame and

animal life is to be looked for in that part of the air which the iron abstracts,
and which rusts it.
(152.) 5th, That causes will very frequently become obvious, by a mere
arrangement of our facts in the order of intensity in which some peculiar
quality subsists; though not of necessity, because counteracting or
modifying causes may be at the same time in action.
(153.) For example: sound consists in impulses communicated to our
ears by the air. If a series of impulses of equal force be communicated to it
at equal intervals of time, at first in slow succession, and by degrees more
and more rapidly, we hear at first a rattling noise, then a low murmur, and
then a hum, which by degrees acquires the character of a musical note,
rising higher and higher in acuteness, till its pitch becomes too high for the
ear to follow. And from this correspondence between the pitch of the note
and the rapidity of succession of the impulse, we conclude that our
sensation of the different pitches of musical notes originates in the different
rapidities with which their impulses are communicated to our ears.
(154.) 6th, That such counteracting or modifying causes may subsist
unperceived, and annul the effects of the cause we seek, in instances which,
but for their action, would have come into our class of favourable facts; and
that, therefore, exceptions may often be made to disappear by removing or
allowing for such counteracting causes. This remark becomes of the
greatest importance, when (as is often the case) a single striking exception
stands out, as it were, against an otherwise unanimous array of facts in
favour of a certain cause.
(155.) Thus, in chemistry, the alkaline quality of the alkaline and earthy
bases is found to be due to the presence of oxygen combined with one or
other of a peculiar set of metals. Ammonia is, however, a violent
outstanding exception, such as here alluded to, being a compound of azote
and hydrogen: but there are almost certain indications that this exception is
not a real one, but assumes that appearance in consequence of some
modifying cause not understood.
(156.) 7th, If we can either find produced by nature, or produce
designedly for ourselves, two instances which agree exactly in all but one
particular, and differ in that one, its influence in producing the
phenomenon, if it have any, must thereby be rendered sensible. If that

particular be present in one instance and wanting altogether in the other, the
production or non-production of the phenomenon will decide whether it be
or be not the only cause: still more evidently, if it be present contrariwise in
the two cases, and the effect be thereby reversed. But if its total presence or
absence only produces a change in the degree or intensity of the
phenomenon, we can then only conclude that it acts as a concurrent cause
or condition with some other to be sought elsewhere. In nature, it is
comparatively rare to find instances pointedly differing in one circumstance
and agreeing in every other; but when we call experiment to our aid, it is
easy to produce them; and this is, in fact, the grand application of
experiments of enquiry in physical researches. They become more valuable,
and their results clearer, in proportion as they possess this quality (of
agreeing exactly in all their circumstances but one), since the question put
to nature becomes thereby more pointed, and its answer more decisive.
(157.) 8th, If we cannot obtain a complete negative or opposition of the
circumstance whose influence we would ascertain, we must endeavour to
find cases where it varies considerably in degree. If this cannot be done, we
may perhaps be able to weaken or exalt its influence by the introduction of
some fresh circumstance, which, abstractedly considered, seems likely to
produce this effect, and thus obtain indirect evidence of its influence. But
then we are always to remember, that the evidence so obtained is indirect,
and that the new circumstance introduced may have a direct influence of its
own, or may exercise a modifying one on some other circumstance.
(158.) 9th, Complicated phenomena, in which several causes
concurring, opposing, or quite independent of each other, operate at once,
so as to produce a compound effect, may be simplified by subducting the
effect of all the known causes, as well as the nature of the case permits,
either by deductive reasoning or by appeal to experience, and thus leaving,
as it were, a residual phenomenon to be explained. It is by this process, in
fact, that science, in its present advanced state, is chiefly promoted. Most of
the phenomena which nature presents are very complicated; and when the
effects of all known causes are estimated with exactness, and subducted, the
residual facts are constantly appearing in the form of phenomena altogether
new, and leading to the most important conclusions.
(159.) For example: the return of the comet predicted by professor
Encke, a great many times in succession, and the general good agreement of

its calculated with its observed place during any one of its periods of
visibility, would lead us to say that its gravitation towards the sun and
planets is the sole and sufficient cause of all the phenomena of its orbitual
motion; but when the effect of this cause is strictly calculated and subducted
from the observed motion, there is found to remain behind a residual
phenomenon, which would never have been otherwise ascertained to exist,
which is a small anticipation of the time of its reappearances or a
diminution of its periodic time, which cannot be accounted for by gravity,
and whose cause is therefore to be enquired into. Such an anticipation
would be caused by the resistance of a medium disseminated through the
celestial regions; and as there are other good reasons for believing this to be
a vera causa, it has therefore been ascribed to such a resistance.
(160.) This 9th observation is of such importance in science, that we
shall exemplify it by another instance or two. M. Arago, having suspended a
magnetic needle by a silk thread, and set it in vibration, observed, that it
came much sooner to a state of rest when suspended over a plate of copper,
than when no such plate was beneath it. Now, in both cases there were two
veræ causæ why it should come at length to rest, viz. the resistance of the
air, which opposes, and at length destroys, all motions performed in it; and
the want of perfect mobility in the silk thread. But the effect of these causes
being exactly known by the observation made in the absence of the copper,
and being thus allowed for and subducted, a residual phenomenon
appeared, in the fact that a retarding influence was exerted by the copper
itself; and this fact, once ascertained, speedily led to the knowledge of an
entirely new and unexpected class of relations. To add one more instance. If
it be true (as M. Fourrier considers it demonstrated to be) that the celestial
regions have a temperature independent of the sun, not greatly inferior to
that at which quicksilver congeals, and much superior to some degrees of
cold which have been artificially produced, two causes suggest themselves:
one is that assigned by the author above mentioned; the radiation of the
stars; another may be proposed in the ether or elastic medium mentioned in
the last section, which the phenomena of light and the resistance of comets
give us reason to believe fills all space, and which, in analogy to all the
elastic media known, may be supposed to possess a temperature and a
specific heat of its own, which it is capable of communicating to bodies
surrounded by it. Now, if we consider that the heat radiated by the sun
follows the same proportion as its light, and regard it as reasonable to admit

with respect to stellar heat what holds good of solar; the effect of stellar
radiation in maintaining a temperature in space should be as much inferior
to that of the radiation of the sun as the light of a moonless midnight is to
that of an equatorial noon; that is to say, almost inconceivably smaller.
Allowing, then, the full effect for this cause, there would still remain a great
residuum due to the presence of the ether.
(161.) Many of the new elements of chemistry have been detected in
the investigation of residual phenomena. Thus, Arfwedson discovered lithia
by perceiving an excess of weight in the sulphate produced from a small
portion of what he considered as magnesia present in a mineral he had
analysed. It is on this principle, too, that the small concentrated residues of
great operations in the arts are almost sure to be the lurking places of new
chemical ingredients: witness iodine, brome, selenium, and the new metals
accompanying platina in the experiments of Wollaston and Tennant. It was a
happy thought of Glauber to examine what every body else threw away.
(162.) Finally, we have to observe, that the detection of a possible
cause, by the comparison of assembled cases, must lead to one of two
things: either, 1st, The detection of a real cause, and of its manner of acting,
so as to furnish a complete explanation of the facts; or, 2dly, The
establishment of an abstract law of nature, pointing out two phenomena of a
general kind as invariably connected; and asserting, that where one is, there
the other will always be found. Such invariable connection is itself a
phenomenon of a higher order than any particular fact; and when many
such are discovered, we may again proceed to classify, combine, and
examine them, with a view to the detection of their causes, or the discovery
of still more general laws, and so on without end.
(163.) Let us now exemplify this inductive search for a cause by one
general example: suppose dew were the phenomenon proposed, whose
cause we would know. In the first place, we must separate dew from rain
and the moisture of fogs, and limit the application of the term to what is
really meant, which is, the spontaneous appearance of moisture on
substances exposed in the open air when no rain or visible wet is falling.
Now, here we have analogous phenomena in the moisture which bedews a
cold metal or stone when we breathe upon it; that which appears on a glass
of water fresh from the well in hot weather; that which appears on the
inside of windows when sudden rain or hail chills the external air; that

which runs down our walls when, after a long frost, a warm moist thaw
comes on: all these instances agree in one point (Rule 2. § 147.), the
coldness of the object dewed, in comparison with the air in contact with it.
(164.) But, in the case of the night dew, is this a real cause—is it a fact
that the object dewed is colder than the air? Certainly not, one would at first
be inclined to say; for what is to make it so? But the analogies are cogent
and unanimous; and, therefore, (pursuant to Rule 3. § 148.) we are not to
discard their indications; and, besides, the experiment is easy: we have only
to lay a thermometer in contact with the dewed substance, and hang one at a
little distance above it out of reach of its influence. The experiment has
been therefore made; the question has been asked, and the answer has been
invariably in the affirmative. Whenever an object contracts dew, it is colder
than the air. Here, then, we have an invariable concomitant circumstance:
but is this chill an effect of dew, or its cause? That dews are accompanied
with a chill is a common remark; but vulgar prejudice would make the cold
the effect rather than the cause. We must, therefore, collect more facts, or,
which comes to the same thing, vary the circumstances; since every
instance in which the circumstances differ is a fresh fact; and, especially, we
must note the contrary or negative cases (Rule 4. § 150.), i. e. where no dew
is produced.
(165.) Now, 1st, no dew is produced on the surface of polished metals,
but it is very copiously on glass, both exposed with their faces upwards, and
in some cases the under side of a horizontal plate of glass is also dewed;
which last circumstance (by Rule 1. § 146.) excludes the fall of moisture
from the sky in an invisible form, which would naturally suggest itself as a
cause. In the cases of polished metal and polished glass, the contrast shows
evidently that the substance has much to do with the phenomenon;
therefore, let the substance alone be diversified as much as possible, by
exposing polished surfaces of various kinds. This done, a scale of intensity
becomes obvious (Rule 5. § 152.). Those polished substances are found to
be most strongly dewed which conduct heat worst; while those which
conduct well resist dew most effectually. Here we encounter a law of the
first degree of generality. But, if we expose rough surfaces, instead of
polished, we sometimes find this law interfered with (Rule 5. § 152.). Thus,
roughened iron, especially if painted over or blackened, becomes dewed
sooner than varnished paper: the kind of surface therefore has a great

influence. Expose, then, the same material in very diversified states as to
surface (Rule 7. § 156.), and another scale of intensity becomes at once
apparent; those surfaces which part with their heat most readily by
radiation are found to contract dew most copiously: and thus we have
detected another law of the same generality with the former, by a
comparison of two classes of facts, one relating to dew, the other to the
radiation of heat from surfaces. Again, the influence ascertained to exist of
substance and surface leads us to consider that of texture: and here, again,
we are presented on trial with remarkable differences, and with a third scale
of intensity, pointing out substances of a close firm texture, such as stones,
metals, &c. as unfavourable, but those of a loose one, as cloth, wool, velvet,
eiderdown, cotton, &c. as eminently favourable, to the contraction of dew:
and these are precisely those which are best adapted for clothing, or for
impeding the free passage of heat from the skin into the air, so as to allow
their outer surfaces to be very cold while they remain warm within.
(166.) Lastly, among the negative instances, (§ 150.) it is observed, that
dew is never copiously deposited in situations much screened from the open
sky, and not at all in a cloudy night; but if the clouds withdraw, even for a
few minutes, and leave a clear opening, a deposition of dew presently
begins, and goes on increasing. Here, then, a cause is distinctly pointed out
by its antecedence to the effect in question (§ 145.). A clear view of the
cloudless sky, then, is an essential condition, or, which comes to the same
thing, clouds or surrounding objects act as opposing causes. This is so
much the case, that dew formed in clear intervals will often even evaporate
again when the sky becomes thickly overcast (Rule 4. § 150.).
(167.) When we now come to assemble these partial inductions so as to
raise from them a general conclusion, we consider, 1st, That all the
conclusions we have come to have a reference to that first general fact—the
cooling of the exposed surface of the body dewed below the temperature of
the air. Those surfaces which part with their heat outwards most readily, and
have it supplied from within most slowly, will, of course, become coldest if
there be an opportunity for their heat to escape, and not be restored to them
from without. Now, a clear sky affords such an opportunity. It is a law well
known to those who are conversant with the nature of heat, that heat is
constantly escaping from all bodies in rays, or by radiation, but is as
constantly restored to them by the similar radiation of others surrounding

them. Clouds and surrounding objects therefore act as opposing causes by
replacing the whole or a great part of the heat so radiated away, which can
escape effectually, without being replaced, only through openings into
infinite space. Thus, at length, we arrive at the general proximate cause of
dew, in the cooling of the dewed surface by radiation faster than its heat can
be restored to it, by communication with the ground, or by counter-
radiation; so as to become colder than the air, and thereby to cause a
condensation of its moisture.
(168.) We have purposely selected this theory of dew, first developed
by the late Dr. Wells, as one of the most beautiful specimens we can call to
mind of inductive experimental enquiry lying within a moderate compass. It
is not possible in so brief a space to do it justice; but we earnestly
43
recommend his work (a short and very entertaining one) for perusal to the
student of natural philosophy, as a model with which he will do well to
become familiar.
(169.) In the analysis above given, the formation of dew is referred to
two more general phenomena; the radiation of heat, and the condensation of
invisible vapour by cold. The cause of the former is a much higher enquiry,
and may be said, indeed, to be totally unknown; that of the latter actually
forms a most important branch of physical enquiry. In such a case, when we
reason upwards till we reach an ultimate fact, we regard a phenomenon as
fully explained; as we consider the branch of a tree to terminate when
traced to its insertion in the trunk, or a twig to its junction with the branch;
or rather, as a rivulet retains its importance and its name till lost in some
larger tributary, or in the main river which delivers it into the ocean. This,
however, always supposes that, on a reconsideration of the case, we see
clearly how the admission of such a fact, with all its attendant laws, will
perfectly account for every particular—as well those which, in the different
stages of the induction, have led us to a knowledge of it, as those which we
had neglected, or considered less minutely than the rest. But, had we no
previous knowledge of the radiation of heat, this same induction would
have made it known to us, and, duly considered, might have led to the
knowledge of many of its laws.
(170.) In the study of nature, we must not, therefore, be scrupulous as
to how we reach to a knowledge of such general facts: provided only we

verify them carefully when once detected, we must be content to seize them
wherever they are to be found. And this brings us to consider the
verification of inductions.
(171.) If, in our induction, every individual case has actually been
present to our minds, we are sure that it will find itself duly represented in
our final conclusion: but this is impossible for such cases as were unknown
to us, and hardly ever happens even with all the known cases; for such is
the tendency of the human mind to speculation, that on the least idea of an
analogy between a few phenomena, it leaps forward, as it were, to a cause
or law, to the temporary neglect of all the rest; so that, in fact, almost all our
principal inductions must be regarded as a series of ascents and descents,
and of conclusions from a few cases, verified by trial on many.
(172.) Whenever, therefore, we think we have been led by induction to
the knowledge of the proximate cause of a phenomenon or of a law of
nature, our next business is to examine deliberately and seriatim all the
cases we have collected of its occurrence, in order to satisfy ourselves that
they are explicable by our cause, or fairly included in the expression of our
law: and in case any exception occurs, it must be carefully noted and set
aside for re-examination at a more advanced period, when, possibly, the
cause of exception may appear, and the exception itself, by allowing for the
effect of that cause, be brought over to the side of our induction; but should
exceptions prove numerous and various in their features, our faith in the
conclusion will be proportionally shaken, and at all events its importance
lessened by the destruction of its universality.
(173.) In the conduct of this verification, we are to consider whether the
cause or law to which we are conducted be one already known and
recognised as a more general one, whose nature is well understood, and of
which the phenomenon in question is but one more case in addition to those
already known, or whether it be one less general, less known, or altogether
new. In the latter case, our verification will suffice, if it merely shows that
all the cases considered are plainly cases in point. But in the former, the
process of verification is of a much more severe and definite kind. We must
trace the action of our cause with distinctness and precision, as modified by
all the circumstances of each case; we must estimate its effects, and show
that nothing unexplained remains behind; at least, in so far as the presence
of unknown modifying causes is not concerned.

(174.) Now, this is precisely the sort of process in which residual
phenomena (such as spoken of in art. 158.) may be expected to occur. If our
induction be really a valid and a comprehensive one, whatever remains
unexplained in the comparison of its conclusion with particular cases, under
all their circumstances, is such a phenomenon, and comes in its turn to be a
subject of inductive reasoning to discover its cause or laws. It is thus that
we may be said to witness facts with the eyes of reason; and it is thus that
we are continually attaining a knowledge of new phenomena and new laws
which lie beneath the surface of things, and give rise to the creation of fresh
branches of science more and more remote from common observation.
(175.) Physical astronomy affords numerous and splendid instances of
this. The law, for example, which asserts that the planets are retained in
their orbits about the sun, and satellites about their primaries, by an
attractive force, decreasing as the square of the distances increases, comes
to be verified in each particular case by deducing from it the exact motions
which, under the circumstances, ought to take place, and comparing them
with fact. This comparison, while it verifies in general the existence of the
law of gravitation as supposed, and its adequacy to explain all the principal
motions of every body in the system, yet leaves some small deviations in
those of the planets, and some very considerable ones in that of the moon
and other satellites, still unaccounted for; residual phenomena, which still
remain to be traced up to causes. By further examining these, their causes
have at length been ascertained, and found to consist in the mutual actions
of the planets on each other, and the disturbing influence of the sun on the
motions of the satellites.
(176.) But a law of nature has not that degree of generality which fits it
for a stepping-stone to greater inductions, unless it be universal in its
application. We cannot rely on its enabling us to extend our views beyond
the circle of instances from which it was obtained, unless we have already
had experience of its power to do so; unless it actually has enabled us
before trial to say what will take place in cases analogous to those originally
contemplated; unless, in short, we have studiously placed ourselves in the
situation of its antagonists, and even perversely endeavoured to find
exceptions to it without success. It is in the precise proportion that a law
once obtained endures this extreme severity of trial, that its value and
importance are to be estimated; and our next step in the verification of an

induction must therefore consist in extending its application to cases not
originally contemplated; in studiously varying the circumstances under
which our causes act, with a view to ascertain whether their effect is
general; and in pushing the application of our laws to extreme cases.
(177.) For example, a fair induction from a great number of facts led
Galileo to conclude that the accelerating power of gravity is the same on all
sorts of bodies, and on great and small masses indifferently; and this he
exemplified by letting bodies of very different natures and weights fall at
the same instant from a high tower, when it was observed that they struck
the ground at the same moment, abating a certain trifling difference, due, as
he justly believed it to be, to the greater proportional resistance of the air to
light than to heavy bodies. The experiment could not, at that time, be fairly
tried with extremely light substances, such as cork, feathers, cotton, &c.
because of the great resistance experienced by these in their fall; no means
being then known of removing this cause of disturbance. It was not,
therefore, till after the invention of the air-pump that this law could be put
to the severe test of an extreme case. A guinea and a downy feather were let
drop at once from the upper part of a tall exhausted glass, and struck the
bottom at the same moment. Let any one make the trial in the air, and he
will perceive the force of an extreme case.
(178.) In the verification of a law whose expression is quantitative, not
only must its generality be established by the trial of it in as various
circumstances as possible, but every such trial must be one of precise
measurement. And in such cases the means taken for subjecting it to trial
ought to be so devised as to repeat and multiply a great number of times any
deviation (if any exist); so that, let it be ever so small, it shall at last become
sensible.
(179.) For instance, let the law to be verified be, that the gravity of
every material body is in the direct proportion of its mass, which is only
another mode of expressing Galileo’s law above mentioned. The time of
falling from any moderate height cannot be measured with precision
enough for our purpose: but if it can be repeated a very great multitude of
times without any loss or gain in the intervals, and the whole amount of the
times of fall so repeated measured by a clock; and if at the same time the
resistance of the air can be rendered exactly alike for all the bodies tried, we
have here Galileo’s trial in a much more refined state; and it is evident that

almost unlimited exactness may be obtained. Now, all this Newton
accomplished by the simple and elegant contrivance of enclosing in a
hollow pendulum the same weights of a great number of substances the
most different that could be found in all respects, as gold, glass, wood,
44
water, wheat, &c. , and ascertaining the time required for the pendulum so
charged to make a great number of oscillations; in each of which it is clear
the weights had to fall, and be raised again successively, without loss of
time, through the same identical spaces. Thus any difference, however
inconsiderable, that might exist in the time of one such fall and rise would
be multiplied and accumulated till they became sensible. And none having
been discovered by so delicate a process in any case, the law was
considered verified both in respect of generality and exactness. This,
however, is nothing to the verifications afforded by astronomical
phenomena, where the deviations, if any, accumulate for thousands of years
instead of a few hours.
(180.) The surest and best characteristic of a well-founded and
extensive induction, however, is when verifications of it spring up, as it
were, spontaneously, into notice, from quarters where they might be least
expected, or even among instances of that very kind which were at first
considered hostile to them. Evidence of this kind is irresistible, and compels
assent with a weight which scarcely any other possesses. To give an
example: M. Mitscherlich had announced a law to this effect—that the
chemical elements of which all bodies consist are susceptible of being
classified in distinct groups, which he termed isomorphous groups; and that
these groups are so related, that when similar combinations are formed of
individuals belonging to two, three, or more of them, such combinations
will crystallize in the same geometrical forms. To this curious and important
law there appeared a remarkable exception. According to professor
Mitscherlich, the arsenic and phosphoric acids are similar combinations
coming under the meaning of his law, and their combinations with soda and
water, forming the salts known to chemists under the names of arseniate and
phosphate of soda, ought, if the law were general, to crystallize in identical
shapes. The fact, however, was understood to be otherwise. But lately, Mr.
Clarke, a British chemist, having examined the two salts attentively,
ascertained the fact that their compositions deviate essentially from that
similarity which M. Mitscherlich’s law requires; and that, therefore, the

exception in question disappears. This was something: but, pursuing the
subject further, the same ingenious enquirer happily succeeded in producing
a new phosphate of soda, differing from that generally known in containing
a different proportion of water, and agreeing in composition exactly with
the arseniate. The crystals of this new salt, when examined, were found by
him to be precisely identical in form with those of the arseniate: thus
verifying, in a most striking and totally unexpected manner, the law in
question, or, as it is called, the law of isomorphism.
(181.) Unexpected and peculiarly striking confirmations of inductive
laws frequently occur in the form of residual phenomena, in the course of
investigations of a widely different nature from those which gave rise to the
inductions themselves. A very elegant example may be cited in the
unexpected confirmation of the law of the developement of heat in elastic
fluids by compression, which is afforded by the phenomena of sound. The
enquiry into the cause of sound had led to conclusions respecting its mode
of propagation, from which its velocity in the air could be precisely
calculated. The calculations were performed; but, when compared with fact,
though the agreement was quite sufficient to show the general correctness
of the cause and mode of propagation assigned, yet the whole velocity could
not be shown to arise from this theory. There was still a residual velocity to
be accounted for, which placed dynamical philosophers for a long time in a
great dilemma. At length Laplace struck on the happy idea, that this might
arise from the heat developed in the act of that condensation which
necessarily takes place at every vibration by which sound is conveyed. The
matter was subjected to exact calculation, and the result was at once the
complete explanation of the residual phenomenon, and a striking
confirmation of the general law of the developement of heat by
compression, under circumstances beyond artificial imitation.
(182.) In extending our inductions to cases not originally contemplated,
there is one step which always strikes the mind with peculiar force, and
with such a sensation of novelty and surprise, as often gives it a weight
beyond its due philosophic value. It is the transition from the little to the
great, and vice versâ, but especially the former. It is so beautiful to see, for
instance, an experiment performed in a watch-glass, or before a blowpipe,
succeed, in a great manufactory, on many tons of matter, or, in the bosom of
a volcano, upon millions of cubic fathoms of lava, that we almost forget

that these great masses are made up of watch-glassfuls, and blowpipe-
beads. We see the enormous intervals between the stars and planets of the
heavens, which afford room for innumerable processes to be carried on, for
light and heat to circulate, and for curious and complicated motions to go
forward among them: we look more attentively, and we see sidereal
systems, probably not less vast and complicated than our own, crowded
apparently into a small space (from the effect of their distance from us), and
forming groups resembling bodies of a substantial appearance, having form
and outline: yet we recoil with incredulous surprise when we are asked why
we cannot conceive the atoms of a grain of sand to be as remote from each
other (proportionally to their sizes) as the stars of the firmament; and why
there may not be going on, in that little microcosm, processes as
complicated and wonderful as those of the great world around us. Yet the
student who makes any progress in natural philosophy will encounter
numberless cases in which this transfer of ideas from the one extreme of
magnitude to the other will be called for: he will find, for instance, the
phenomena of the propagation of winds referred to the same laws which
regulate the propagation of motions through the smallest masses of air;
those of lightning assimilated to the mere communication of an electric
spark, and those of earthquakes to the tremors of a stretched wire: in short,
he must lay his account to finding the distinction of great and little
altogether annihilated in nature: and it is well for man that such is the case,
and that the same laws, which he can discover and verify in his own
circumscribed sphere of power, should prove available to him when he
comes to apply them on the greatest scale; since it is thus only that he is
enabled to become an exciting cause in operations of any considerable
magnitude, and to vindicate his importance in creation.
(183.) But the business of induction does not end here: its final result
must be followed out into all its consequences, and applied to all those
cases which seem even remotely to bear upon the subject of enquiry. Every
new addition to our stock of causes becomes a means of fresh attack with
new vantage ground upon all those unexplained parts of former phenomena
which have resisted previous efforts. It can hardly be pressed forcibly
enough on the attention of the student of nature, that there is scarcely any
natural phenomenon which can be fully and completely explained in all its
circumstances, without a union of several, perhaps of all, the sciences. The
great phenomena of astronomy, indeed, may be considered exceptions; but

this is merely because their scale is so vast that one only of the most widely
extending forces of nature takes the lead, and all those agents whose sphere
of action is limited to narrower bounds, and which determine the production
of phenomena nearer at hand, are thrown into the back ground, and become
merged and lost in comparative insignificance. But in the more intimate
phenomena which surround us it is far otherwise. Into what a complication
of different branches of science are we not led by the consideration of such
a phenomenon as rain, for instance, or flame, or a thousand others, which
are constantly going on before our eyes? Hence, it is hardly possible to
arrive at the knowledge of a law of any degree of generality in any branch
of science, but it immediately furnishes us with a means of extending our
knowledge of innumerable others, the most remote from the point we set
out from; so that, when once embarked in any physical research, it is
impossible for any one to predict where it may ultimately lead him.
(184.) This remark rather belongs to the inverse or deductive process,
by which we pursue laws into their remote consequences. But it is very
important to observe, that the successful process of scientific enquiry
demands continually the alternate use of both the inductive and deductive
method. The path by which we rise to knowledge must be made smooth and
beaten in its lower steps, and often ascended and descended, before we can
scale our way to any eminence, much less climb to the summit. The
achievement is too great for a single effort; stations must be established,
and communications kept open with all below. To quit metaphor; there is
nothing so instructive, or so likely to lead to the acquisition of general
views, as this pursuit of the consequences of a law once arrived at into
every subject where it may seem likely to have an influence. The discovery
of a new law of nature, a new ultimate fact, or one that even temporarily
puts on that appearance, is like the discovery of a new element in chemistry.
Thus, selenium was hardly discovered by Berzelius in the vitriol works of
Fahlun, when it presently made its appearance in the sublimates of
Stromboli, and the rare and curious products of the Hungarian mines. And
thus it is with every new law, or general fact. It is hardly announced before
its traces are found every where, and every one is astonished at its having so
long remained concealed. And hence it happens that unexpected lights are
shed at length over parts of science that had been abandoned in despair, and
given over to hopeless obscurity.

(185.) The verification of quantitative laws has been already spoken of
(178.); but their importance in physical science is so very great, inasmuch
as they alone afford a handle to strict mathematical deductive application,
that something ought to be said of the nature of the inductions by which
they are to be arrived at. In their simplest or least general stages (of which
alone we speak at present) they usually express some numerical relation
between two quantities dependent on each other, either as collateral effects
of a common cause, or as the amount of its effect under given numerical
circumstances or data. For example, the law of refraction before noticed (§
22.) expresses, by a very simple relation, the amount of angular deviation of
a ray of light from its course, when the angle at which it is inclined to the
refracting surface is known, viz. that the sine of the angle which the
incident ray makes with a perpendicular to the surface is always to that of
the angle made by the refracted ray with the same perpendicular, in a
constant proportion, so long as the refracting substance is the same. To
arrive inductively at laws of this kind, where one quantity depends on or
varies with another, all that is required is a series of careful and exact
measures in every different state of the datum and quæsitum. Here,
however, the mathematical form of the law being of the highest importance,
the greatest attention must be given to the extreme cases as well as to all
those points where the one quantity changes rapidly with a small change of
45
the other. The results must be set down in a table in which the datum
gradually increases in magnitude from the lowest to the highest limit of
which it is susceptible. It will depend then entirely on our habit of treating
mathematical subjects, how far we may be able to include such a table in
the distinct statement of a mathematical law. The discovery of such laws is
often remarkably facilitated by the contemplation of a class of phenomena
to be noticed further on, under the head of Collective Instances, (see § 194.)
in which the nature of the mathematical expression in which the law sought
is comprehended, is pointed out by the figure of some curve brought under
inspection by a proper mode of experimenting.
(186.) After all, unless our induction embraces a series of cases which
absolutely include the whole scale of variation of which the quantities in
question admit, the mathematical expression so obtained cannot be
depended upon as the true one, and if the scale actually embraced be small,
the extension of laws so derived to extreme cases will in all probability be

exceedingly fallacious. For example, air is an elastic fluid, and as such, if
enclosed in a confined space and squeezed, its bulk diminishes: now, from a
great number of trials made in cases where the air has been compressed into
a half, a third, &c. even as far as a fiftieth of its bulk, or less, it has been
concluded that “the density of air is proportional to the compressing force,”
or the bulk it occupies inversely as that force; and when the air is rarefied
by taking off part of its natural pressure, the same is found to be the case,
within very extensive limits. Yet it is impossible that this should be, strictly
or mathematically speaking, the true law; for, if it were so, there could be
no limit to the condensation of air, while yet we have the strongest
analogies to show that long before it had reached any very enormous pitch
the air would be reduced into a liquid, and even, perhaps, if pressed yet
more violently, into a solid form.
(187.) Laws thus derived, by the direct process of including in
mathematical formulæ the results of a greater or less number of
measurements, are called “empirical laws.” A good example of such a law
is that given by Dr. Young (Phil. Trans. 1826,) for the decrement of life, or
the law of mortality. Empirical laws in this state are evidently unverified
inductions, and are to be received and reasoned on with the utmost reserve.
No confidence can ever be placed in them beyond the limits of the data
from which they are derived; and even within those limits they require a
special and severe scrutiny to examine how nearly they do represent the
observed facts; that is to say, whether, in the comparison of their results
with the observed quantities, the differences are such as may fairly be
attributed to error of observation. When so carefully examined, they
become, however, most valuable; and frequently, when afterwards verified
theoretically by a deductive process (as will be explained in our next
chapter), turn out to be rigorous laws of nature, and afford the noblest and
most convincing supports of which theories themselves are susceptible. The
finest instances of this kind are the great laws of the planetary motions
deduced by Kepler, entirely from a comparison of observations with each
other, with no assistance from theory. These laws, viz. that the planets move
in ellipses round the sun; that each describes about the sun’s centre equal
areas in equal times; and that in the orbits of different planets the squares of
the periodical times are proportional to the cubes of the distances; were the
results of inconceivable labour of calculation and comparison: but they
amply repaid the labour bestowed on them, by affording afterwards the

most conclusive and unanswerable proofs of the Newtonian system. On the
other hand, when empirical laws are unduly relied on beyond the limits of
the observations from which they were deduced, there is no more fertile
source of fatal mistakes. The formulæ which have been empirically deduced
for the elasticity of steam (till very recently), and those for the resistance of
fluids, and other similar subjects, have almost invariably failed to support
the theoretical structures which have been erected on them.
(188.) It is a remarkable and happy fact, that the shortest and most
direct of all inductions should be that which has led at once, or by very few
steps, to the highest of all natural laws,—we mean those of motion and
force. Nothing can be more simple, precise, and general, than the
enunciation of these laws; and, as we have once before observed, their
application to particular facts in the descending or deductive method is
limited by nothing but the limited extent of our mathematics. It would
seem, then, that dynamical science were taken thenceforward out of the pale
of induction, and transformed into a matter of absolute à priori reasoning,
as much as geometry; and so it would be, were our mathematics perfect,
and all the data known. Unhappily, the first is so far from being the case,
that in many of the most interesting branches of dynamical enquiry they
leave us completely at a loss. In what relates to the motions of fluids, for
instance, this is severely felt. We can include our problems, it is true, in
algebraical equations, and we can demonstrate that they contain the
solutions; but the equations themselves are so intractable, and present such
insuperable difficulties, that they often leave us quite as much in the dark as
before. But even were these difficulties overcome, recourse to experience
must still be had, to establish the data on which particular applications are
to depend; and although mathematical analysis affords very powerful means
of representing in general terms the data of any proposed case, and
afterwards, by comparison of its results with fact, determining what those
data must be to explain the observed phenomena, still, in any mode of
considering the matter, an appeal to experience in every particular instance
of application is unavoidable, even when the general principles are regarded
as sufficiently established without it. Now, in all such cases of difficulty we
must recur to our inductive processes, and regard the branches of dynamical
science where this takes place as purely experimental. By this we gain an
immense advantage, viz. that in all those points of them where the abstract
dynamical principles do afford distinct conclusions, we obtain verifications

for our inductions of the highest and finest possible kind. When we work
our way up inductively to one of these results, we cannot help feeling the
strongest assurance of the validity of the induction.
(189.) The necessity of this appeal to experiment in every thing relating
to the motions of fluids on the large scale has long been felt. Newton
himself, who laid the first foundations of hydrodynamical science (so this
branch of dynamics is called), distinctly perceived it, and set the example of
laborious and exact experiments on their resistance to motion, and other
particulars. Venturi, Bernoulli, and many others, have applied the method of
experiment to the motions of fluids in pipes and canals; and recently the
brothers Weber have published an elaborate and excellent experimental
enquiry into the phenomena of waves. One of the greatest and most
successful attempts, however, to bring an important, and till then very
obscure, branch of dynamical enquiry back to the dominion of experiment,
has been made by Chladni and Savart in the case of sound and vibratory
motion in general; and it is greatly to be wished that the example may be
followed in many others hardly less abstruse and impracticable when
theoretically treated. In such cases the inductive and deductive methods of
enquiry may be said to go hand in hand, the one verifying the conclusions
deduced by the other; and the combination of experiment and theory, which
may thus be brought to bear in such cases, forms an engine of discovery
infinitely more powerful than either taken separately. This state of any
department of science is perhaps of all others the most interesting, and that
which promises the most to research.
(190.) It can hardly be expected that we should terminate this division
of our subject without some mention of the “prerogatives of instances” of
Bacon, by which he understands characteristic phenomena, selected from
the great miscellaneous mass of facts which occur in nature, and which, by
their number, indistinctness, and complication, tend rather to confuse than
to direct the mind in its search for causes and general heads of induction.
Phenomena so selected on account of some peculiarly forcible way in
which they strike the reason, and impress us with a kind of sense of
causation, or a particular aptitude for generalization, he considers, and
justly, as holding a kind of prerogative dignity, and claiming our first and
especial attention in physical enquiries.

(191.) We have already observed that, in forming inductions, it will
most commonly happen that we are led to our conclusions by the especial
force of some two or three strongly impressive facts, rather than by
affording the whole mass of cases a regular consideration; and hence the
need of cautious verification. Indeed, so strong is this propensity of the
human mind, that there is hardly a more common thing than to find persons
ready to assign a cause for every thing they see, and, in so doing, to join
things the most incongruous, by analogies the most fanciful. This being the
case, it is evidently of great importance that these first ready impulses of the
mind should be made on the contemplation of the cases most likely to lead
to good inductions. The misfortune, however, is, in natural philosophy, that
the choice does not rest with us. We must take the instances as nature
presents them. Even if we are furnished with a list of them in tabular order,
we must understand and compare them with each other, before we can tell
which are the instances thus deservedly entitled to the highest
consideration. And, after all, after much labour in vain, and groping in the
dark, accident or casual observation will present a case which strikes us at
once with a full insight into a subject, before we can even have time to
determine to what class its prerogative belongs. For example, the laws of
crystallography were obscure, and its causes still more so, till Haüy
fortunately dropped a beautiful crystal of calcareous spar on a stone
pavement, and broke it. In piecing together the fragments, he observed their
facets not to correspond with those of the crystal in its entire state, but to
belong to another form; and, following out the hint offered by a “glaring
instance” thus casually obtruded on his notice, he discovered the beautiful
laws of the cleavage, and the primitive forms of minerals.
(192.) It has always appeared to us, we must confess, that the help
which the classification of instances, under their different titles of
prerogative, affords to inductions, however just such classification may be
in itself, is yet more apparent than real. The force of the instance must be
felt in the mind, before it can be referred to its place in the system; and,
before it can be either referred or appretiated, it must be known; and when it
is appretiated, we are ready enough to interweave it in our web of induction,
without greatly troubling ourselves with enquiring whence it derives the
weight we acknowledge it to have in our decisions. However, since much
importance is usually attached to this part of Bacon’s work, we shall here
give a few examples to illustrate the nature of some of his principal cases.

One, of what he calls “glaring instances,” has just been mentioned. In these,
the nature or cause enquired into, (which in this case is the cause of the
assumption of a peculiar external form, or the internal structure of a
crystal,) “stands naked and alone, and this in an eminent manner, or in the
highest degree of its power.” No doubt, such instances as these are highly
instructive; but the difficulty in physics is to find such, not to perceive their
force when found.
(193.) The contrary of glaring are “clandestine instances,” where “the
nature sought is exhibited in its weakest and most imperfect state.” Of this,
Bacon himself has given an admirable example in the cohesion of fluids, as
a clandestine instance of the “nature or quality of consistence, or solidity.”
Yet here, again, the same acute discrimination which enabled Bacon to
perceive the analogy which connects fluids with solids, through the
common property of cohesive attraction, would, at the same time, have
enabled him to draw from it, if properly supported, every consequence
necessary to forming just notions of the cohesive force; nor does its
reference to the class of clandestine instances at all assist in bringing
forward and maturing the final results. When, however, the final result is
obtained,—when our induction is complete, and we would verify it,—this
class of instances is of great use, being, in fact, frequently no other than that
of extreme cases, such as we have already spoken of (in § 177.); which, by
placing our conclusions, as it were, in violent circumstances, try their
temper, and bring their vigour to the test.
(194.) Bacon’s “collective instances” (instantiæ unionis), are no other
than general facts, or laws of some degree of generality, and are themselves
the results of induction. But there is a species of collective instance which
Bacon does not seem to have contemplated, of a peculiarly instructive
character; and that is, where particular cases are offered to our observation
in such numbers at once as to make the induction of their law a matter of
ocular inspection. For example, the parabolic form assumed by a jet of
water spouted from a round hole, is a collective instance of the velocities
and directions of the motions of all the particles which compose it seen at
once, and which thus leads us, without trouble, to recognize the law of the
motion of a projectile. Again, the beautiful figures exhibited by sand
strewed on regular plates of glass or metal set in vibration, are collective
instances of an infinite number of points which remain at rest while the

remainder of the plate vibrates; and in consequence afford us, as it were, a
sight of the law which regulates their arrangement and sequence throughout
the whole surface. The beautifully coloured lemniscates seen around the
optic axes of crystals exposed to polarized light afford a superb example of
the same kind, pointing at once to the general mathematical expression of
46
the law which regulates their production. Of such collective instances as
these, it is easy to see the importance, and its reason. They lead us to a
general law by an induction which offers itself spontaneously, and thus
furnish advanced points in our enquiries; and when we start from these,
already “a thousand steps are lost.”
(195.) A fine example of a collective instance is that of the system of
Jupiter or Saturn with its satellites. We have here, in miniature, and seen at
one view, a system similar to that of the planets about the sun; of which,
from the circumstance of our being involved in it, and unfavourably
situated for seeing it otherwise than in detail, we are incapacitated from
forming a general idea but by slow progressive efforts of reason.
Accordingly, the contemplation of the circumjovial planets (as they were
called) most materially assisted in securing the admission of the Copernican
system.
(196.) Of “Crucial instances” we have also already spoken, as affording
the readiest and securest means of eliminating extraneous causes, and
deciding between rival hypotheses. Owing to the disposition of the mind to
form hypotheses, and to prejudge cases, it constantly happens that, among
all the possible suppositions which may occur, two or three principal ones
occupy us, to the exclusion of the rest; or it may be that, if we have been
less precipitate, out of a great multitude rejected for obvious inapplicability
to some one or other case, two or three of better claims remain for decision;
and this such instances enable us to do. One of the instances cited by Bacon
in illustration of his crucial class is very remarkable, being neither more nor
less than the proposal of a direct experiment to determine whether the
tendency of heavy bodies downwards is a result of some peculiar
mechanism in themselves, or of the attraction of the earth “by the corporeal
mass thereof, as by a collection of bodies of the same nature.” If it be so, he
says, “it will follow that the nearer all bodies approach to the earth, the
stronger and with the greater force and velocity they will tend to it; but the
farther they are, the weaker and slower:” and his experiment consists in

comparing the effect of a spring and a weight in keeping up the motions of
two “clocks,” regulated together, and removed alternately to the tops of
high buildings and into the deepest mines. By clocks he could not have
meant pendulum clocks, which were not then known, (the first made in
England was in 1662,) fly-clocks, so that the comparison, though too coarse,
was not contrary to sound mechanical principles. In short, its principle was
the comparison of the effect of a spring with that of a weight, in producing
certain motions in certain times, on heights and in mines. Now, this is the
very same thing that has really been done in the recent experiments of
professors Airy and Whewell in Dolcoath mine: a pendulum (a weight
moved by gravity) has been compared with a chronometer balance, moved
and regulated by a spring. In his 37th aphorism, Bacon also speaks of
gravity as an incorporeal power, acting at a distance, and requiring time for
its transmission; a consideration which occurred at a later period to
Laplace, in one of his most delicate investigations.
(197.) A well chosen and strongly marked crucial instance is,
sometimes, of the highest importance; when two theories, which run
parallel to each other (as is sometimes the case) in their explanation of great
classes of phenomena, at length come to be placed at issue upon a single
fact. A beautiful instance of this will be cited in the next section. We may
add to the examples above given of such instances, that of the application of
chemical tests, which are almost universally crucial experiments.
(198.) Bacon’s “travelling instances” are those in which the nature or
quality under investigation “travels,” or varies in degree; and thus
(according to § 152.) afford an indication of a cause by a gradation of
intensity in the effect. One of his instances is very happy, being that of
“paper, which is white when dry, but proves less so when wet, and comes
nearer to the state of transparency upon the exclusion of the air, and
admission of water.” In reading this, and many other instances in the
Novum Organum, one would almost suppose (had it been written) that its
author had taken them from Newton’s Optics.
(199.) The travelling instances, as well as what Bacon terms “frontier
instances,” are cases in which we are enabled to trace that general law
which seems to pervade all nature—the law, as it is termed, of continuity,
and which is expressed in the well known sentence, “Natura non agit per
saltum.” The pursuit of this law into cases where its application is not at

first sight obvious, has proved a fertile source of physical discovery, and led
us to the knowledge of an analogy and intimate connection of phenomena
between which at first we should never have expected to find any.
(200.) For example, the transparency of gold leaf, which permits a
bluish-green light to pass through it, is a frontier instance between the
transparency of pellucid bodies and the opacity of metals, and it prevents a
breach of the law of continuity between transparent and opake bodies, by
exhibiting a body of the class generally regarded the most opake in nature,
as still possessed of some slight degree of transparency. It thus proves that
the quality of opacity is not a contrary or antagonist quality to that of
transparency, but only its extreme lowest degree.

CHAP. VII.
OF THE HIGHER DEGREES OF INDUCTIVE
GENERALIZATION, AND OF THE FORMATION AND
VERIFICATION OF THEORIES.

(201.) As particular inductions and laws of the first degree of generality are
obtained from the consideration of individual facts, so Theories result from
a consideration of these laws, and of the proximate causes brought into
view in the previous process, regarded all together as constituting a new set
of phenomena, the creatures of reason rather than of sense, and each
representing under general language innumerable particular facts. In raising
these higher inductions, therefore, more scope is given to the exercise of
pure reason than in slowly groping out our first results. The mind is more
disencumbered of matter, and moves as it were in its own element. What is
now before it, it perceives more intimately, and less through the medium of
sense, or at least not in the same manner as when actually at work on the
immediate objects of sense. But it must not be therefore supposed that, in
the formation of theories, we are abandoned to the unrestrained exercise of
imagination, or at liberty to lay down arbitrary principles, or assume the
existence of mere fanciful causes. The liberty of speculation which we
possess in the domains of theory is not like the wild licence of the slave
broke loose from his fetters, but rather like that of the freeman who has
learned the lessons of self-restraint in the school of just subordination. The
ultimate objects we pursue in the highest theories are the same as those of
the lowest inductions; and the means by which we can most securely attain
them bear a close analogy to those which we have found successful in such
inferior cases.
(202.) The immediate object we propose to ourselves in physical
theories is the analysis of phenomena, and the knowledge of the hidden

processes of nature in their production, so far as they can be traced by us.
An important part of this knowledge consists in a discovery of the actual
structure or mechanism of the universe and its parts, through which, and by
which, those processes are executed; and of the agents which are concerned
in their performance. Now, the mechanism of nature is for the most part
either on too large or too small a scale to be immediately cognizable by our
senses; and her agents in like manner elude direct observation, and become
known to us only by their effects. It is in vain therefore that we desire to
become witnesses to the processes carried on with such means, and to be
admitted into the secret recesses and laboratories where they are effected.
Microscopes have been constructed which magnify more than a thousand
times in linear dimension, so that the smallest visible grain of sand may be
enlarged to the appearance of one a thousand million times more bulky; yet
the only impression we receive by viewing it through such a magnifier is,
that it reminds us of some vast fragment of a rock, while the intimate
structure on which depend its colour, its hardness, and its chemical
properties, remains still concealed: we do not seem to have made even an
approach to a closer analysis of it by any such scrutiny.
(203.) On the other hand, the mechanism of the great system of which
our planet forms a part escapes immediate observation by the immensity of
its scale, nay, even by the slowness of its evolutions. The motion of the
minute hand of a watch can hardly be perceived without the closest
attention, and that of the hour hand not at all. But what are these, in respect
of the impression of slowness they produce in our minds, compared with a
revolving movement which takes a whole year, or twelve, thirty, or eighty
years to complete, as is the case with the planets in their revolutions round
the sun. Yet no sooner do we come to reflect on the linear dimensions of
these orbs, (which however we do not see, nor can we measure them but by
a long, circuitous, and difficult process,) than we are lost in astonishment at
47
the swiftness of the very motions which before seemed so slow. The
motion of the sails of a windmill offers (on a small scale) an illustrative
case. At a distance the rotation seems slow and steady—but when we stand
close to one of the sails in its sweep, we are surprised at the swiftness with
which it rushes by us.
(204.) Again, the agents employed by nature to act on material
structures are invisible, and only to be traced by the effects they produce.

Heat dilates matter with an irresistible force; but what heat is, remains yet a
problem. A current of electricity passing along a wire moves a magnetized
needle at a distance; but except from this effect we perceive no difference
between the condition of the wire when it conveys and when it does not
convey the stream: and we apply the terms current, or stream, to the
electricity only because in some of its relations it reminds us of something
we have observed in a stream of air or water. In like manner we see that the
moon circulates about the earth; and because we believe it to be a solid
mass, and have never seen one solid substance revolve round another within
our reach to handle and examine unless retained by a force or united by a
tie, we conclude that there is a force, and a mode of connection, between
the moon and the earth; though, what that mode can be, we have no
conception, nor can imagine how such a force can be exerted at a distance,
and with empty space, or at most an invisible fluid, between. (See § 148.)
(205.) Yet are we not to despair, since we see regular and beautiful
results brought about in human works by means which nobody would, at
first sight, think could have any thing to do with them. A sheet of blank
paper is placed upon a frame, and shoved forwards, and after winding its
way successively over and under half a dozen rollers, and performing many
other strange evolutions, comes out printed on both sides. And, after all, the
acting cause in this process is nothing more than a few gallons of water
boiled in an iron vessel, at a distance from the scene of operations. But why
the water so boiled should be capable of producing the active energy which
sets the whole apparatus in motion is, and will probably long remain, a
secret to us.
(206.) This, however, does not at all prevent our having a very perfect
comprehension of the whole subsequent process. We might frequent
printing-houses, and form a theory of printing, and having worked our way
up to the point where the mechanical action commenced (the boiler of the
steam-engine), and verified it by taking to pieces, and putting together
again, the train of wheels and the presses, and by sound theoretical
examination of all the transfers of motion from one part to another; we
should, at length, pronounce our theory good, and declare that we
understood printing thoroughly. Nay, we might even go away and apply the
principles of mechanism we had learned in this enquiry to other widely
different purposes; construct other machines, and put them in motion by the

same moving power, and all without arriving at any correct idea as to the
ultimate source of the force employed. But, if we were inclined to theorize
farther, we might do so; and it is easy to imagine how two theorists might
form very different hypotheses as to the origin of the power which
alternately raised and depressed the piston-rod of the engine. One, for
example, might maintain that the boiler (whose contents we will suppose
that neither theorist has been permitted to examine) was the den of some
powerful unknown animal, and he would not be without plausible analogies
in the warmth, the supply of fuel and water, the breathing noises, the
smoke, and above all, the mechanical power exerted. He would say (not
without a show of reason), that where there is a positive and wonderful
effect, and many strong analogies, such as materials consumed, and all the
usual signs of life maintained, we are not to deny the existence of animal
life because we know no animal that consumes such food. Nay, he might
observe with truth, that the fuel actually consists of the chemical ingredients
which constitute the chief food of all animals, &c.; while, on the other hand,
his brother theorist, who caught a glimpse of the fire, and detected the
peculiar sounds of ebullition, might acquire a better notion of the case, and
form a theory more in consonance with fact.
(207.) Now, nothing is more common in physics than to find two, or
even many, theories maintained as to the origin of a natural phenomenon.
For instance, in the case of heat itself, one considers it as a really existing
material fluid, of such exceeding subtlety as to penetrate all bodies, and
even to be capable of combining with them chemically; while another
regards it as nothing but a rapid vibratory or rotatory motion in the ultimate
particles of the bodies heated; and produces a singularly ingenious train of
mechanical reasoning to show, that there is nothing contradictory to sound
dynamical principles in such a doctrine. Thus, again, with light: one
considers it as consisting in actual particles darted forth from luminous
bodies, and acted upon in their progress by forces of extreme intensity
residing in the substances on which they strike; another, in the vibratory
motion of the particles of luminous bodies, communicated to a peculiar
subtle and highly elastic ethereal medium, filling all space, and conveyed
through it into our eyes, as sounds are to our ears, by the undulations of the
air.

(208.) Now, are we to be deterred from framing hypotheses and
constructing theories, because we meet with such dilemmas, and find
ourselves frequently beyond our depth? Undoubtedly not. Est quodam
prodire tenus si non datur ultra. Hypotheses, with respect to theories, are
what presumed proximate causes are with respect to particular inductions:
they afford us motives for searching into analogies; grounds of citation to
bring before us all the cases which seem to bear upon them, for
examination. A well imagined hypothesis, if it have been suggested by a fair
inductive consideration of general laws, can hardly fail at least of enabling
us to generalize a step farther, and group together several such laws under a
more universal expression. But this is taking a very limited view of the
value and importance of hypotheses: it may happen (and it has happened in
the case of the undulatory doctrine of light) that such a weight of analogy
and probability may become accumulated on the side of an hypothesis, that
we are compelled to admit one of two things; either that it is an actual
statement of what really passes in nature, or that the reality, whatever it be,
must run so close a parallel with it, as to admit of some mode of expression
common to both, at least in so far as the phenomena actually known are
concerned. Now, this is a very great step, not only for its own sake, as
leading us to a high point in philosophical speculation, but for its
applications; because whatever conclusions we deduce from an hypothesis
so supported must have at least a strong presumption in their favour: and we
may be thus led to the trial of many curious experiments, and to the
imagining of many useful and important contrivances, which we should
never otherwise have thought of, and which, at all events, if verified in
practice, are real additions to our stock of knowledge and to the arts of life.
(209.) In framing a theory which shall render a rational account of any
natural phenomenon, we have first to consider the agents on which it
depends, or the causes to which we regard it as ultimately referable. These
agents are not to be arbitrarily assumed; they must be such as we have good
inductive grounds to believe do exist in nature, and do perform a part in
phenomena analogous to those we would render an account of; or such,
whose presence in the actual case can be demonstrated by unequivocal
signs. They must be veræ causæ, in short, which we can not only show to
exist and to act, but the laws of whose action we can derive independently,
by direct induction, from experiments purposely instituted; or at least make
such suppositions respecting them as shall not be contrary to our

experience, and which will remain to be verified by the coincidence of the
conclusions we shall deduce from them, with facts. For example, in the
theory of gravitation we suppose an agent,—viz. force, or mechanical
power,—to act on any material body which is placed in the presence of any
other, and to urge the two mutually towards each other. This is a vera causa;
for heavy bodies (that is, all bodies, but some more, some less,) tend to, or
endeavour to reach, the earth, and require the exertion of force to counteract
this endeavour, or to keep them up. Now, that which opposes and
neutralizes force is force. And again, a plumb-line, which, when allowed to
hang freely, always hangs perpendicularly; is found to hang observably
aside from the perpendicular when in the neighbourhood of a considerable
mountain; thereby proving that a force is exerted upon it, which draws it
towards the mountain. Moreover, since it is a fact that the moon does
circulate about the earth, it must be drawn towards the earth by a force; for
if there were no force acting upon it, it would go on in a straight line
without turning aside to circulate in an orbit, and would, therefore, soon go
away and be lost in space. This force, then, which we call the force of
gravity, is a real cause.
(210.) We have next to consider the laws which regulate the action of
these our primary agents; and these we can only arrive at in three ways: 1st,
By inductive reasoning; that is, by examining all the cases in which we
know them to be exercised, inferring, as well as circumstances will permit,
its amount or intensity in each particular case, and then piecing together, as
it were, these disjecta membra, generalizing from them, and so arriving at
the laws desired; 2dly, By forming at once a bold hypothesis, particularizing
the law, and trying the truth of it by following out its consequences and
comparing them with facts; or, 3dly, By a process partaking of both these,
and combining the advantages of both without their defects, viz. by
assuming indeed the laws we would discover, but so generally expressed,
that they shall include an unlimited variety of particular laws;—following
out the consequences of this assumption, by the application of such general
principles as the case admits;—comparing them in succession with all the
particular cases within our knowledge; and, lastly, on this comparison, so
modifying and restricting the general enunciation of our laws as to make the
results agree.

(211.) All these three processes for the discovery of those general
elementary laws on which the higher theories are grounded are applicable
with different advantage in different circumstances. We might exemplify
their successive application to the case of gravitation: but as this would
rather lead into a disquisition too particular for the objects of this discourse,
and carry us too much into the domain of technical mathematics, we shall
content ourselves with remarking, that the method last mentioned is that
which mathematicians (especially such as have a considerable command of
those general modes of representing and reasoning on quantity, which
constitute the higher analysis,) find the most universally applicable, and the
most efficacious; and that it is applicable with especial advantage in cases
where subordinate inductions of the kind described in the last section have
already led to laws of a certain generality admitting of mathematical
expression. Such a case, for instance, is the elliptic motion of a planet,
which is a general proposition including the statement of an infinite number
of particular places, in which the laws of its motion allow it to be some time
or other found, and for which, of course, the law of force must be so
assumed as to account.
(212.) With regard to the first process of the three above enumerated, it
is in fact an induction of the kind described in § 185.; and all the remarks
we there made on that kind of induction apply to it in this stage. The direct
assumption of a particular hypothesis has been occasionally practised very
successfully. As examples, we may mention Coulomb’s and Poisson’s
theories of electricity and magnetism, in both which, phenomena of a very
complicated and interesting nature are referred to the actions of attractive
and repulsive forces, following a law similar in its expression to the law of
gravitation. But the difficulty and labour, which, in the greater theories,
always attends the pursuit of a fundamental law into its remote
consequences, effectually precludes this method from being commonly
resorted to as a means of discovery, unless we have some good reason, from
analogy or otherwise, for believing that the attempt will prove successful, or
have been first led by partial inductions to particular laws which naturally
point it out for trial.
(213.) In this case the law assumes all the characters of a general
phenomenon resulting from an induction of particulars, but not yet verified
by comparison with all the particulars, nor extended to all that it is capable

of including. (See § 171.) It is the verification of such inductions which
constitutes theory in its largest sense, and which embraces an estimation of
the influence of all such circumstances as may modify the effect of the
cause whose laws of action we have arrived at and would verify. To return
to our example: particular inductions drawn from the motions of the several
planets about the sun, and of the satellites round their primaries, &c. having
led us to the general conception of an attractive force exerted by every
particle of matter in the universe on every other according to the law to
which we attach the name of gravitation; when we would verify this
induction, we must set out with assuming this law, considering the whole
system as subjected to its influence and implicitly obeying it, and nothing
interfering with its action; we then, for the first time, perceive a train of
modifying circumstances which had not occurred to us when reasoning
upwards from particulars to obtain the fundamental law; we perceive that
all the planets must attract each other, must therefore draw each other out
of the orbits which they would have if acted on only by the sun; and as this
was never contemplated in the inductive process, its validity becomes a
question, which can only be determined by ascertaining precisely how great
a deviation this new class of mutual actions will produce. To do this is no
easy task, or rather, it is the most difficult task which the genius of man has
ever yet accomplished: still, it has been accomplished by the mere
application of the general laws of dynamics; and the result (undoubtedly a
most beautiful and satisfactory one) is, that all those observed deviations in
the motions of our system which stood out as exceptions (§ 154.), or were
noticed as residual phenomena and reserved for further enquiry (§ 158.), in
that imperfect view of the subject which we got in the subordinate process
by which we rose to our general conclusion, prove to be the immediate
consequences of the above-mentioned mutual actions. As such, they are
neither exceptions nor residual facts, but fulfilments of general rules, and
essential features in the statement of the case, without which our induction
would be invalid, and the law of gravitation positively untrue.
(214.) In the theory of gravitation, the law is all in all, applying itself at
once to the materials, and directly producing the result. But in many other
cases we have to consider not merely the laws which regulate the actions of
our ultimate causes, but a system of mechanism, or a structure of parts,
through the intervention of which their effects become sensible to us. Thus,
in the delicate and curious electro-dynamic theory of Ampere, the mutual

attraction or repulsion of two magnets is referred to a more universal
phenomenon, the mutual action of electric currents, according to a certain
fundamental law. But, in order to bring the case of a magnet within the
range of this law, he is obliged to make a supposition of a peculiar structure
or mechanism, which constitutes a body a magnet, viz. that around each
particle of the body there shall be constantly circulating, in a certain stated
direction, a small current of electric fluid.
(215.) This, we may say, is too complex; it is artificial, and cannot be
granted: yet, if the admission of this or any other structure tenfold more
artificial and complicated will enable any one to present in a general point
of view a great number of particular facts,—to make them a part of one
system, and enable us to reason from the known to the unknown, and
actually to predict facts before trial,—we would ask, why should it not be
granted? When we examine those instances of nature’s workmanship which
we can take to pieces and understand, we find them in the highest degree
artificial in our own sense of the word. Take, for example, the structure of
an eye, or of the skeleton of an animal,—what complexity and what
artifice! In the one, a pellucid muscle; a lens formed with elliptical surfaces;
a circular aperture capable of enlargement or contraction without loss of
form. In the other, a framework of the most curious carpentry; in which
occurs not a single straight line, nor any known geometrical curve, yet all
evidently systematic, and constructed by rules which defy our research. Or
examine a crystallized mineral, which we can in some measure dissect, and
thus obtain direct evidence of an internal structure. Neither artifice nor
complication are here wanting; and though it is easy to assert that these
appearances are, after all, produced by something which would be very
simple, if we did but know it, it is plain that the same might be said of a
steam-engine executing the most complicated movements, previous to any
investigation of its nature, or any knowledge of the source of its power.
(216.) In estimating, however, the value of a theory, we are not to look,
in the first instance, to the question, whether it establishes satisfactorily, or
not, a particular process or mechanism; for of this, after all, we can never
obtain more than that indirect evidence which consists in its leading to the
same results. What, in the actual state of science, is far more important for
us to know, is whether our theory truly represent all the facts, and include
all the laws, to which observation and induction lead. A theory which did

this would, no doubt, go a great way to establish any hypothesis of
mechanism or structure, which might form an essential part of it: but this is
very far from being the case, except in a few limited instances; and, till it is
so, to lay any great stress on hypotheses of the kind, except in as much as
they serve as a scaffold for the erection of general laws, is to “quite mistake
the scaffold for the pile.” Regarded in this light, hypotheses have often an
eminent use: and a facility in framing them, if attended with an equal
facility in laying them aside when they have served their turn, is one of the
most valuable qualities a philosopher can possess; while, on the other hand,
a bigoted adherence to them, or indeed to peculiar views of any kind, in
opposition to the tenor of facts as they arise, is the bane of all philosophy.
(217.) There is no doubt, however, that the safest course, when it can be
followed, is to rise by inductions carried on among laws, as among facts,
from law to law, perceiving, as we go on, how laws which we have looked
upon as unconnected become particular cases, either one of the other, or all
of one still more general, and, at length, blend altogether in the point of
view from which we learn to regard them. An example will illustrate what
we mean. It is a general law, that all hot bodies throw out or radiate heat in
all directions, (by which we mean, not that heat is an actual substance
darted out from hot bodies, but only that the laws of the transmission of
heat to distant objects are similar to those which would regulate the
distribution of particles thrown forth in all directions,) and that other colder
bodies placed in their neighbourhood become hot, as if they received the
heat so radiated. Again, all solid bodies which become heated in one part
conduct, or diffuse, the heat from that part through their whole substance.
Here we have two modes of communicating heat,—by radiation, and by
conduction; and both these have their peculiar, and, to all appearance, very
different laws. Now, let us bring a hot and a cold body (of the same
substance) gradually nearer and nearer together,—as they approach, the heat
will be communicated from the hot to the cold one by the laws of radiation;
and from the nearer to the farther part of the colder one, as it gradually
grows warm, by those of conduction. Let their distance be diminished till
they just lightly touch. How does the heat now pass from one to the other?
Doubtless, by radiation; for it may be proved, that in such a contact there is
yet an interval. Let them then be forced together, and it will seem clear that
it must now be by conduction. Yet their interval must diminish gradually, as
the force by which they are pressed together increases, till they actually

cohere, and form one. The law of continuity, then, of which we have before
spoken (§ 199.), forbids us to suppose that the intimate nature of the process
of communication is changed in this transition from light to violent contact,
and from that to actual union. If so, we might ask, at what point does the
change happen? Especially since it is also demonstrable, that the particles
of the most solid body are not, really, in contact. Therefore, the laws of
conduction and radiation have a mutual dependence, and the former are
only extreme cases of the latter. If, then, we would rightly understand what
passes, or what is the process of nature in the slow communication of heat
through the substance of a solid, we must ground our enquiries upon what
takes place at a distance, and then urge the laws to which we have arrived,
up to their extreme case.
(218.) When two theories run parallel to each other, and each explains a
great many facts in common with the other, any experiment which affords a
crucial instance to decide between them, or by which one or other must fall,
is of great importance. In thus verifying theories, since they are grounded
on general laws, we may appeal, not merely to particular cases, but to
whole classes of facts; and we therefore have a great range among the
individuals of these for the selection of some particular effect which ought
to take place oppositely in the event of one of the two suppositions at issue
being right and the other wrong. A curious example is given by M. Fresnel,
as decisive, in his mind, of the question between the two great opinions on
the nature of light, which, since the time of Newton and Huyghens, have
divided philosophers. (See § 207.) When two very clean glasses are laid one
on the other, if they be not perfectly flat, but one or both in an almost
imperceptible degree convex or prominent, beautiful and vivid colours will
be seen between them; and if these be viewed through a red glass, their
appearance will be that of alternate dark and bright stripes. These stripes are
formed between the two surfaces in apparent contact, as any one may
satisfy himself by using, instead of a flat plate of glass for the upper one, a
triangular-shaped piece, called a prism, like a three-cornered stick, and
looking through the inclined side of it next the eye, by which arrangement
the reflection of light from the upper surface is prevented from intermixing
with that from the surfaces in contact. Now, the coloured stripes thus
produced are explicable on both theories, and are appealed to by both as
strong confirmatory facts; but there is a difference in one circumstance
according as one or the other theory is employed to explain them. In the

case of the Huyghenian doctrine, the intervals between the bright stripes
ought to appear absolutely black; in the other, half bright, when so viewed
through a prism. This curious case of difference was tried as soon as the
opposing consequences of the two theories were noted by M. Fresnel, and
the result is stated by him to be decisive in favour of that theory which
makes light to consist in the vibrations of an elastic medium.
(219.) Theories are best arrived at by the consideration of general laws;
but most securely verified by comparing them with particular facts, because
this serves as a verification of the whole train of induction, from the lowest
term to the highest. But then, the comparison must be made with facts
purposely selected so as to include every variety of case, not omitting
extreme ones, and in sufficient number to afford every reasonable
probability of detecting error. A single numerical coincidence in a final
conclusion, however striking the coincidence or important the subject, is
not sufficient. Newton’s theory of sound, for example, leads to a numerical
expression for the actual velocity of sound, differing but little from that
afforded by the correct theory afterwards explained by Lagrange, and (when
certain considerations not contemplated by him are allowed for) agreeing
with fact; yet this coincidence is no verification of Newton’s view of the
general subject of sound, which is defective in an essential point, as the
great geometer last named has very satisfactorily shown. This example is
sufficient to inspire caution in resting the verification of theories upon any
thing but a very extensive comparison with a great mass of observed facts.
(220.) But, on the other hand, when a theory will bear the test of such
extensive comparison, it matters little how it has been originally framed.
However strange and, at first sight, inadmissible its postulates may appear,
or however singular it may seem that such postulates should have been
fixed upon,—if they only lead us, by legitimate reasonings, to conclusions
in exact accordance with numerous observations purposely made under
such a variety of circumstances as fairly to embrace the whole range of the
phenomena which the theory is intended to account for, we cannot refuse to
admit them; or if we still hesitate to regard them as demonstrated truths, we
cannot, at least, object to receive them as temporary substitutes for such
truths, until the latter shall become known. If they suffice to explain all the
phenomena known, it becomes highly improbable that they will not explain
more; and if all their conclusions we have tried have proved correct, it is

probable that others yet untried will be found so too; so that in rejecting
them altogether, we should reject all the discoveries to which they may lead.
(221.) In all theories which profess to give a true account of the process
of nature in the production of any class of phenomena, by referring them to
general laws, or to the action of general causes, through a train of
modifying circumstances; before we can apply those laws, or trace the
action of those causes in any assigned case, we require to know the
circumstances: we must have data whereon to ground their application.
Now, these can be learned only from observation; and it may seem to be
arguing in a vicious circle to have recourse to observation for any part of
those theoretical conclusions, by whose comparison with fact the theory
itself is to be tried. The consideration of an example will enable us to
remove this difficulty. The most general law which has yet been discovered
in chemistry is this, that all the elementary substances in nature are
susceptible of entering into combination with each other only in fixed or
definite proportions by weight, and not arbitrarily; so that when any two
substances are put together with a view to unite them, if their weights are
not in some certain determinate proportion, a complete combination will not
take place, but some part of one or the other ingredient will remain over and
above, and uncombined. Suppose, now, we have found a substance having
all the outward characters of a homogeneous or unmixed body, but which,
on analysis, we discover to consist of sulphur, and lead in the proportion of
20 parts of the former to 130 of the latter ingredient; and we would know
whether this is to be regarded as a verification of the law of definite
proportions or an exception to it. The question is reduced to this, whether
the proportion 20 to 130 be or be not that fixed and definite proportion, (or
one of them, if there be more than one proportion possible,) in which,
according to the law in question, sulphur and lead can combine; now, this
can never be decided by merely looking at the law in all its generality. It is
clear, that when particularized by restricting its expression to sulphur and
lead, the law should state what are those particular fixed proportions in
which these bodies can combine. That is to say, there must be certain data
or numbers, by which these are distinguished from all other bodies in
nature, and which require to be known before we can apply the general law
to the particular case. To determine such data, observation must be
consulted; and if we were to have recourse to that of the combination of the
two substances in question with each other, no doubt there would be ground

for the logical objection of a vicious circle: but this is not done; the
determination of these numerical data is derived from experiments
purposely made on a great variety of different combinations, among which
that under consideration does not of necessity occur, and all these being
found, independently of each other, to agree in giving the same results, they
are therefore safely assumed as part of the system. Thus, the law of definite
proportions, when applied to the actual state of nature, requires two separate
statements, the one announcing the general law of combination, the other
particularizing the numbers appropriate to the several elements of which
natural bodies consist, or the data of nature. Among these data, if arranged
in a list, there will be found opposite to the element sulphur the number 16,
48
and opposite to lead, 104 ; and since 20 is to 130 in the exact proportion of
16 to 104, it appears that the combination in question affords a satisfactory
verification of the law.
(222.) The great importance of physical data of this description, and the
advantage of having them well determined, will be obvious, if we consider,
that a list of them, when taken in combination with the general law, affords
the means of determining at once the exact proportion of the ingredients of
all natural compounds, if we only know the place they hold in the system.
In chemistry, the number of admitted elements is between fifty and sixty,
and new ones are added continually as the science advances. Now, the
moment the number corresponding to any new substance added to the list is
determined, we have, in fact, ascertained all the proportions in which it can
enter into combination with all the others, so that a careful experiment made
with the object of determining this number is, in fact, equivalent to as many
different experiments as there are binary, ternary, or yet more complicated
combinations capable of existing, into which the new substance may enter,
as an ingredient.
(223.) The importance of obtaining exact physical data can scarcely be
too much insisted on, for without them the most elaborate theories are little
better than mere inapplicable forms of words. It would be of little
consequence to be informed, abstractedly, that the sun and planets attract
each other, with forces proportional to their masses, and inversely as the
squares of their distances: but, as soon as we know the data of our system,
as soon as we have an accurate statement (no matter how obtained) of the
distances, masses, and actual motions of the several bodies which compose

it, we need no more to enable us to predict all the movements of its several
parts, and the changes that will happen in it for thousands of years to come;
and even to extend our views backwards into time, and recover from the
past, phenomena, which no observation has noted, and no history recorded,
and which yet (it is possible) may have left indelible traces of their
existence in their influence on the state of nature in our own globe, and
those of the other planets.
(224.) The proof, too, that our data are correctly assumed, is involved
in the general verification of the whole theory, of which, when once
assumed, they form a part; and the same comparison with observation
which enables us to decide on the truth of the abstract principle, enables us,
at the same time, to ascertain whether we have fixed the values of our data
in accordance with the actual state of nature. If not, it becomes an important
question, whether the assumed values can be corrected, so as to bring the
results of theory to agree with facts? Thus it happens, that as theories
approach to their perfection, a more and more exact determination of data
becomes requisite. Deviations from observed fact, which, in a first or
approximative verification, may be disregarded as trifling, become
important when a high degree of precision is attained. A difference between
the calculated and observed places of a planet, which would have been
disregarded by Kepler in his verification of the law of elliptic motion,
would now be considered fatal to the theory of gravity, unless it could be
shown to arise from an erroneous assumption of some of the numerical data
of our system.
(225.) The observations most appropriate for the ready and exact
determination of physical data are, therefore, those which it is most
necessary to have performed with exactness and perseverance. Hence it is,
that their performance, in many cases, becomes a national concern, and
observatories are erected and maintained, and expeditions despatched to
distant regions, at an expense which, to a superficial view, would appear
most disproportioned to their objects. But it may very reasonably be asked
why the direct assistance afforded by governments to the execution of
continued series of observations adapted to this especial end should
continue to be, as it has hitherto almost exclusively been, confined to
astronomy.

(226.) Physical data intended to be employed as elements of calculation
in extensive theories, require to be known with a much greater degree of
exactness than any single observation possesses, not only on account of
their dignity and importance, as affording the means of representing an
indefinite multitude of facts; but because, in the variety of combinations
that may arise, or in the changes that circumstances may undergo, cases will
occur when any trifling error in one of the data may become enormously
magnified in the final result to be compared with observation. Thus, in the
case of an eclipse of the sun, when the moon enters very obliquely upon the
sun’s disc, a trifling error in the diameter of either the sun or moon may
make a great one in the time when the eclipse shall be announced to
commence. It ought to be remarked, that these are, of all others, the
conjunctures where observations are most available for the determination of
data; for, by the same rule that a small change in the data will, in such cases,
produce a great one in the thing to be observed; so, vice versâ, any
moderate amount of error, committed in an observation undertaken for
ascertaining its value, can produce but a very trifling one in the reverse
calculation from which the data come to be determined by observation. This
remark extends to every description of physical data in every department of
science, and is never to be overlooked when the object in view is the
determination of data with the last degree of precision.
(227.) But how, it may be asked, are we to ascertain by observation,
data more precise than observation itself? How are we to conclude the value
of that which we do not see, with greater certainty than that of quantities
which we actually see and measure? It is the number of observations which
may be brought to bear on the determination of data that enables us to do
this. Whatever error we may commit in a single determination, it is highly
improbable that we should always err the same way, so that, when we come
to take an average of a great number of determinations, (unless there be
some constant cause which gives a bias one way or the other,) we cannot
fail, at length, to obtain a very near approximation to the truth, and, even
allowing a bias, to come much nearer to it than can fairly be expected from
any single observation, liable to be influenced by the same bias.
(228.) This useful and valuable property of the average of a great many
observations, that it brings us nearer to the truth than any single observation
can be relied on as doing, renders it the most constant resource in all

physical enquiries where accuracy is desired. And it is surprising what a
rapid effect, in equalizing fluctuations and destroying deviations, a
moderate multiplication of individual observations has. A better example
can hardly be taken than the average height of the quicksilver in the
common barometer, which measures the pressure of the air, and whose
fluctuations are proverbial. Nevertheless, if we only observe it regularly
every day, and, at the end of each month, take an average of the observed
heights, we shall find the fluctuations surprisingly diminished in amount;
and if we go on for a whole year, or for many years in succession, the
annual averages will be found to agree with still greater exactness. This
equalizing power of averages, by destroying all such fluctuations as are
irregular or accidental, frequently enables us to obtain evidence of
fluctuations really regular, periodic in their recurrence, and so much smaller
in their amount than the accidental ones, that, but for this mode of
proceeding, they never would have become apparent. Thus, if the height of
the barometer be observed four times a day, constantly, for a few months,
and the averages taken, it will be seen that a regular daily fluctuation, of
very small amount, takes place, the quicksilver rising and falling twice in
the four-and-twenty hours. It is by such observations that we are enabled to
ascertain—what no single measure (unless by a fortunate coincidence),
could give us any idea, and never any certain knowledge of—the true sea
level at any part of the coast, or the height at which the water of the ocean
would stand, if perfectly undisturbed by winds, waves, or tides: a subject of
very great importance, and upon which it would be highly desirable to
possess an extensive series of observations, at a great many points on the
coasts of the principal continents and islands over the whole globe.
(229.) In all cases where there is a direct and simple relation between
the phenomenon observed and a single datum on which it depends, every
single observation will give a value of this quantity, and the average of all
(under certain restrictions) will be its exact value. We say, under certain
restrictions; for, if the circumstances under which the observations are made
be not alike, they may not all be equally favourable to exactness, and it
would be doing injustice to those most advantageous, to class them with the
rest. In such cases as these, as well as in cases where the data are numerous
and complicated together, so as not to admit of single, separate
determination (a thing of continual occurrence), we have to enter into very
nice, and often not a little intricate, considerations respecting the probable

accuracy of our results, or the limits of error within which it is probable
they lie. In so doing we are obliged to have recourse to a refined and
curious branch of mathematical enquiry, called the doctrine of probabilities,
the object of which (as its name imports) is to reduce our estimation of the
probability of any conclusion to calculation, so as to be able to give more
than a mere guess at the degree of reliance which ought to be placed in it.
(230.) To give some general idea of the considerations which such
computations involve, let us imagine a person firing with a pistol at a wafer
on a wall ten yards distant: we might, in a general way, take it for granted,
that he would hit the wall, but not the wafer, at the first shot; but if we
would form any thing like a probable conjecture of how near he would
come to it, we must first have an idea of his skill. No better way of judging
could be devised than by letting him fire a hundred shots at it, and marking
where they all struck. Suppose this done,—suppose the wafer has been hit
once or twice, that a certain number of balls have hit the wall within an inch
of it, a certain number between one and two inches, and so on, and that one
or two have been some feet wide of the mark. Still the question arises, what
estimate are we thence to form of his skill? how near (or nearer) may we,
after this experience, safely, or at least not unfairly, bet that he will come to
the mark the next subsequent shot? This the laws of probability enable us
on such data to say. Again, suppose, before we were allowed to measure the
distances, the wafer were to have been taken away, and we were called
upon, on the mere evidence of the marks on the wall, to say where it had
been placed; it is clear that no reasoning would enable any one to say with
certainty; yet there is assuredly one place which we may fix on with greater
probability of being right than any other. Now, this is a very similar case to
that of an observer—an astronomer for example—who would determine the
exact place of a heavenly body. He points to it his telescope, and obtains a
series of results disagreeing among themselves, but yet all agreeing within
certain limits, and only a comparatively small number of them deviating
considerably from the mean of all; and from these he is called upon to say,
definitively, what he shall consider to have been the most probable place of
his star at the moment. Just so in the calculation of physical data; where no
two results agree exactly, and where all come within limits, some wide,
some close, what have we to guide us when we would make up our minds
what to conclude respecting them? It is evident that any system of
calculation that can be shown to lead of necessity to the most probable

conclusion where certainty is not to be had must be valuable. However, as
this doctrine is one of the most difficult and delicate among the applications
of mathematics to natural philosophy, this slight mention of it must suffice
at present.
(231.) In the foregoing pages we have endeavoured to explain the spirit
of the methods to which, since the revival of philosophy, natural science has
been indebted for the great and splendid advances it has made. What we
have all along most earnestly desired to impress on the student is, that
natural philosophy is essentially united in all its departments, through all
which one spirit reigns and one method of enquiry applies. It cannot,
however, be studied as a whole, without subdivision into parts; and, in the
remainder of this discourse, we shall therefore take a summary view of the
progress which has been made in the different branches into which it may
be most advantageously so subdivided, and endeavour to give a general
idea of the nature of each, and of its relations to the rest. In the course of
this, we shall have frequent opportunity to point out the influence of those
general principles we have above endeavoured to explain, on the progress
of discovery. But this we shall only do as cases arise, without entering into
any regular analysis of the history of each department with that view. Such
an analysis would, indeed, be a most useful and valuable work, but would
far exceed our present limits. We are not, however, without a hope that this
great desideratum in science will, ere long, be supplied from a quarter every
way calculated to do it justice.

PART III.
OF THE SUBDIVISION OF PHYSICS INTO DISTINCT BRANCHES,
AND THEIR MUTUAL RELATIONS.

CHAPTER I.
OF THE PHENOMENA OF FORCE, AND OF THE CONSTITUTION OF
NATURAL BODIES.

(232.) Natural History may be considered in two very different lights:
either, 1st, as a collection of facts and objects presented by nature, from the
examination, analysis, and combination of which we acquire whatever
knowledge we are capable of attaining both of the order of nature, and of
the agents she employs for producing her ends, and from which, therefore,
all sciences arise; or, 2dly, as an assemblage of phenomena to be explained;
of effects to be deduced from causes; and of materials prepared to our
hands, for the application of our principles to useful purposes. Natural
history, therefore, considered in the one or the other of these points of view,
is either the beginning or the end of physical science. As it offers to us, in a
confused and interwoven mass, the elements of all our knowledge, our
business is to disentangle, to arrange, and to present them in a separate and
distinct state: and to this end we are called upon to resolve the important but
complicated problem,—Given the effect, or assemblage of effects, to find
the causes. The principles on which this enquiry relies are those which
constitute the relation of cause and effect, as it exists with reference to our
minds; and their rules and mode of application have been attempted to be
sketched out, (though in far less detail than the intrinsic interest of the
subject, both in a logical and practical point of view, would demand,) in the
foregoing pages. It remains now to bring together, in a summary statement,
the results of the general examination of nature, so far as it has been
prosecuted to the discovery of natural agents, and the mode in which they
act.
(233.) The first great agent which the analysis of natural phenomena
offers to our consideration, more frequently and prominently than any other,

is force. Its effects are either, 1st, to counteract the exertion of opposing
force, and thereby to maintain equilibrium; or, 2dly, to produce motion in
matter.
(234.) Matter, or that, whatever it be, of which all the objects in nature
which manifest themselves directly to our senses consist, presents us with
two general qualities, which at first sight appear to stand in contradiction to
each other—activity and inertness. Its activity is proved by its power of
spontaneously setting other matter in motion, and of itself obeying their
mutual impulse, and moving under the influence of its own and other force;
inertness, in refusing to move unless obliged to do so by a force impressed
externally, or mutually exerted between itself and other matter, and by
persisting in its state of motion or rest unless disturbed by some external
cause. Yet in reality this contradiction is only apparent. Force being the
cause, and motion the effect produced by it on matter, to say that matter is
inert, or has inertia, as it is termed, is only to say that the cause is expended
in producing its effect, and that the same cause cannot (without renewal)
produce double or triple its own proper effect. In this point of view,
equilibrium may be conceived as a continual production of two opposite
effects, each undoing at every instant what the other has done.
(235.) However, if this should appear too metaphysical, at all events
this difference of effects gives rise to two great divisions of the science of
force, which are commonly known by the names of Statics and Dynamics;
the latter term, which is general, and has been used by us before in its
general sense, being usually confined to the doctrine of motion, as produced
and modified by force. Each of these great divisions again branches out into
distinct subdivisions, according as we consider the equilibrium or motion of
matter in the three distinct states in which it is presented to us in nature, the
solid, liquid, and aëriform state, to which, perhaps, ought to be added the
viscous, as a state intermediate between that of solidity and fluidity, the
consideration of which, though very obscure and difficult, offers a high
degree of interest on a variety of accounts.

Statics and Dynamics.

(236.) The principles have been definitively fixed by Galileo and his
successors, down to Newton, on a basis of sound induction; and as they are
perfectly general, and apply to every case, they are competent, as we have
already before observed, to the solution of every problem that can occur in
the deductive processes, by which phenomena are to be explained, or
effects calculated. Hence, they include every question that can arise
respecting the motions and rest of the smallest particles of matter, as well as
of the largest masses. But the mode of reasoning from these general
principles differs materially, whether we consider them as applied to masses
of matter of a sensible size, or to those excessively minute, and perhaps
indivisible, molecules of which such masses are composed. The
investigations which relate to the latter subject are extremely intricate, as
they necessarily involve the consideration of the hypotheses we may form
respecting the intimate constitution of the several sorts of bodies above
enumerated.
(237.) On the other hand, those which respect the equilibrium and
motions of sensible masses of matter are happily capable of being so
managed as to render unnecessary the adoption of any particular hypothesis
of structure. Thus, in reasoning respecting the application of forces to a
solid mass, we suppose its parts indissolubly and unalterably connected; it
matters not by what tie, provided this condition be satisfied, that one point
of it cannot be moved without setting all the rest in motion, so that the
relative situation of the parts one among another be not changed. This is the
abstract notion of a solid which the mechanician employs in his reasonings.
And their conclusions will apply to natural bodies, of course, only so far as
they conform to such a definition. In strictness of speaking, however, there
are no bodies which absolutely conform to it. No substance is known whose
parts are absolutely incapable of yielding one among another; but the
amount by which they do yield is so excessively small as to be
demonstrably incapable, in most cases, of having any influence on the
results: and in those where it has such influence, an especial investigation of
its amount can always be made. This gives rise to two subdivisions of the
application of mechanical reasonings to solid masses. Those which refer to
the action of forces on flexible or elastic, and on inflexible or rigid, bodies,
comprehending under the latter all such whose resistance to flexure or
fracture is so very great as to permit our adoption of the language and ideas
of the extreme case without fear of material error.

(238.) In like manner, when we reason respecting the action of forces
on a fluid mass, all we have occasion to assume is, that its parts are freely
moveable one among the other. If, besides this, we choose to regard a fluid
as incompressible, and deduce conclusions on this supposition, they will
hold good only so far as there may be found such fluids in nature. Now, in
strictness, there are none such; but, practically speaking, in the greater
number of cases their resistance to compression is so very great that the
result of the reasoning so carried on is not sensibly vitiated; and, in the
remaining cases, the same general principles enable us to enter on a special
enquiry directed to this point: and hence the division of fluids, in
mechanical language, into compressible and incompressible, the latter being
only the extreme or limiting case of the former.
(239.) As we propose here, however, only to consider what is the actual
constitution of nature, we shall regard all bodies, as they really are, more or
less flexible and yielding. We know for certain, that the space which any
material body appears to occupy is not entirely filled by it; because there is
none which by the application of a sufficient force may not be compressed
or forced into a smaller space, and which, either wholly, as in air or liquids,
or in part, as in the greater number of solids, will not recover its former
dimensions when the force is taken off. In the case of air, this condensation
may be urged to almost any extent; and not only does a mass of air so
condensed completely recover its original bulk, when the applied pressure
is removed, but if that ordinary pressure under which it exists at the earth’s
surface (and which arises from the weight of the atmosphere) be also
removed by an air-pump, it will still further dilate itself without limit so far
as we have yet been able to try it. Hence we are led to the conclusion that
the particles of air are mutually elastic, and have a tendency to recede from
one another, which can only be counteracted by force, and therefore is itself
a force of the repulsive kind. Nevertheless, as air is heavy, and as
gravitation is a universal property of matter, there is no doubt that this
repulsive tendency must have a limit, and that there is a distance to which,
if the particles of the air could be removed from each other, their mutual
repulsion would cease, and an attraction take its place. This limit is
probably attained at some very great height above the earth’s surface,
beyond which, of course, its atmosphere cannot extend.

(240.) What, however, we can only conclude by this or similar
reasoning respecting air, we see distinctly in liquids. They are all, though in
a small degree, compressible, and recover their former dimensions
completely when the pressure is removed; but they cannot be dilated (by
mechanical means), and have no tendency, while they remain liquids, to
enlarge themselves beyond a certain limit, and therefore they assume a
determinate surface while at rest, and their parts actually resist further
separation with a considerable force, thus giving rise to the phenomenon of
the cohesion of liquids.
(241.) Both in air and in liquids, however, the most perfect freedom of
motion of the parts among each other subsists, which could hardly be the
case if they were not separate and independent of each other. And from this,
combined with the foregoing considerations, it has been concluded that they
do not actually touch, but are kept asunder at determinate distances from
each other, by the constant action of the two forces of attraction and
repulsion, which are supposed to balance and counteract each other at the
ordinary distances of the particles, but to prevail, the one, or the other,
according as they are forcibly urged together or pulled asunder.
(242.) In solids, however, the case is very different. The mutual free
motion of their parts inter se is powerfully impeded, and in some almost
destroyed. In some, a slow and gradual change of figure may be produced
to a great extent, by pressure or blows, as for instance in the metals, clay,
butter, &c.; in others, fracture is the consequence of any attempt to change
the figure by violence beyond a certain very small limit. In solids, then, it is
evident, that the consideration of their intimate structure has a very great
influence in modifying the general results of the action of such attractive
and repulsive forces as may be assumed to account for the phenomena they
present; yet the general facts that their parts cohere with a certain energy,
and that they resist displacement or intrusion on the part of other bodies, are
sufficient to demonstrate at least the existence of such forces, whatever
obscurity may subsist as to their mode of action.
(243.) This division of bodies into airs, liquids, and solids, gives rise,
then, to three distinct branches of mechanical science, in each of which the
general principles of equilibrium and motion have their peculiar mode of
application; viz. pneumatics, hydrostatics, and what might, without
impropriety, be termed stereostatics.

Pneumatics.

(244.) Pneumatics relates to the equilibrium or movements of aërial
fluids under all circumstances of pressure, density, and elasticity. The
weight of the air, and its pressure on all the bodies on the earth’s surface,
were quite unknown to the ancients, and only first perceived by Galileo, on
the occasion of a sucking-pump refusing to draw water above a certain
height. Before his time it had always been supposed that water rose by
suction in a pipe, in consequence of a certain natural abhorrence of a
vacuum or empty space, which obliged the water to enter by way of
supplying the place of the air sucked out. But if any such abhorrence
existed, and had the force of an acting cause, which could urge water a
single foot into a pipe, there is no reason why the same principle should not
carry it up two, three, or any number of feet; none why it should suddenly
stop short at a certain height, and refuse to rise higher, however violent the
suction might be, nay, even fall back, if purposely forced up too high.
(245.) Galileo, however, at first contented himself with the conclusion,
that the natural abhorrence of a vacuum was not strong enough to sustain
the water more than about thirty-two feet above its level; and, although the
true cause of the phenomenon at length occurred to him, in the pressure of
the air on the general surface, it was not satisfactorily demonstrated till his
pupil, Torricelli, conceived the happy idea of instituting an experiment on a
small scale by the use of a much heavier liquid, mercury, instead of water,
and, in place of sucking out the air from above, employing the much more
effectual method of filling a long glass tube with mercury, and inverting it
into a basin of the same metal. It was then at once seen, as by a glaring
instance, that the maintenance of the mercury in the tube (which is nothing
else than the common barometer) was the effect of a perfectly definite
external cause, while its fluctuations from day to day, with the varying state
of the atmosphere, strongly corroborated the notion of its being due to the
pressure of the external air on the surface of the mercury in the reservoir.
(246.) The discovery of Torricelli was, however, at first much
misconceived, and even disputed, till the question was finally decided by
appeal to a crucial instance, one of the first, if not the very first on record in
physics, and for which we are indebted to the celebrated Pascal. His
acuteness perceived that if the weight of the incumbent air be the direct

cause of the elevation of the mercury, it must be measured by the amount of
that elevation, and therefore that, by carrying a barometer up a high
mountain, and so ascending into the atmosphere above a large portion of the
incumbent air, the pressure, as well as the length of the column sustained by
it, must be diminished; while, on the other hand, if the phenomenon were
due to the cause originally assigned, no difference could be expected to take
place, whether the observation were made on a mountain or on the plain.
Perhaps the decisive effect of the experiment which he caused to be
instituted for the purpose, on the Puy de Dôme, a high mountain in
Auvergne, while it convinced every one of the truth of Torricelli’s views,
tended more powerfully than any thing which had previously been done in
science to confirm, in the minds of men, that disposition to experimental
verification which had scarcely yet taken full and secure root.
(247.) Immediately on this discovery followed that of the air-pump, by
Otto von Guericke of Magdeburgh, whose aim seems to have been to
decide the question, whether a vacuum could or could not exist, by
endeavouring to make one. The imperfection of his mechanism enabled him
only to diminish the aërial contents of his receivers, not entirely to empty
them; but the curious effects produced by even a partial exhaustion of air
speedily excited attention, and induced our illustrious countryman, Robert
Boyle, to the prosecution of those experiments which terminated in his
hands, and in those of Hauksbee, Hooke, Mariotte, and others, in a
satisfactory knowledge of the general law of the equilibrium of the air
under the influence of greater or less pressures. These discoveries have
since been extended to all the various descriptions of aërial fluids which
chemistry has shown to exist, and to maintain their aëriform state under
artificial pressure, and even to those which may be produced from liquids
reduced to a state of vapour by heat, so long as they retain that state.
(248.) The manner in which the observed law of equilibrium of an
elastic fluid, like air, may be considered to originate in the mutual repulsion
of its particles, has been investigated by Newton, and the actual statement
of the law itself, as announced by Mariotte, “that the density of the air, or
the quantity of it contained in the same space, is, cæteris paribus,
proportional to the pressure it supports,” has recently been verified within
very extensive limits by direct experiment, by a committee of the Royal
Academy of Paris. This law contains the principle of solution of every

dynamical question that can occur relative to the equilibrium of elastic
fluids, and is therefore to be regarded as one of the highest axioms in the
science of pneumatics.

Hydrostatics.

(249.) The principles of the equilibrium of liquids, understanding by
this word such fluids as do not, though quite at liberty, attempt to dilate
themselves beyond a certain point, are at once few and simple. The first
steps towards a knowledge of them were made by Archimedes, who
established the general fact, that a solid immersed in a liquid loses a portion
of its weight equal to that of the liquid it displaces. It seems very
astonishing, after this, that it should not have been at once concluded that
the weight thus said to be lost is only counteracted by the upward pressure
of the liquid, and that, therefore, a portion of any liquid, surrounded on all
sides by a liquid of the same kind, does really exert its weight in keeping its
place. Yet the prejudice that “liquids do not gravitate in their natural place”
kept its ground, and was only dispelled with the mass of error and absurdity
which the introduction of a rational and experimental philosophy by Galileo
swept away.
(250.) The hydrostatical law of the equal pressure of liquids in all
directions, with its train of curious and important consequences, is an
immediate conclusion from the perfect mobility of their parts among one
another, in consequence of which each of them tends to recede from an
excess of pressure on one side, and thus bears upon the rest, and distributes
the pressure among its neighbours. In this form it was laid down by
Newton, and has proved one of the most useful and fertile principles of
physico-mathematical reasoning on the equilibrium of fluid masses, as
affording a means of tracing the action of a force applied at any point of a
liquid through its whole extent. It applies, too, without any modification, to
expansible fluids as well as to liquids; and, in the applications of geometry
to this subject, enables us to dispense with any minute and intricate
enquiries as to the mode in which individual particles act on each other.
(251.) In a practical point of view, this law is remarkable for the
directness of its application to useful purposes. The immediate and perfect

distribution of a pressure applied on any one part, however small, of a fluid
surface through the whole mass, enables us to communicate at one instant
the same pressure to any number of such parts by merely increasing the
surface of the fluid, which may be done by enlarging the containing vessel;
and if the vessel be so constructed that a large portion of its surface shall be
moveable together, the pressures on all the similar parts of this portion will
be united into one consentient force, which may thus be increased to any
extent we please. The hydraulic press, invented by Bramah, (or rather
applied by him after a much more ancient inventor, Stevin,) is constructed
on this principle. A small quantity of water is driven by sufficient pressure
into a vessel already full, and provided with a moveable surface or piston of
great size. Under such circumstances something must give way; the great
surface of the piston accumulates the pressure on it to such an extent that
nothing can resist its violence. Thus, trees are torn up by the roots; piles
extracted from the earth; woollen and cotton goods compressed into the
most portable dimensions; and even hay, for military service, reduced to
such a state of coercion as to be easily packed on board transports.
(252.) Liquids differ from aëriform fluids by their cohesion, which may
be regarded as a kind of approach to a solid state, and was so regarded by
Bacon (193.). Indeed, there can be little doubt that the solid, liquid, and
aëriform states of bodies are merely stages in a progress of gradual
transition from one extreme to the other; and that, however strongly marked
the distinctions between them may appear, they will ultimately turn out to
be separated by no sudden or violent line of demarcation, but shade into
each other by insensible gradations. The late experiments of Baron Cagnard
de la Tour may be regarded as a first step towards the full demonstration of
this (199.). But the cohesion of liquids is not, like that of solids, so modified
by their structure in other respects as to destroy the mobility of their parts
one among another (unless in those cases of nearer approach to the solid
state which obtain in viscid or gummy liquids). On the contrary, the two
qualities co-exist, and give rise to a number of curious and intricate
phenomena.
(253.) One of the most remarkable of these is capillary attraction, or
capillarity as it is sometimes called. Every body has remarked the adhesion
of water to glass. The elevation of the general surface of the liquid where it
is in contact with the containing vessel; the form of a drop suspended at the

under side of a solid: these are instances of capillary attraction. If a small
glass tube with a bore as fine as a hair be immersed in water, the water will
be observed to rise in it to a certain height, and to assume a concave surface
at its upper extremity. The attraction of the glass on the water, and the
cohesion of the parts of the water to each other, are no doubt the joint
causes of this curious effect; but the mode of action is at once obscure and
complex; and although the researches of Laplace and Young have thrown
great light on it, further investigation seems necessary before we can be said
distinctly to understand it.
(254.) As the capillarity and cohesion of the parts of liquids shows
them to possess the power of mutual attraction, so their elasticity
demonstrates that they also possess that of repulsion when forcibly brought
nearer than their natural state. From the extremely small extent to which the
compression of liquids can be carried by any force we can employ,
compared with that of air, we must conclude that this repulsion is much
more violent in the former than in the latter, but counteracted also by a more
powerful force of attraction. So much more powerful, indeed, is the
resistance of liquids to compression, that they were usually regarded as
incompressible; an opinion corroborated by a celebrated experiment made
at Florence, in which water was forced through the pores (as it was said) of
a golden ball. More recent experiments by Canton, and since by Perkins,
Oërsted, and others, have demonstrated however the contrary, and assigned
the amount of compression.
(255.) The consideration of the motions of fluids, whether liquid or
expansible, is infinitely more complicated than that of their equilibrium.
When their motions are slow, it is reasonable to suppose that the law of the
equable distribution of pressure obtains; but in very rapid displacements of
their parts one among the other, it is not easy to see how such an equable
distribution can be accomplished, and some phenomena exist which seem to
indicate a contrary conclusion.
(256.) Independent of this, there are difficulties of an almost
insuperable nature to the regular deductive application of the general
principles of mechanics to this subject, which arise from the excessive
intricacy of the pure mathematical enquiries to which its investigation leads.
It was Newton who set the example of a first attempt to draw any
conclusions respecting the motion of fluid masses by direct reasoning from

dynamical principles, and thus laid the foundation of Hydrodynamics; but it
was not till the time of D’Alembert that the method of reducing any
question respecting the motions of fluids under the action of forces to strict
mathematical investigation could be said to be completely understood. But
the cases even now in which this mode of treating such questions can be
applied with full satisfaction are few in comparison of those in which the
experimental method of enquiry as already observed (189.) is preferable.
Such, for example, is that of the resistance of fluids to bodies moving
through them; a knowledge of which is of great importance in naval
architecture and in gunnery, where the resistance of the air acts to an
enormous extent. Such, too, among the practical subjects which depend
mainly on this branch of science, are the use of sails in navigation; the
construction of windmills, and water-wheels; the transmission of water
through pipes and channels; the construction of docks and harbours, &c.

Nature of Solids in general.

(257.) The intimate constitution of solids is, in all probability, very
complicated, and we cannot be said to know much of it. By some recent
delicate experiments on the dimensions of wires violently strained, it has
been shown that they are to a certain small extent capable of being dilated
by tension, as they are also of being compressed by pressure, but within
limits even narrower than those of liquids. Usually, when strained too far,
they break, and refuse to re-unite; or, if compressed too forcibly, take a
permanent contraction of dimension. Thus, wood may be indented by a
blow, and metals rendered denser and heavier by hammering or rolling.
There is a certain degree of confusion prevalent in ordinary language about
the hardness, elasticity, and other similar qualities, of solids, which it may
be well to remove. Hardness is that disposition of a solid which renders it
difficult to displace its parts among themselves. Thus, steel is harder than
iron; and diamond almost infinitely harder than any other substance in
nature: but the compressibility of steel, or the extent to which it will yield to
a given pressure and recover itself, is not much less than that of soft iron,
and that of ice is very nearly the same with that of water.

(258.) Again, we call Indian rubber a very elastic body, and so it is; but
in a different sense from steel. Its parts admit of great mutual displacement
without permanent dislocation; however distorted, it recovers its figure
readily, but with a small force. Yet, if Indian rubber were to be enclosed in a
space that it just filled, so as not to permit its parts to yield laterally,
doubtless it would resist actual compression with great violence. Here, then,
we have an instance of two kinds of elasticity in one substance; a feebler
effort of recovery from distorted figure, and a more violent one from a state
of altered dimension. Both, however, originate in the same causes, and are
referable to the same principles; the former being in fact only a modified
case of the latter, as the effort of a steel spring, when bent, to recover its
former shape, is referable to the same forces which give to steel its hardness
and strength to resist actual compression and fracture.
(259.) The toughness of a solid, or that quality by which it will endure
heavy blows without breaking, is again distinct from hardness though often
confounded with it. It consists in a certain yielding of parts with a powerful
general cohesion, and is compatible with various degrees of elasticity.
Malleability is again another quality of solids, especially metals, quite
distinct from toughness, and depends on their capability of being deprived
of their figure without an effort to recover it and without fracture.
(260.) Tenacity, again, is a property of solids more directly depending
on the cohesion of their parts than toughness. It consists in their power of
resisting separation by a strain steadily applied, while the quality of
toughness is materially influenced by their disposition to communicate
through their substance the jarring effect of a blow. Accordingly, the
tenacity of a solid is a direct measure of the cohesive attraction of its parts,
and is the best proof of the existence of such a power.

Crystallography.

(261.) It cannot be supposed that these and many other tangible
qualities, as they may be called, should subsist in solids without a
corresponding mechanism in their internal structure. That they have such a
mechanism, and that a very curious and intricate one, the phenomena of
crystallography sufficiently show. This interesting and beautiful department

of natural science is of comparatively very modern date. That many natural
substances affected certain forms must have been known from the earliest
times. Pliny appears to have been acquainted with this fact, at least in some
instances, as he describes the forms of quartz and diamond. But till the time
of Linnæus no material attention seems to have been bestowed on the
subject. He, however, observed, and described with care, the crystalline
forms of a variety of substances, and even regarded them as so definite a
character of the solids which assumed them, that he supposed every
particular form to be generated by a particular salt. Romé de l’Isle pursued
the study of the crystalline forms of bodies yet farther. He first ascertained
the important fact of the constancy of the angles at which their faces meet;
and observing further that many of them appear in several different shapes,
first conceived the idea that these shapes might be reducible to one,
appropriated in a peculiar manner to each substance, and modified by strict
geometrical laws. Bergmann, reasoning on a fact imparted to him by his
pupil Gahn, made a yet greater step, and showed how at least one species of
crystal might be built up of thin laminæ ranged in a certain order, and
following certain rules of superposition. He failed, however, in deducing
just and general conclusions from this remark, which, correctly viewed, is
the foundation of the most important law of crystallography, that which
connects the primitive form with other forms capable of being exhibited by
the same substance, by a certain fixed relation. An idea may be formed of
what is meant by this sort of connection of one form with another, by
considering a pointed pyramid built up of cubic stones, disposed in layers,
each of which separately is a square plate of the thickness of a single stone.
These layers, laid horizontally one on the other, and decreasing regularly in
size from the bottom to the top, produce a pyramidal form with a rough or
channeled surface; and if the layers are so extremely thin that the channels
cease to be visible to the eye, the pyramid will seem smooth and perfect.
(262.) Very shortly after this, and without knowledge of what had been
done by Gahn and Bergmann, the Abbé Haüy, instructed by the accidental
fracture of a fine group of crystals, made the remark noticed already (in
67.), and reasoning on it with more caution and success, and pursuing it into
all its detail, developed the general laws which regulate the superposition of
the layers of particles of which he supposes all crystals to be built up, and
which enable us, from knowing their primitive forms, to discover, previous
to trial, what other forms they are capable of assuming; and which,

according to this idea, are called derivative or secondary forms. Mohs and
others have since imagined processes and systems by which the derivation
of forms from each other is facilitated, and have corrected some errors of
over-hasty generalization into which their predecessors had fallen, as well
as advanced, by an extraordinary diligence of research, our knowledge of
the forms which the various substances which occur in nature and art
actually do assume.
(263.) In what manner a variety in point of external form may originate
in a variety of figures in the ultimate particles of which a solid is composed,
may very readily be imagined by considering what would happen if the
bricks of which an edifice is constructed had all a certain leaning or bias in
one direction out of the perpendicular. Suppose every brick, for instance,
when laid flat on its face, with its longer edges north and south, had its
eastern and western faces upright, but its northern and southern ones
leaning southwards at a certain inclination the same for each brick; a house
built of such bricks would lean the same way, if the bricks fitted well
together. If, besides this, the eastern and western faces of the bricks, instead
of being truly upright, had an inclination eastward, the house would have a
similar one, and all its four corners, instead of being upright, would lean to
the south-east. Suppose, instead of a house, a pyramid were built of such
oblique bricks, with the sides of its base directed to the four points of the
compass; then its point, instead of being situated vertically over the centre
of its base, would stand perpendicularly over some point to the south-east of
that centre, and the pyramid itself would have its sides facing the south and
the east, more highly inclined to the horizon than those towards the north
and west.
(264.) Whatever conception we may form of the manner in which the
particles of a crystal cohere and form masses, it is next to impossible to
divest ourselves of the idea of a determinate figure common to them all.
Any other supposition, indeed, would be incompatible with that exact
similarity in all other respects which the phenomena of chemistry may be
considered as having demonstrated. However, it must be borne in mind that
this idea, plausible as it may appear, is yet in some degree hypothetical, and
that the laws of crystallography, as determined from inductive observation,
are quite independent of any supposition of the kind, or even of the
existence of such things as ultimate particles or atoms at all.

(265.) Still, that peculiar internal constitution of solid bodies, whatever
it be, which is indicated by the assumption of determinate figures, by their
splitting easier in some directions than in others, and by their presenting
glittering plane surfaces when broken into fragments, cannot but have an
important influence on all their relations to external agents, as well as to
their internal movements and the mutual actions of their parts on one
another. Accordingly, the division of bodies into crystallized and
uncrystallized, or imperfectly crystallized, is one of the most universal
importance; and almost all the phenomena produced by those more intimate
natural causes which act within small limits, and as it were on the
immediate mechanism of solid substances, are remarkably modified by
their crystalline structure. Thus, in transparent solids, the course taken by
the rays of light, in traversing them, as well as the properties impressed
upon them in so doing, are intimately connected with this structure. The
recent experiments of M. Savart, too, have proved that this is also the case
with their power of resistance to external force, on which depends their
elasticity. Crystallized substances, according to the results of these
experiments, resist compression with different degrees of elastic force,
according to the direction in which it is attempted to compress them; and all
the phenomena dependent on their elasticity are affected by this cause,
especially those which relate to their vibratory movements and their
conveyance of sound.
(266.) There can be little doubt that modifications, similarly depending
on the internal structure of crystals, will be traced through every department
of physics. In that interesting one which relates to the action of heat in
expanding the dimensions of substances, a beginning has already been
made by Professor Mitscherlich. It had long been known that all substances
are dilated by heat, and no exception to this law has been found, so long as
we regard the bulk of the heated body. Thus, an iron rod when hot is both
longer and thicker than when cold; and the difference of dimension, though
but trifling in itself, is yet capable of being made sensible, and is of
considerable consequence in engineering. Thus, too, the quicksilver in a
common thermometer occupies a larger space when hot than when cold;
and being confined by the glass ball, (which also expands, but not so much
in proportion,) it is forced to rise in the tube. These and similar facts had
long been known; and accurate measures of the total amount of dilatation of
a variety of different bodies, under similar accessions of heat, had been

obtained and registered in tables. But no one had suspected the important
fact, that this expansion in crystallized bodies takes place under totally
different circumstances from what obtains in uncrystallized ones. M.
Mitscherlich has lately shown that such substances expand differently in
different directions, and has even produced a case in which expansion in
one direction is actually accompanied with contraction in another. This step,
the most important beyond a doubt which has yet been made in pyrometry,
can however only be regarded as the first in a series of researches which
will occupy the next generation, and which promises to afford an abundant
harvest of new facts, as well as the elucidation of some of the most obscure
and interesting points in the doctrine of heat.
(267.) From what has been said, it is clear that if we look upon solid
bodies as collections of particles or atoms, held together and kept in their
places by the perpetual action of attractive and repulsive forces, we cannot
suppose these forces, at least in crystallized substances, to act alike in all
directions. Hence arises the conception of polarity, of which we see an
instance, on a great scale, in the magnetic needle, but which, under
modified forms, there is nothing to prevent us from conceiving to act
among the ultimate atoms of solid or even fluid bodies, and to produce all
the phenomena which they exhibit in their crystallized state, either when
acting on each other, or on light, heat, &c. It is not difficult, if we give the
reins to imagination, to conceive how attractive and repulsive atoms, bound
together by some unknown tie, may form little machines or compound
particles, which shall have many of the properties which we refer to
polarity; and accordingly many ingenious suppositions have been made to
that effect: but in the actual state of science it is certainly safest to wave
these hypotheses, without however absolutely rejecting them, and regard
the polarity of matter as one of the ultimate phenomena to which the
analysis of nature leads us, and of which it is our business fully to
investigate the laws, before we endeavour to ascertain its causes, or trace
the mechanism by which it is produced.
(268.) The mutual attractions and repulsions of the particles of matter,
then, and their polarity, whether regarded as an original or a derivative
property, are the forces which, acting with great energy, and within very
confined limits, we must look to as the principles on which the intimate
constitution of all bodies and many of their mutual actions depend. These

are what are understood by the general term of molecular forces. Molecular
attraction has been attempted to be confounded by some with the general
attraction of gravity, which all matter exerts on all other matter; but this
idea is refuted by the plainest facts.

CHAP. II.
OF THE COMMUNICATION OF MOTION THROUGH BODIES.—OF
SOUND AND LIGHT.

(269.) The propagation of motion through all substances, whether of a
single impulse, as a blow or thrust, or of one frequently and regularly
repeated, such as a jarring or vibratory movement, depends wholly on these
molecular forces; and it is on such propagation that sound and very
probably light depend. To conceive the manner in which a motion may be
conveyed from one part of a substance to another, whether solid or fluid, we
may attend to what takes place when a wave is made to run along a
stretched string, or the surface of still water. Every part of the string, or
water, is in succession moved from its place, and agitated with a motion
similar to that of the original impulse, leaving its place and returning to it,
and when one part ceases to move the next receives as it were the
impression, and forwards it onward. This may seem a slow and circuitous
process in description; but when sound, for example, is conveyed through
the air, we are to consider, 1st, that the air, the substance actually in motion,
is extremely light and acted upon by a very powerful elasticity, so that the
force which propagates the motion, or by which the particles adjacent act
on, and urge forward, each other, is very great, compared with the quantity
of materials set in motion by it: and the same is true, even in a greater
degree, in liquids and solids; for in these the elastic forces are even greater,
in proportion to the weight, than in air.
(270.) A general notion of the mode in which sounds are conveyed
through the air was not altogether deficient among the ancients; but it is to
Newton that we owe the first attempt to analyze the process, and show
correctly what takes place in the communication of motion from particle to
particle. Reasoning on the properties of the air as an elastic body, he

showed the effect of an impulse on any portion of it to consist in a
condensation of the air immediately adjacent in the direction of the impulse,
which then, re-acting by its spring, drives back the portion which had
advanced to its original place, and at the same time urges forward the
portion before it, in the direction of the impulse, so that every particle
alternately advances and retreats. But, in pursuing this idea into its details,
Newton fell into some errors which were pointed out by Cramer, though
their origin was not traced, nor the reasoning corrected, till the subject was
resumed by Lagrange and Euler; nor is this any impeachment of the
penetration of our immortal countryman. The mathematical theory of the
propagation of sound, and of vibratory and undulatory motions in general,
is one of the utmost intricacy; and, in spite of every exertion on the part of
the most expert geometers, continues to this day to give continual occasion
for fresh researches; while phenomena are constantly presenting
themselves, which show how far we are from being able to deduce all the
particulars, even of cases comparatively simple, by any direct reasoning
from first principles.
(271.) Whenever an impulse of any kind is conveyed by the air, to our
ears, it produces the impression of sound; but when such an impulse is
regularly and uniformly repeated in extremely rapid succession, it gives us
that of a musical note, the pitch of the note depending on the rapidity of the
succession (see art. 153.). The sense of harmony, too, depends on the
periodical recurrence of coincident impulses on the ear, and affords,
perhaps, the only instance of a sensation for whose pleasing impression a
distinct and intelligible reason can be assigned.
(272.) Acoustics, then, or the science of sound, is a very considerable
branch of physics, and one which has been cultivated from the earliest ages.
Even Pythagoras and Aristotle were not ignorant of the general mode of its
transmission through the air, and of the nature of harmony; but as a branch
of science, independent of its delightful application in the art of music, it
could be hardly said to exist, till its nature and laws became a matter of
experimental enquiry to Bacon and Galileo, Mersenne and Wallis; and of
mathematical investigation to Newton, and his illustrious successors,
Lagrange and Euler. From that time its progress, as a branch both of
mathematical and experimental science, has been constant and accelerated.
A curious and beautiful method of observation, due to Chladni, consists in

the happy device of strewing sand over the surfaces of bodies in a state of
sonorous vibration, and marking the figures it assumes. This has made their
motions susceptible of ocular examination, and has been lately much
improved on, and varied in its application, by M. Savart, to whom we also
owe a succession of instructive researches on every point connected with
the subject of sound, which may rank among the finest specimens of
modern experimental enquiry. But the subject is far from being exhausted;
and, indeed, there are few branches of physics which promise at once so
much amusing interest, and such important consequences, in its bearings on
other subjects, and especially, through the medium of strong analogies, on
that of light.

Light and Vision.

(273.) The nature of light has always been involved in considerable
doubt and mystery. The ancients could scarcely be said to have any opinion
on the subject, unless, indeed, it could be considered such to affirm that
distant bodies could not be put into communication without an
intermedium; and that, therefore, there must be something between the eye
and the thing seen. What that something is, however, they could only form
crude and vague conjectures. One supposed that the eyes themselves emit
rays or emanations of some unknown kind, by which distant objects are as
it were felt; a singularly unfortunate idea, since it gives no reason why
objects should not be equally well seen in the dark—no account, in short, of
the part performed by light in vision. Others imagined that all visible
objects are constantly throwing out from them, in all directions, some sort
of resemblances or spectral forms of themselves, which, when received by
the eyes, produce an impression of the objects. Vague and clumsy as this
hypothesis obviously is, it assigns to the object a power, and to light a
diffusive propagation in all directions, which are, the one and the other,
independent of our eyes, and therefore goes to separate the phenomena of
light from those of vision.
(274.) The hypothesis of Newton is a refinement and improvement on
this idea. Instead of spectra or resemblances, he supposes luminous objects
actually to dart out from them in all directions, particles, of inconceivable

minuteness (as indeed they must be, having such an enormous velocity (see
17.), not to dash in pieces every thing they strike upon). These particles he
supposes to be acted upon by attractive and repulsive forces, residing in all
material bodies, the latter extending to some very small distance beyond
their surfaces; and by the action of these forces to be turned aside from their
natural straight-lined course, without ever coming in actual contact with the
particles themselves of the bodies on which they fall, but either being
turned back and reflected by the repulsive forces before they reach them, or
penetrating between their intervals, as a bird may be supposed to fly
through the branches of a forest, and undergoing all their actions, to take at
quitting them a direction finally determined by the position of the surface at
which they emerge with respect to their course.
(275.) This hypothesis, which was discussed and reasoned upon by
Newton in a manner worthy of himself, affords, by the application of the
same dynamical laws which he had applied with so much success to the
explanation of the planetary motions, not merely a plausible, but a perfectly
reasonable and fair explanation of all the usual phenomena of light known
in his time. His own beautiful discoveries, too, of the different
refrangibilities of the differently coloured rays, were perfectly well
represented in this theory, by simply admitting a difference of velocity in
the particles, which produce in the eye the sensations of different colours.
And had the properties of light remained confined to these, there would
have been no occasion to have resorted to any other mode of conceiving it.
(276.) A very different hypothesis had, however, been suggested about
the same period by Huyghens, who supposed light to be produced in the
same manner with sound, by the communication of a vibratory motion from
the luminous body to a highly elastic fluid, which he imagined as filling all
space, and as being less condensed within the limits of space occupied by
matter, and that to a greater or less extent, according to the nature of the
occupying substance. Thus, in place of any thing actually thrown off, he
substituted waves, or vibrations, propagated in all directions from luminous
bodies, through this medium, or ether, as he called it. Huyghens, being
himself a consummate mathematician, was enabled to trace many of the
consequences of this hypothesis, and to show that the ordinary laws of
reflection and refraction were represented or accounted for by it, as well as
by Newton’s. But the hypothesis of Huyghens has not been fully successful

in accounting for what may be considered the chief of all optical facts, the
production of colours in the ordinary refraction of light by a prism, of
which the theory of Newton gives a complete and elegant explanation; and
the discovery of which by him marks one of the greatest epochs in the
annals of experimental science. This, which has been often urged in
objection to it, remains still, if not quite unanswered, at least only
imperfectly removed.
(277.) Other phenomena, however, were not wanting to afford a further
trial of the explanatory powers of either hypothesis. The diffraction or
inflection of light, discovered by Grimaldi, a Jesuit of Bologna, seemed to
indicate that the rays of light were turned aside from their straight course by
merely passing near bodies of every description. These phenomena, which
are very curious and beautiful, were minutely examined by Newton, and
referred by him to the action of repulsive forces extending to a sensible
distance from the surfaces of bodies; and his explanation, so far as the facts
known to him are concerned, appears as satisfactory as could reasonably be
then expected; and much more so than any thing which could at that time be
produced on the side of the hypothesis of Huyghens, which, in fact, seemed
incapable of giving any account whatever of them.
(278.) Another class of delicate and splendid optical phenomena, which
had begun to attract attention somewhat previous to Newton’s time, seemed
to leave both hypotheses equally at a loss. These were the colours exhibited
by very thin films, either of a liquid (such as a soap-bubble), or of air, as
when two glasses are laid together with only air between them. These
colours were examined by Newton with a minuteness and care altogether
unexampled in experimental philosophy at that time, and with which few
researches undertaken since will bear to stand in competition. Their result
was a theory of a very singular nature, which he grounded on an hypothesis
of what he termed fits of easy transmission and reflection; and which
supposed each ray of light to pass in its progress periodically through a
succession of states such as would alternately dispose it to penetrate or be
reflected back from the surface of a body on which it might fall. The
simplest way in which the reader may conceive this hypothesis, is to regard
every particle of light as a sort of little magnet revolving rapidly about its
own centre while it advances in its course, and thus alternately presenting
its attractive and repulsive pole, so that when it arrives at the surface of a

body with its repulsive pole foremost, it is repelled and reflected; and when
the contrary, attracted, so as to enter the surface. Newton, however, very
cautiously avoided announcing his theory in this or any similar form,
confining himself entirely to general language. In consequence, it has been
confidently asserted by all his followers, that the doctrine of fits of easy
reflection and transmission, as laid down by him, is substantially nothing
more than a statement of facts. Were it so, it is clear that any other theory
which should offer a just account of the same phenomena must ultimately
involve and coincide with that of Newton. But this, as we shall presently
see, is not the case; and this instance ought to serve to make us extremely
cautious how we employ, in stating physical laws derived from experiment,
language which involves any thing in the slightest degree theoretical, if we
would present the laws themselves in a form which no future research shall
modify or subvert.
(279.) A third class of optical phenomena, which were likewise
discovered while Newton was yet engaged in his optical researches, was
that exhibited by doubly refracting crystals. In what the phenomenon of
double refraction consists, we have already had occasion to explain. The
fact itself was first noticed by Erasmus Bartolin in the crystal called Iceland
spar; and was studied with attention by Huyghens, who ascertained its laws,
and referred it with remarkable ingenuity and success to his theory of light,
by the additional hypothesis of such a constitution of his ethereal medium
within the crystal as should enable it to convey an impulse faster in one
direction than another: as if, for example’s sake, we should suppose a sound
conveyed through the air with different degrees of rapidity in a vertical and
horizontal direction.
(280.) Some remarkable facts accompanying the double refraction
produced by Iceland spar, which Bartolin, Huyghens, and Newton, had
observed, led the latter to conceive the singular idea that a ray of light after
its emergence from such a crystal acquires sides, that is to say, distinct
relations to surrounding space, which it carries with it through its whole
subsequent course, and which give rise to all those curious and complicated
phenomena which are now known under the name of the polarization of
light. These results, however, appeared so extraordinary, and offered so
little handle for further enquiry, that their examination dropped, as if by
common consent; Newton himself resting content with urging strongly the

apparent incompatibility of these properties with the Huyghenian doctrine,
but without making any attempt to explain them by his own.
(281.) From the period of Newton’s optical discoveries to the
commencement of the present century, no great accession to our knowledge
of the nature of light was made, if we except one, which, from its invaluable
practical application, must ever hold a prominent place in the annals both of
art and science: we mean, the discovery of the principle of the achromatic
telescope, which originated in a discussion between the celebrated geometer
Euler, Klingenstierna, an eminent Swedish philosopher, and our own
countryman, the admirable optician Dollond, on the occasion of certain
abstract theoretical investigations of the former, which led him to speculate
on its possibility, and which ultimately terminated in its complete and
happy execution by the latter; a memorable case in science, though not a
singular one, where the speculative geometer in his chamber, apart from the
world, and existing among abstractions, has originated views of the noblest
49
practical application.
(282.) The explanation which our knowledge of optical laws affords of
the mechanism of the eye, and the process by which vision is performed, is
as complete and satisfactory as that of hearing by the propagation of motion
through the air. The camera obscura, invented by Baptista Porta in 1560,
gave the first idea how the actual images of external objects might be
conveyed into the eye, but it was not till after a considerable interval that
Kepler, the immortal discoverer of those great laws which regulate the
periods and motions of the planets, pointed out distinctly the offices
performed by the several parts of the eye in the act of vision. From this to
the invention of the telescope and microscope there would seem but a small
step, but it is to accident rather than design that it is due; and its re-
invention by Galileo, on a mere description of its effects, may serve, among
a thousand similar instances, to show that inestimable practical applications
lie open to us, if we can only once bring ourselves to conceive their
possibility, a lesson which the invention of the achromatic telescope itself,
as we have above related it, not less strongly exemplifies.
(283.) The little instrument with which Galileo’s splendid discoveries
were made was hardly superior in power to an ordinary finder of the present
day; but it was rapidly improved on, and in the hands of Huyghens attained

to gigantic dimensions and very great power. It was to obviate the necessity
of the enormous length required for these telescopes, and yet secure the
same power, that Gregory and Newton devised the reflecting telescope,
which has since become a much more powerful instrument than its original
inventors probably ever contemplated.
(284.) The telescope, as it exists at present, with the improvements in
its structure and execution which modern artists have effected, must
assuredly be ranked among the highest and most refined productions of
human art; that in which man has been able to approximate most closely to
the workmanship of nature, and which has conferred upon him, if not
another sense, at least an exaltation of one already possessed by him that
merits almost to be regarded as a new one. Nor does it appear yet to have
reached its ultimate perfection, to which indeed it is difficult to assign any
bounds, when we take into consideration the wonderful progress which
workmanship of every kind is making, and the delicacy, far superior to that
of former times, with which materials may now be wrought, as well as the
ingenious inventions and combinations which every year is bringing forth
50
for accomplishing the same ends by means hitherto unattempted.
(285.) After a long torpor, the knowledge of the properties of light
began to make fresh progress about the end of the last century, advancing
with an accelerated rapidity, which has continued unabated to the present
time. The example was set by our late admirable and lamented countryman,
Dr. Wollaston, who re-examined and verified the laws of double refraction
in Iceland spar announced by Huyghens. Attention being thus drawn to the
subject, the geometry of Laplace soon found a means of explaining at least
one portion of the mystery of this singular phenomenon, by the Newtonian
theory of light, applied under certain supposed conditions; and the
reasoning which led him to the result (at that time quite unexpected), may
justly be regarded as one of his happiest efforts. The prosecution of the
subject, which had now acquired a high degree of interest, was encouraged
by the offer of a prize on the part of the French Academy of Sciences; and it
was in a memoir which received this honourable reward on that occasion, in
1810, that Malus, a retired officer of engineers in the French army,
announced the great discovery of the polarization of light by ordinary
reflection at the surface of a transparent body.

(286.) Malus found that when a beam of light is reflected from the
surface of such a body at a certain angle, it acquires precisely the same
singular property which is impressed upon it in the act of double refraction,
and which Newton had before expressed by saying that it possessed sides.
This was the first circumstance which pointed out a connection between
that hitherto mysterious phenomenon and any of the ordinary modifications
of light; and it proved ultimately the means of bringing the whole within the
limits, if not of a complete explanation, at least of a highly plausible
theoretical representation. So true is, in science, the remark of Bacon, that
no natural phenomenon can be adequately studied in itself alone, but, to be
understood, must be considered as it stands connected with all nature.
(287.) The new class of phenomena thus disclosed were immediately
studied with diligence and success, both abroad by Malus and Arago, and at
home by our countryman Dr. Brewster, and their laws investigated with a
care proportioned to their importance; when another and apparently still
more extraordinary class of phenomena presented itself in the production of
the most vivid and beautiful colours (every way resembling those observed
by Newton in thin films of air or liquids, only infinitely more developed
and striking,) in certain transparent crystallized substances, when divided
into flat plates in particular directions, and exposed in a beam of polarized
light. The attentive examination of these colours by Wollaston, Biot, and
Arago, but more especially by Brewster, speedily led to the disclosure of a
series of optical phenomena so various, so brilliant, and evidently so closely
connected with the most important points relating to the intimate structure
of crystallized bodies, as to excite the highest interest,—that sort of interest
which is raised when we feel we are on the eve of some extraordinary
discovery, and expect every moment that some leading fact will turn up,
which will throw light on all that appears obscure, and reduce into order all
that seems anomalous.
(288.) This expectation was not disappointed. So long before the time
we are speaking of as the first year of the present century, our illustrious
countryman, the late Dr. Thomas Young, had established a principle in
optics, which, regarded as a physical law, has hardly its equal for beauty,
simplicity, and extent of application, in the whole circle of science.
Considering the manner in which the vibrations of two musical sounds
arriving at once at the ear affect the sense with an impression of sound or

silence according as they conspire or oppose each other’s effects, he was led
to the idea that the same ought to hold good with light as with sound, if the
theory which makes light analogous to sound be the true one; and that,
therefore, two rays of light, setting off from the same origin, at the same
instant, and arriving at the same place by different routes, ought to
strengthen or wholly or partially destroy each other’s effects according to
the difference in length of the routes described by them. That two lights
should in any circumstances combine to produce darkness may be
considered strange, but is literally true; and it had even been noticed long
ago as a singular and unaccountable fact by Grimaldi, in his experiments on
the inflection of light. The experimental means by which Dr. Young
confirmed this principle, which is known in optics by the name of the
interference of the rays of light, were as simple and satisfactory as the
principle itself is beautiful; but the verifications of it, drawn from the
explanation it affords of phenomena apparently the most remote, are still
more so. Newton’s colours of thin films were the first phenomena to which
its author applied it with full success. Its next remarkable application was to
those of diffraction, of which, in the hands of M. Fresnel, a late eminent
French geometer, it also furnished a complete explanation, and that, too, in
cases to which Newton’s hypothesis could not apparently be made to apply,
and through a complication of circumstances which might afford a very
severe test of any hypothesis.
(289.) A simple and beautiful experiment on the interferences of
polarized light due to Fresnel and Arago enabled them to bring Dr. Young’s
law to bear on the colours produced by crystallized plates in a polarized
beam, and by so doing afforded a key to all the intricacies of these
magnificent but complex phenomena. Nothing now was wanting to a
rational theory of double refraction but to frame an hypothesis of some
mode in which light might be conceived to be propagated through the
elastic medium supposed to convey it in such a way as not to be
contradictory to any of the facts, nor to the general laws of dynamics. This
essential idea, without which every thing that had been before done would
have been incomplete, was also furnished by Dr. Young, who, with a
sagacity which would have done honour to Newton himself, had declared,
that to accommodate the doctrine of Huyghens to the phenomena of
polarized light it is necessary to conceive the mode of propagation of a
luminous impulse through the ether, differently from that of a sonorous one

through the air. In the latter, the particles of the air advance and recede; in
the former, those of the ether must be supposed to tremble laterally.
(290.) Taking this as the groundwork of his reasoning, Fresnel
succeeded in erecting on it a theory of polarization and double refraction, so
happy in its adaptation to facts, and in the coincidence with experience of
results deduced from it by the most intricate analysis, that it is difficult to
conceive it unfounded. If it be so, it is at least the most curiously artificial
system that science has yet witnessed; and whether it be so or not, so long
as it serves to group together in one comprehensive point of view a mass of
facts almost infinite in number and variety, to reason from one to another,
and to establish analogies and relations between them; on whatever
hypothesis it may be founded, or whatever arbitrary assumptions it may
make respecting structures and modes of action, it can never be regarded as
other than a most real and important accession to our knowledge.
(291.) Still, it is by no means impossible that the Newtonian theory of
light, if cultivated with equal diligence with the Huyghenian, might lead to
an equally plausible explanation of phenomena now regarded as beyond its
reach. M. Biot is the author of the hypothesis we have already mentioned of
a rotatory motion of the particles of light about their axes. He has employed
it only for a very limited purpose; but it might doubtless be carried much
farther; and by admitting only the regular emission of the luminous particles
at equal intervals of time, and in similar states of motion from the shining
body, which does not seem a very forced supposition, all the phenomena of
interference at least would be readily enough explained without the
admission of an ether.
(292.) The optical examination of crystallized substances affords one
among many fine examples of the elucidation which every branch of
science is capable of affording to every other. The indefatigable researches
of Dr. Brewster and others have shown that the phenomena exhibited by
polarized light in its transmission through crystals afford a certain
indication of the most important points relating to the structure of the
crystals themselves, and thus become most valuable characters by which to
recognise their internal constitution. It was Newton who first showed of
what importance as a physical character,—as the indication of other
properties,—the action of a body on light might become; but the characters
afforded by the use of polarized light as an instrument of experimental

enquiry are so marked and intimate, that they may almost be said to have
furnished us with a kind of intellectual sense, by which we are enabled to
scrutinize the internal arrangement of those wonderful structures which
Nature builds up by her refined and invisible architecture, with a delicacy
eluding our conception, yet with a symmetry and beauty which we are
never weary of admiring. In this point of view the science of optics has
rendered to mineralogy and crystallography services not less important than
to astronomy by the invention of the telescope, or to natural history by that
of the microscope; while the relations which have been discovered to exist
between the optical properties of bodies and their crystalline forms, and
even their chemical habitudes, have afforded numerous and beautiful
instances of general laws concluded from laborious and painful induction,
and curiously exemplifying the simplicity of nature as it emerges slowly
from an entangled mass of particulars in which, at first, neither order nor
connection can be traced.

CHAP. III.
OF COSMICAL PHENOMENA.

Astronomy and Celestial Mechanics.

(293.) Astronomy, as has been observed in the former part of this
discourse, as a science of observation, had made considerable progress
among the ancients: indeed, it was the only branch of physical science
which could be regarded as having been cultivated by them with any degree
of assiduity or real success. The Chaldean and Egyptian records had
furnished materials from which the motions of the sun and moon could be
calculated with sufficient exactness for the prediction of eclipses; and some
remarkable cycles, or periods of years in which the lunar eclipses return in
very nearly the same order, had been ascertained by observation.
Considering the extreme imperfection of their means of measuring time and
space, this was, perhaps, as much as could have been expected at that early
period, and it was followed up for a while in a philosophical spirit of just
speculation, which, if continued, could hardly have failed to lead to sound
and important conclusions.
(294.) Unfortunately, however, the philosophy of Aristotle laid it down
as a principle, that the celestial motions were regulated by laws proper to
themselves, and bearing no affinity to those which prevail on earth. By thus
drawing a broad and impassable line of separation between celestial and
terrestrial mechanics, it placed the former altogether out of the pale of
experimental research, while it at the same time impeded the progress of the
latter by the assumption of principles respecting natural and unnatural
motions, hastily adopted from the most superficial and cursory remark,
undeserving even the name of observation. Astronomy, therefore, continued
for ages a science of mere record, in which theory had no part, except in so

far as it attempted to conciliate the inequalities of the celestial motions with
that assumed law of uniform circular revolution which was alone
considered consistent with the perfection of the heavenly mechanism.
Hence arose an unwieldy, if not self-contradictory, mass of hypothetical
motions of sun, moon, and planets, in circles, whose centres were carried
round in other circles, and these again in others without end,—“cycle on
epicycle, orb on orb,”—till at length, as observation grew more exact, and
fresh epicycles were continually added, the absurdity of so cumbrous a
mechanism became too palpable to be borne. Doubts were expressed, to
51
which the sarcasm of a monarch gave a currency they might not have
obtained in a period when men scarcely dared trust themselves to think; and
at length Copernicus, promulgating his own, or reviving the Pythagorean
doctrine, which places the sun in the centre of our system, gave to
astronomy a simplicity which, contrasted with the complication of the
preceding views, at once commanded assent.
52
(295.) An elegant writer , whom we have before had occasion to
quote, has briefly and neatly accounted for the confused notions which so
long prevailed respecting the constitution of our system, and the difficulty
experienced in acquiring a true notion of the disposition of its parts. “We
see it,” he observes, “not in plan, but in section.” The reason of this is, that
our point of observation lies in its general plane, but the notion we aim at
forming of it is not that of its section, but of its plan. This is as if we should
attempt to read a book, or make out the countries on a map, with the eye on
a level with the paper. We can only judge directly of the distances of objects
by their sizes, or rather of their change of distance by their change of size;
neither have we any means of ascertaining, otherwise than indirectly, even
their positions, one among the other, from their apparent places as seen by
us. Now, the variations in apparent size of the sun and moon are too small
to admit of exact measure without the use of the telescope, and the bodies
of the planets cannot even be distinguished as having any distinct size with
the naked eye.
(296.) The Copernican system once admitted, however, this difficulty
of conception, at least, is effectually got over, and it becomes a mere
problem of geometry and calculation to determine, from the observed
places of a planet, its real orbit about the sun, and the other circumstances

of its motion. This Kepler accomplished for the orbit of Mars, which he
ascertained to be an ellipse having the sun in one of its foci; and the same
law, being extended by inductive analogy to all the planets, was found to be
verified in the case of each. This with the other remarkable laws which are
usually cited in physical astronomy by the name of Kepler’s laws, constitute
undoubtedly the most important and beautiful system of geometrical
relations which have ever been discovered by a mere inductive process,
independent of any consideration of a theoretical kind. They comprise
within them a compendium of the motions of all the planets, and enable us
to assign their places in their orbits at any instant of time past or to come
(disregarding their mutual perturbations), provided certain purely
geometrical problems can be numerically resolved.
(297.) It was not, however, till long after Kepler’s time that the real
importance of these laws could be felt. Regarded in themselves, they
offered, it is true, a fine example of regular and harmonious disposition in
the greatest of all the works of creation, and a striking contrast to the
cumbersome mechanism of the cycles and epicycles which preceded them;
but there their utility seemed to terminate, and, indeed, Kepler was
reproached, and not without a semblance of reason, with having rendered
the actual calculation of the places of the planets more difficult than before,
the resources of geometry being then inadequate to resolve the problems to
which the strict application of his laws gave rise.
(298.) The first result of the invention of the telescope and its
application to astronomical purposes, by Galileo, was the discovery of
Jupiter’s disc and satellites,—of a system offering a beautiful miniature of
that greater one of which it forms a portion, and presenting to the eye of
sense, at a single glance, that disposition of parts which in the planetary
system itself is discerned only by the eye of reason and imagination (see
195.). Kepler had the satisfaction of seeing it ascertained, that the law
which he had discovered to connect the times of revolution of the planets
with their distances from the sun, holds good also when applied to the
periods of circulation of these little attendants round the centre of their
principal; thus demonstrating it to be something more than a mere empirical
rule, and to depend on the intimate nature of planetary motion itself.
(299.) It had been objected to the doctrine of Copernicus, that, were it
true, Venus should appear sometimes horned like the moon. To this he

answered by admitting the conclusion, and averring that, should we ever be
able to see its actual shape, it would appear so. It is easy to imagine with
what force the application would strike every mind when the telescope
confirmed this prediction, and showed the planet just as both the
philosopher and his objectors had agreed it ought to appear. The history of
science affords perhaps only one instance analogous to this. When Dr.
Hutton expounded his theory of the consolidation of rocks by the
application of heat, at a great depth below the bed of the ocean, and
especially of that of marble by actual fusion; it was objected that, whatever
might be the case with others, with calcareous or marble rocks, at least, it
was impossible to grant such a cause of consolidation, since heat
decomposes their substance and converts it into quicklime, by driving off
the carbonic acid, and leaving a substance perfectly infusible, and incapable
even of agglutination by heat. To this he replied, that the pressure under
which the heat was applied would prevent the escape of the carbonic acid;
and that being retained, it might be expected to give that fusibility to the
compound which the simple quicklime wanted. The next generation saw
this anticipation converted into an observed fact, and verified by the direct
experiments of Sir James Hall, who actually succeeded in melting marble,
by retaining its carbonic acid under violent pressure.
(300.) Kepler, among a number of vague and even wild speculations on
the causes of the motions whose laws he had developed so beautifully and
with so much patient labour, had obtained a glimpse of the general law of
the inertia of matter, as applicable to the great masses of the heavenly
bodies as well as to those with which we are conversant on the earth. After
Kepler, Galileo, while he gave the finishing blow to the Aristotelian dogmas
which erected a barrier between the laws of celestial and terrestrial motion,
by his powerful argument and caustic ridicule, contributed, by his
investigations of the laws of falling bodies and the motions of projectiles, to
lay the foundation of a true system of dynamics, by which motions could be
determined from a knowledge of the forces producing them, and forces
from the motions they produce. Hooke went yet farther, and obtained a
view so distinct of the mode in which the planets might be retained in their
orbits by the sun’s attraction, that, had his mathematical attainments been
equal to his philosophical acumen, and his scientific pursuits been less
various and desultory, it can hardly be doubted that he would have arrived
at a knowledge of the law of gravitation.

(301.) But every thing which had been done towards this great end,
before Newton, could only be regarded as smoothing some first obstacles,
and preparing a state of knowledge, in which powers like his could be
effectually exerted. His wonderful combination of mathematical skill with
physical research enabled him to invent, at pleasure, new and unheard-of
methods of investigating the effects of those causes which his clear and
penetrating mind detected in operation. Whatever department of science he
touched, he may be said to have formed afresh. Ascending by a series of
close-compacted inductive arguments to the highest axioms of dynamical
science, he succeeded in applying them to the complete explanation of all
the great astronomical phenomena, and many of the minuter and more
enigmatical ones. In doing this, he had every thing to create: the
mathematics of his age proved totally inadequate to grapple with the
numerous difficulties which were to be overcome; but this, so far from
discouraging him, served only to afford new opportunities for the exertion
of his genius, which, in the invention of the method of fluxions, or, as it is
now more generally called, the differential calculus, has supplied a means
of discovery, bearing the same proportion to the methods previously in use,
that the steam-engine does to the mechanical powers employed before its
invention. Of the optical discoveries of Newton we have already spoken;
and if the magnitude of the objects of his astronomical discoveries excite
our admiration of the mental powers which could so familiarly grasp them,
the minuteness of the researches into which he there set the first example of
entering, is no less calculated to produce a corresponding impression.
Whichever way we turn our view, we find ourselves compelled to bow
before his genius, and to assign to the name of Newton a place in our
veneration which belongs to no other in the annals of science. His era marks
the accomplished maturity of the human reason as applied to such objects.
Every thing which went before might be more properly compared to the
first imperfect attempts of childhood, or the essays of inexpert, though
promising, adolescence. Whatever has been since performed, however great
in itself, and worthy of so splendid and auspicious a beginning, has never,
in point of intellectual effort, surpassed that astonishing one which
produced the Principia.
(302.) In this great work, Newton shows all the celestial motions
known in his time to be consequences of the simple law, that every particle
of matter attracts every other particle in the universe with a force

proportional to the product of their masses directly, and the square of their
mutual distance inversely, and is itself attracted with an equal force. Setting
out from this, he explains how an attraction arises between the great
spherical masses of which our system consists, regulated by a law precisely
similar in its expression; how the elliptic motions of planets about the sun,
and of satellites about their primaries, according to the exact rules
inductively arrived at by Kepler, result as necessary consequences from the
same general law of force; and how the orbits of comets themselves are
only particular cases of planetary movements. Thence proceeding to
applications of greater difficulty, he explains how the perplexing
inequalities of the moon’s motion result from the sun’s disturbing action;
how tides arise from the unequal attraction of the sun as well as of the moon
on the earth, and the ocean which surrounds it; and, lastly, how the
precession of the equinoxes is a necessary consequence of the very same
law.
(303.) The immediate successors of Newton found full occupation in
verifying his discoveries, and in extending and improving the mathematical
methods which it had now become manifest were to prove the keys to an
inexhaustible treasure of knowledge. The simultaneous but independent
discovery of a method of mathematical investigation in every respect
similar to that of Newton, by Leibnitz, while it created a degree of national
jealousy which can now only be regretted, had the effect of stimulating the
continental geometers to its cultivation, and impressing on it a character
more entirely independent of the ancient geometry, to which Newton was
peculiarly attached. It was fortunate for science that it did so; for it was
speedily found that (with one fine exception on the part of our countryman
Maclaurin, followed up, after a long interval, by the late Professor Robison
of Edinburgh, with equal elegance,) the geometry of Newton was like the
bow of Ulysses, which none but its master could bend; and that, to render
his methods available beyond the points to which he himself carried them, it
was necessary to strip them of every vestige of that antique dress in which
he had delighted to clothe them. This, however, the countrymen of Newton
were very unwilling to do; and they paid the penalty in finding themselves
condemned to the situation of lookers on, while their continental neighbours
both in Germany and France were pushing forward in the career of
mathematico-physical discovery with emulous rapidity.

(304.) The legacy of research which Newton may be said to have left to
his successors was truly immense. To pursue, through all its intricacies, the
consequences of the law of gravitation; to account for all the inequalities of
the planetary movements, and the infinitely more complicated, and to us
more important ones, of the moon; and to give, what Newton himself
certainly never entertained a conception of, a demonstration of the stability
and permanence of the system, under all the accumulating influence of its
internal perturbations; this labour, and this triumph, were reserved for the
succeeding age, and have been shared in succession by Clairaut,
D’Alembert, Euler, Lagrange and Laplace. Yet so extensive is the subject,
and so difficult and intricate the purely mathematical enquiries to which it
leads, that another century may yet be required to go through with the task.
The recent discoveries of astronomers have supplied matter for
investigation, to the geometers of this and the next generation, of a
difficulty far surpassing any thing that had before occurred. Five primary
planets have been added to our system; four of them since the
commencement of the present century, and these, singularly deviating from
the general analogy of the others, and offering cases of difficulty in theory,
which no one had before contemplated. Yet even the intricate questions to
which these bodies have given rise seem likely to be surpassed by those
which have come into view, with the discovery of several comets revolving
in elliptic orbits, like the planets, round the sun, in very moderate periods.
But the resources of modern geometry seem, so far from being exhausted,
to increase with the difficulties they have to encounter, and already, among
the successors of Lagrange and Laplace, the present generation has to
enumerate a powerful array of names, which promise to render it not less
celebrated in the annals of physico-mathematical research than that which
has just passed away.
(305.) Meanwhile the positions, figures, and dimensions of all the
planetary orbits, are now well known, and their variations from century to
century in great measure determined; and it has been generally
demonstrated, that all the changes which the mutual actions of the planets
on each other can produce in the course of indefinite ages, are periodical,
that is to say, increasing to a certain extent (and that never a very great one),
and then again decreasing; so that the system can never be destroyed or
subverted by the mutual action of its parts, but keeps constantly oscillating,
as it were, round a certain mean state, from which it can never deviate to

any ruinous extent. In particular the researches of Laplace, Lagrange, and
Poisson, have shown the ultimate invariability of the mean distance of each
planet from the sun, and consequently of its periodic time. Relying on these
grand discoveries, we are enabled to look forward, from the point of time
which we now occupy, many thousands of years into futurity, and predict
the state of our system without fear of material error, but such as may arise
from causes whose existence at present we have no reason to suppose, or
from interference which we have no right to anticipate.
(306.) A correct enumeration and description of the fixed stars in
catalogues, and an exact knowledge of their position, supply the only
effectual means we can have of ascertaining what changes they are liable to,
and what motions, too slow to deprive them of their usual epithet, fixed, yet
sufficient to produce a sensible change in the lapse of ages, may exist
among them. Previous to the invention of the compass, they served as
guides to the navigator by night; but for this purpose, a very moderate
knowledge of a few of the principal ones sufficed. Hipparchus was the first
astronomer, who, excited by the appearance of a new star, conceived the
idea of forming a catalogue of the stars, with a view to its use as an
astronomical record, “by which,” says Pliny, “posterity will be able to
discover, not only whether they are born and die, but also whether they
change their places, and whether they increase or decrease.” His catalogue,
containing more than 1000 stars, was constructed about 128 years before
Christ. It was in the course of the laborious discussion of his own and
former observations of them, undertaken with a view to the formation of
this catalogue, that he first recognised the fact of that slow, general advance
of all the stars eastward, when compared with the place of the equinox,
which is known under the name of the precession of the equinoxes, and
which Newton succeeded in referring to a motion in the earth’s axis,
produced by the attraction of the sun and moon.
(307.) Since Hipparchus, at various periods in the history of astronomy,
catalogues of stars have been formed, among which that of Ulugh Begh,
comprising about 1000 stars, constructed in 1437, is remarkable as the
production of a sovereign prince, working personally in conjunction with
his astronomers; and that of Tycho Brahe, containing 777 stars, constructed
in 1600, as having originated in a phenomenon similar to that which drew
the attention of Hipparchus. In more recent times, astronomers provided

with the finest instruments their respective eras could supply, and
established in observatories, munificently endowed by the sovereigns and
governments of different European nations, have vied and are still vying
with each other, in extending the number of registered stars, and giving the
utmost possible degree of accuracy to the determination of their places.
Among these, it would be ungrateful not to claim especial notice for the
superb series of observations which, under a succession of indefatigable and
meritorious astronomers, has, for a very long period, continued to emanate
from our own national observatory of Greenwich.
(308.) The distance of the fixed stars is so immense, that every attempt
to assign a limit, within which it must fall, has hitherto failed. The enquiries
of astronomers of all ages have been directed to ascertain this distance, by
taking the dimensions of our own particular system of sun and planets, or of
the earth itself, as the unit of a scale on which it might be measured. But
although many have imagined that their observations afforded grounds for
the decision of this interesting point, it has uniformly happened either that
the phenomena on which they relied have proved to be referable to other
causes not previously known, and which the superior accuracy of their
researches has for the first time brought to light; or to errors arising from
instrumental imperfections and unavoidable defects of the observations
themselves.
(309.) The only indication we can expect to obtain of the actual
distance of a star, would consist in an annual change in its apparent place
corresponding to the motion of the earth round the sun, called its annual
parallax, and which is nothing more than the measure of the apparent size
of the earth’s orbit as seen from the star. Many observers have thought they
have detected a measurable amount of this parallax; but as astronomical
instruments have advanced in perfection, the quantity which they have
successively assigned to it has been continually reduced within narrower
and narrower limits, and has invariably been commensurate with the errors
to which the instruments used might fairly be considered liable. The
conclusion this strongly presses on us is, that it is really a quantity too small
to admit of distinct measurement in the present state of our means for that
purpose; and that, therefore, the distance of the stars must be a magnitude of
such an order as the imagination almost shrinks from contemplating. But
this increase in our scale of dimension calls for a corresponding

enlargement of conception in all other respects. The same reasoning which
places the stars at such immeasurable remoteness, exalts them at the same
time into glorious bodies, similar to, and even far surpassing, our own sun,
the centres perhaps of other planetary systems, or fulfilling purposes of
which we can have no idea, from any analogy in what passes immediately
around us.
(310.) The comparison of catalogues, published at different periods, has
given occasion to many curious remarks, respecting changes both of place
and brightness among the stars, to the discovery of variable ones which lose
and recover their lustre periodically, and to that of the disappearance of
several from the heavens so completely as to have left no vestige
discernible even by powerful telescopes. In proportion as the construction
of astronomical and optical instruments has gone on improving, our
knowledge of the contents of the heavens has undergone a corresponding
extension, and, at the same time, attained a degree of precision which could
not have been anticipated in former ages. The places of all the principal
stars in the northern hemisphere, and of a great many in the southern, are
now known to a degree of nicety which must infallibly detect any real
motions that may exist among them, and has in fact done so, in a great
many instances, some of them very remarkable ones.
(311.) It is only since a comparatively recent date, however, that any
great attention has been bestowed on the smaller stars, among which there
can be no doubt of the most interesting and instructive phenomena being
sooner or later brought to light. The minute examination of them with
powerful telescopes, and with delicate instruments for the determination of
their places, has, indeed, already produced immense catalogues and masses
of observations, in which thousands of stars invisible to the naked eye are
registered; and has led to the discovery of innumerable important and
curious facts, and disclosed the existence of whole classes of celestial
objects, of a nature so wonderful as to give room for unbounded speculation
on the extent and construction of the universe.
(312.) Among these, perhaps the most remarkable are the revolving
double stars, or stars which, to the naked eye or to inferior telescopes,
appear single; but, if examined with high magnifying powers, are found to
consist of two individuals placed almost close together, and which, when
carefully watched, are (many of them) found to revolve in regular elliptic

orbits about each other; and so far as we have yet been able to ascertain, to
obey the same laws which regulate the planetary movements. There is
nothing calculated to give a grander idea of the scale on which the sidereal
heavens are constructed than these beautiful systems. When we see such
magnificent bodies united in pairs, undoubtedly by the same bond of mutual
gravitation which holds together our own system, and sweeping over their
enormous orbits, in periods comprehending many centuries, we admit at
once that they must be accomplishing ends in creation which will remain
for ever unknown to man; and that we have here attained a point in science
where the human intellect is compelled to acknowledge its weakness, and to
feel that no conception the wildest imagination can form will bear the least
comparison with the intrinsic greatness of the subject.

Geology.

(313.) The researches of physical astronomy are confessedly
incompetent to carry us back to the origin of our system, or to a period
when its state was, in any great essential, different from what it is at
present. So far as the causes now in action go, and so far as our calculations
will enable us to estimate their effects, we are equally unable to perceive in
the general phenomena of the planetary system either the evidence of a
beginning, or the prospect of an end. Geometers, as already stated, have
demonstrated that, in the midst of all the fluctuations which can possibly
take place in the elements of the orbits of the planets, by reason of their
mutual attraction, the general balance of the parts of the system will always
be preserved, and every departure from a mean state periodically
compensated. But neither the researches of the physical astronomer, nor
those of the geologist, give us any ground for regarding our system, or the
globe we inhabit, as of eternal duration. On the contrary, there are
circumstances in the physical constitution of our own planet which at least
obscurely point to an origin and a formation, however remote, since it has
been found that the figure of the earth is not globular but elliptical, and that
its attraction is such as requires us to admit the interior to be more dense
than the exterior, and the density to increase with some degree of regularity
from the surface towards the centre, and that, in layers arranged elliptically

round the centre, circumstances which could scarcely happen without some
such successive deposition of materials as would enable pressure to be
propagated with a certain degree of freedom from one part of the mass to
another, even if we should hesitate to admit a state of primitive fluidity.
(314.) But from such indications nothing distinct can be concluded; and
if we would speculate to any purpose on a former state of our globe and on
the succession of events which from time to time may have changed the
condition and form of its surface, we must confine our views within limits
far more restricted, and to subjects much more within the reach of our
capacity, than either the creation of the world or its assumption of its
present figure. These, indeed, were favourite speculations with a race of
geologists now extinct; but the science itself has undergone a total change
of character, even within the last half century, and is brought, at length,
effectually within the list of the inductive sciences. Geologists now no
longer bewilder their imaginations with wild theories of the formation of
the globe from chaos, or its passage through a series of hypothetical
transformations, but rather aim at a careful and accurate examination of the
records of its former state, which they find indelibly impressed on the great
features of its actual surface, and to the evidences of former life and
habitation which organised remains imbedded and preserved in its strata
indisputably afford.
(315.) Records of this kind are neither few nor vague; and though the
obsoleteness of their language when we endeavour to interpret it too
minutely, may, and no doubt often does, lead to misapprehension, still its
general meaning is, on the whole, unequivocal and satisfactory. Such
records teach us, in terms too plain to be misunderstood, that the whole or
nearly the whole of our present lands and continents were formerly at the
bottom of the sea, where they received deposits of materials from the
wearing and degradation of other lands not now existing, and furnished
receptacles for the remains of marine animals and plants inhabiting the
ocean above them, as well as for similar spoils of the land washed down
into its bosom.
(316.) These remains are occasionally brought to light; and their
examination has afforded indubitable evidence of the former existence of a
state of animated nature widely different from what now obtains on the
globe, and of a period anterior to that in which it has been the habitation of

man, or rather, indeed, of a series of periods, of unknown duration, in which
both land and sea teemed with forms of animal and vegetable life, which
have successively disappeared and given place to others, and these again to
new races approximating gradually more and more nearly to those which
now inhabit them, and at length comprehending species which have their
counterparts existing.
(317.) These wrecks of a former state of nature, thus wonderfully
preserved (like ancient medals and inscriptions in the ruins of an empire),
afford a sort of rude chronology, by whose aid the successive depositions of
the strata in which they are found may be marked out in epochs more or less
definitely terminated, and each characterized by some peculiarity which
enables us to recognise the deposits of any period, in whatever part of the
world they may be found. And, so far as has been hitherto investigated, the
order of succession in which these deposits have been formed appears to
have been the same in every part of the globe.
(318.) Many of the strata which thus bear evident marks of having been
deposited at the bottom of the sea, and of course in a horizontal state, are
now found in a position highly inclined to the horizon, and even
occasionally vertical. And they often bear no less evident marks of violence,
in their bending and fracture, the dislocation of parts which were once
contiguous, and the existence of vast collections of broken fragments which
afford every proof of great violence having been used in accomplishing
some at least of the changes which have taken place.
(319.) Besides the rocks which carry this internal evidence of
submarine deposition, are many which exhibit no such proofs, but on the
contrary hold out every appearance of owing their origin to volcanoes or to
some other mode of igneous action; and in every part of the world, and
among strata of all ages, there occur evidences of such action so abundant,
and on such a scale, as to point out the volcano and the earthquake as agents
which may have been instrumental in the production of those changes of
level, and those violent dislocations which we perceive to have taken place.
(320.) At all events, in accounting for those changes, geologists have no
longer recourse, as formerly, to causes purely hypothetical, such as a
shifting of the earth’s axis of rotation, bringing the sea to overflow the land,
by a change in the place of the longer and shorter diameters of the

spheroidal figure, nor to tides produced by the attraction of comets
suddenly approaching very near the earth, nor to any other fanciful and
arbitrarily assumed hypotheses; but rather endeavour to confine themselves
to a careful consideration of causes evidently in action at present, with a
view to ascertain how far they, in the first instance, are capable of
accounting for the facts observed, and thus legitimately bringing into view,
as residual phenomena, those effects which cannot be so accounted for.
When this shall have been in some measure accomplished, we shall be able
to pronounce with greater security than at present respecting the necessity
of admitting a long succession of tremendous and ravaging catastrophes and
cataclysms,—epochs of terrific confusion and violence which many
geologists (perhaps with justice) regard as indispensable to the explanation
of the existing features of the world. We shall learn to distinguish between
the effects which require for their production the sudden application of
convulsive and fracturing efforts, and those, probably not less extensive,
changes which may have been produced by forces equally or more
powerful, but acting with less irregularity, and so distributed over time as to
produce none of those interregnums of chaotic anarchy which we are apt to
think (perhaps erroneously) great disfigurements of an order so beautiful
and harmonious as that of nature.
(321.) But to estimate justly the effects of causes now in action in
geology is no easy task. There is no à priori or deductive process by which
we can estimate the amount of the annual erosion, for instance, of a
continent by the action of meteoric agents, rain, wind, frost, &c., nor the
quantity of destruction produced on its coasts by the direct violence of the
sea, nor the quantity of lava thrown up per annum by volcanoes over the
whole surface of the earth, nor any similar effect. And to consult experience
on all such points is a slow and painful process if rightly gone into, and a
very fallible one if only partially executed. Much, then, at present must be
left to opinion, and to that sort of clear-judging tact which sometimes
anticipates experience; but this ought not to stand in the way of our making
every possible effort to obtain accurate information on such points, by
which alone geology can be rendered, if not an experimental science, at
least a science of that kind of active observation which forms the nearest
approach to it, where actual experiment is impossible.

(322.) Let us take, for example, the question, “What is the actual
direction in which changes of relative level are taking place between the
existing continents and seas?” If we consult partial experience, that is, all
the information that we possess respecting ancient sea-marks, soundings,
&c., we shall only find ourselves bewildered in a mass of conflicting,
because imperfect, evidence. It is obvious that the only way to decide the
point is to ascertain, by very precise and careful observations at proper
stations on coasts, selected at points where there exist natural marks not
liable to change in the course of at least a century, the true elevation of such
marks above the mean level of the sea, and to multiply these stations
sufficiently over the whole globe to be capable of affording real available
knowledge. Now, this is not a very easy operation (considering the accuracy
required); for the mean level of the sea can be determined by no single
observation, any more than the mean height of the barometer at a given
station, being affected both by periodical and accidental fluctuations due to
tides, winds, waves, and currents. Yet if an instrument adapted for the
purpose were constructed, and rendered easily attainable, and rules for its
use carefully drawn up, there is little doubt we should soon (by the industry
of observers scattered over the world) be in possession of a most valuable
mass of information, which could not fail to afford a point of departure for
the next generation, and furnish ground for the only kind of argument which
ever can be conclusive on such subjects.
(323.) Geology, in the magnitude and sublimity of the objects of which
it treats, undoubtedly ranks, in the scale of the sciences, next to astronomy;
like astronomy, too, its progress depends on the continual accumulation of
observations carried on for ages. But, unlike astronomy, the observations on
which it depends, when the whole extent of the subject to be explored is
taken into consideration, can hardly yet be said to be more than
commenced. Yet, to make up for this, there is another important difference,
that while in the latter science it is impossible to recall the past or anticipate
the future, and observation is in consequence limited to a single fact in a
single moment; in the former, the records of the past are always present;—
they may be examined and re-examined as often as we please, and require
nothing but diligence and judgment to put us in possession of their whole
contents. Only a very small part of the surface of our globe has, however,
been accurately examined in detail, and of that small portion we are only
able to scratch the mere exterior, for so we must consider those excavations

which we are apt to regard as searching the bowels of the earth; since the
deepest mines which have been sunk penetrate to a depth hardly surpassing
the ten thousandth part of the distance between its surface and its centre. Of
course inductions founded on such limited examination can only be
regarded as provisional, except in those remarkable cases where the same
great formations in the same order have been recognised in very distant
quarters, and without exception. This, however, cannot long be the case.
The spirit with which the subject has been prosecuted for many years in our
own country has been rewarded with so rich a harvest of surprising and
unexpected discoveries, and has carried the investigation of our island into
such detail, as to have excited a corresponding spirit among our continental
neighbours; while the same zeal which animates our countrymen on their
native shore accompanies them in their sojourns abroad, and has already
begun to supply a fund of information respecting the geology of our Indian
possessions, as well as of every other point where English intellect and
research can penetrate.
(324.) Nothing can be more desirable than that every possible facility
and encouragement should be afforded for such researches, and indeed to
the pursuits of the enlightened resident or traveller in every department of
science, by the representatives of our national authority wherever our power
extends. By these only can our knowledge of the actual state of the surface
of the globe, and that of the animals and vegetables of the ancient
continents and seas, be extended and perfected, while more complete
information than we at present possess of the habits of those actually
existing, and the influence of changes of climate, food, and circumstances,
on them, may be expected to render material assistance to our speculations
respecting those which have become extinct.

CHAP. IV.
OF THE EXAMINATION OF THE MATERIAL CONSTITUENTS OF
THE WORLD.

Mineralogy.

(325.) The consideration of the history and structure of our globe, and the
examination of the fossil contents of its strata, lead us naturally to consider
the materials of which it consists. The history of these materials, their
properties as objects of philosophical enquiry, and their application to the
useful arts and the embellishments of life, with the characters by which they
can be certainly distinguished one from another, form the object of
mineralogy, taken in its most extended sense.
(326.) There is no branch of science which presents so many points of
contact with other departments of physical research, and serves as a
connecting link between so many distant points of philosophical
speculation, as this. To the geologist, the chemist, the optician, the
crystallographer, the physician, it offers especially the very elements of
their knowledge, and a field for many of their most curious and important
enquiries. Nor, with the exception of chemistry, is there any which has
undergone more revolutions, or been exhibited in a greater variety of forms.
To the ancients it could scarcely be said to be at all known, and up to a
comparatively recent period, nothing could be more imperfect than its
descriptions, or more inartificial and unnatural than its classification. The
more important minerals in the arts, indeed, those used for economical
purposes and those from which metals were extracted, had a certain degree
of attention paid to them, for the sake of their utility and commercial value,
and the precious stones for that of ornament. But until their crystalline
forms were attentively observed and shown to be determinate characters on

which dependence could be placed, no mineralogist could give any correct
account of the real distinction between one mineral and another.
(327.) It was only, however, when chemical analysis had acquired a
certain degree of precision and universal applicability that the importance
of mineralogy as a science began to be recognized, and the connection
between the external characters of a stone and its ingredient constituents
brought into distinct notice. Among these characters, however, none were
found to possess that eminent distinctness which the crystalline form offers;
a character, in the highest degree geometrical, and affording, as might be
naturally supposed, the strongest evidence of its necessary connection with
the intimate constitution of the substance. The full importance of this
character was, however, not felt until its connection with the texture or
cleavage of a mineral was pointed out, and even then it required numerous
and striking instances of the critical discernment of Haüy and other eminent
mineralogists in predicting from the measurements of the angles of crystals
which had been confounded together that differences would be found to
exist in their chemical composition, all which proved fully justified in their
result before the essential value of this character was acknowledged. This
was no doubt in great measure owing to the high importance set by the
German mineralogists on those external characters of touch, sight, weight,
colour, and other sensible qualities, which are little susceptible, with the
exception of weight, of exact determination, and which are subject to
material variations in different specimens of the same mineral. By degrees,
however, the necessity of ascribing great weight to a character so definite
was admitted, especially when it was considered that the same step which
pointed out the intimate connection of external form with internal structure
furnished the mineralogist with the means of reducing all the forms of
which a mineral is susceptible under one general type, or primitive form,
and afforded grounds for an elegant theoretical account of the assumption
of definite figures ab initio.
(328.) A simple and elegant invention of Dr. Wollaston, the reflecting
goniometer, gave a fresh impulse to that view of mineralogy which makes
the crystalline form the essential or leading character, by putting it in the
power of every one, by the examination of even the smallest portion of a
broken crystal, to ascertain and verify that essential character on which the
identity of a mineral in the system of Haüy was made to depend. The

application of so ready and exact a method speedily led to important results,
and to a still nicer discrimination of mineral species than could before be
attained; and the confirmation given to these results by chemical analysis
stamped them with a scientific and decided character which they have
retained ever since.
(329.) Meanwhile the progress made in chemical analysis had led to the
important conclusion that every chemical compound susceptible of
assuming the solid state assumed with it a determinate crystalline form; and
the progress of optical science had shown that the fundamental crystalline
form, in the case at least of transparent bodies, drew with it a series of
optical properties no less curious than important in relation to the affections
of light in its passage through such substances. Thus, in every point of view,
additional importance became added to this character; and the study of the
crystalline forms of bodies in general assumed the form of a separate and
independent branch of science, of which the geometrical forms of the
mineral world constituted only a particular case. Mineralogy, however, as a
branch of natural history, remains still distinct either from optics or
crystallography. The mineralogist is content, and thinks he has performed
his task, if not as a natural historian at least as a classifier and arranger, if he
only gives such a characteristic description of a mineral as shall effectually
distinguish it from every other, and shall enable any one who may
encounter such a body in any part of the world to impose on it its name,
assign it a place in his system, and turn to his books for a further description
of all that the chemist, the optician, the lapidary, or the artist, may require to
know. Still this is no easy matter: the laborious researches of the most
eminent mineralogists can hardly yet be said to have effectually
accomplished it; and its difficulty may be appreciated by the small number
of simple minerals, or minerals of perfectly definite and well-marked
characters, which have been hitherto made out. Nor can this indeed be
wondered at, when we consider that by far the greater portion of the rocks
and stones which compose the external crust of the globe consists of
nothing more than the accumulated detritus of older rocks, in which the
fragments and powder of an infinite variety of substances are mingled
together, in all sorts of varying proportions, and in such a way as to defy
separation. Many of these rocks, however, so compounded, occur with
sufficient frequency and uniformity of character to have acquired names
and to have been usefully applied; indeed, in the latter respect, minerals of

this description far surpass all the others. As objects of natural history,
therefore, they are well worthy of attention, however difficult it may be to
assign them a place in any artificial arrangement.
(330.) This paucity of simple minerals, however, is probably rather
apparent than real, and in proportion as the researches of the chemist and
crystallographer shall be extended throughout nature, they will no doubt
become much more numerous. Indeed, in the great laboratories of nature it
can hardly be doubted that almost every kind of chemical process is going
forwards, by which compounds of every description are continually
forming. Accordingly, it is remarked, that the lavas and ejected scoriæ of
volcanoes are receptacles in which mineral products previously unknown
are constantly discovered, and that the primitive formations, as they are
called in geology, which bear no marks of having been produced by the
destruction of others, are also remarkable for the beauty and distinctness of
character of their minerals.
(331.) The great difficulty which has been experienced in attempts to
classify mineral substances by their chemical constituents has arisen from
the observed presence, in some specimens of minerals bearing that general
resemblance in other respects as well as agreement in form which would
seem to entitle them to be considered as alike, of ingredients foreign to the
usual composition of the species, and that occasionally in so large a
proportion as to render it unjustifiable to refer their occurrence to accidental
impurities. These cases, as well as some anomalies observed in the
classification of minerals by their crystalline forms, which seemed to show
that the same substance might occasionally appear under two distinct forms,
as well as some remarkable coincidences between the forms of substances
quite distinct from each other in a chemical point of view, have within a
recent period given rise to a branch of the science of crystallography of a
very curious and important nature. The isomorphism of certain groups of
chemical elements has already afforded us an example illustrative of the
manner in which inductions sometimes receive unexpected verifications
(see 180.). The laws and relations thus brought to light are among the most
curious and interesting parts of modern science, and seem likely in their
further developement to afford ample scope for the exercise of chemical
and mineralogical research. They have already afforded innumerable fine
examples of that important step in science by which anomalies disappear,

and occasional incongruities become reconciled under more general
expressions of physical laws, and thus unite in affording support to those
very views which they promised, when first observed, to overset. Nothing,
indeed, can be more striking than to see the very ingredient which every
previous chemist and mineralogist would agree to disregard and reject as a
mere casual impurity brought forward and appealed to in support of a
theory expressly directed to the object of rescuing science from the
imputation of disregarding, under any circumstances, the plain results of
direct experiment.

Chemistry.

(332.) The laws which concern the intimate constitution of bodies, not
as respects their structure or the manner in which their parts are put
together, but as regards their materials or the ingredients of which those
parts are composed, form the objects of chemistry. A solid body may be
regarded as a fabric, more or less regularly and artificially constructed, in
which the materials and the workmanship may be separately considered,
and in which, though the latter be ruined and confounded by violence, the
former remain unchanged in their nature, though differently arranged. In
liquid or aërial bodies, too, though there prevails a less degree of difference
in point of structure, and a greater facility of dispersion and dissipation,
than in solids, yet an equal diversity of materials subsists, giving to them
properties differing extremely from each other.
(333.) The inherent activity of matter is proved not only by the
production of motion by the mutual attractions and repulsions of distant or
contiguous masses, but by the changes and apparent transformations which
different substances undergo in their sensible qualities by mere mixture. If
water be added to water, or salt to salt, the effect is an increase of quantity,
but no change of quality. In this case, the mutual action of the particles is
entirely mechanical. Again, if a blue powder and a yellow one, each
perfectly dry, be mixed and well shaken together, a green powder will be
produced; but this is a mere effect arising in the eye from the intimate
mixture of the yellow and blue light separately and independently reflected
from the minute particles of each; and the proof is had by examining the

mixture with a microscope, when the yellow and blue grains will be seen
separate and each quite unaltered. If the same experiment be tried with
coloured liquids, which are susceptible of mixing without chemical action,
a compound colour is likewise produced, but no examination with
magnifiers is in that case sufficient to detect the ingredients; the reason
obviously being, the excessive minuteness of the parts, and their perfect
intermixture, produced by agitating two liquids together. From the mixture
of two powders, extreme patience would enable any one, by picking out
with a magnifier grain after grain, to separate the ingredients. But when
liquids are mixed, no mechanical separation is any longer practicable; the
particles are so minute as to elude all search. Yet this does not hinder us
from regarding such a compound as still a mere mixture, and its properties
are accordingly intermediate between those of the liquids mixed. But this is
far from being the case with all liquids. When a solution of potash, for
example, and another of tartaric acid, each perfectly liquid, are mixed
together in proper proportions, a great quantity of a solid saline substance
falls to the bottom of the containing vessel, which is quite different from
either potash or tartaric acid, and the liquid from which it subsided offers no
indications by its taste or other sensible qualities of the ingredients mixed,
but of something totally different from either. It is evident that this is a
phenomenon widely different from that of mere mixture; there has taken
place a great and radical change in the intimate nature of the ingredients, by
which a new substance is produced which had no existence before. And it
has been produced by the union of the ingredients presented to each other;
for when examined it is found that nothing has been lost, the weight of the
whole mixture being the sum of the weights mixed. Yet the potash and
tartaric acid have disappeared entirely, and the weight of the new product is
found to be exactly equal to that of the tartaric acid and potash employed,
taken together, abating a small portion held in solution in the liquid, which
may be obtained however by evaporation. They have therefore combined,
and adhere to one another with a cohesive force sufficient to form a solid
out of a liquid; a force which has thus been called into action by merely
presenting them to each other in a state of solution.
(334.) It is the business of chemistry to investigate these and similar
changes, or the reverse of such changes, where a single substance is
resolved into two or more others, having different properties from it, and
from each other, and to enquire into all the circumstances which can

influence them; and either determine, modify, or suspend their
accomplishment, whether such influence be exercised by heat or cold, by
time and rest, or by agitation or pressure, or by any of those agents of which
we have acquired a knowledge, such as electricity, light, magnetism, &c.
(335.) The wonderful and sudden transformations with which chemistry
is conversant, the violent activity often assumed by substances usually
considered the most inert and sluggish, and, above all, the insight it gives
into the nature of innumerable operations which we see daily carried on
around us, have contributed to render it the most popular, as it is one of the
most extensively useful, of the sciences; and we shall, accordingly, find
none which have sprung forward, during the last century, with such
extraordinary vigour, and have had such extensive influence in promoting
corresponding progress in others. One of the chief causes of its popularity
is, perhaps, to be sought for in this, that it is, of all the sciences, perhaps, the
most completely an experimental one; and even its theories are, for the most
part, of that generally intelligible and readily applicable kind, which
demand no intense concentration of thought, and lead to no profound
mathematical researches. The simple process of inductive generalization,
grounded on the examination of numerous facts, all of them presenting
considerable intrinsic interest, has sufficed, in most instances, to lead, by a
clear and direct road, to its highest laws yet known. But, on the other hand,
these laws, when stated, are not yet fully sufficient to lead us, except in very
limited cases, to a deductive knowledge of particulars never before
examined, at least, not without great caution, and constant appeal to
experiment as a check on our reasoning; so that we are justified in regarding
the axioms of chemistry, the true handles of deductive reasoning, as still
unknown, and, perhaps, likely long to remain so. This is no fault of its
cultivators, who have comprised in their list the highest and most varied
talents and industry, but of the inherent complexity of the subject, and the
infinite multitude of causes which are concerned in the production of every,
even the simplest, chemical phenomenon.
(336.) The history of chemistry (on which, however, we are not about
to enlarge,) is one of great interest to those who delight to trace the steps by
which mankind advance to the discovery of truth through a series of
mistakes and failures. It may be divided, 1st, into the period of the
alchemists, a lamentable epoch in the annals of intellectual wandering; 2dly,

that of the phlogistic doctrines of Beccher and Stahl, in which, as if to prove
the perversity of the human mind, of two possible roads the wrong was
chosen; and a theory obtained universal credence on the strength of an
induction, valid as such, but wrongly interpreted, which is negatived, in
every instance, by an appeal to the balance. This, too, happened, not by
reason of unlucky coincidences, or individual oversights, but of necessity,
and from an inherent defect of the theory itself, which thus impeded the
progress of the science, as far as a science of experiment can be impeded by
a false theory, by perplexing its cultivators with the appearance of
contradictions in their experiments where none really subsisted, by
destroying all their confidence in the numerical exactness of their own
results, and by involving the subject in a mist of visionary and hypothetical
causes in place of the true acting principles. Thus, in the combustion of any
substance which is incapable of flying away in fumes, an increase of weight
takes place,—the ashes are heavier than the fuel. Whenever this was
observed, however, it was passed carelessly over as arising from the escape
of phlogiston, or the principle of inflammability, which was considered as
being either the element of fire itself, or in some way combined with it, and
thus essentially light. It is now known that the increase of weight is owing
to the absorption of, and combination with, a quantity of a peculiar
ingredient called oxygen, from the air, a principle essentially heavy. So far
as weight is concerned, it makes no difference whether a body having
weight enters, or one having levity escapes; but there is this plain difference
in a philosophical point of view, that oxygen is a real producible substance,
and phlogiston is no such thing: the former is a vera causa, the latter an
hypothetical being, introduced to account for what the other accounts for
much better.
(337.) The third age of chemistry—that which may be called
emphatically modern chemistry—commenced (in 1786) when Lavoisier, by
a series of memorable experiments, extinguished for ever this error, and
placed chemistry in the rank of one of the exact sciences,—a science of
number, weight, and measure. From that epoch to the present day it has
constantly advanced with an accelerated progress, and at this moment may
be regarded as more progressive than ever. The principal features in this
progress may be comprised under the following general heads:—

1. The discovery of the proximate, if not the ultimate, elements of
all bodies, and the enlargement of the list of known elements
to its present extent of between fifty and sixty substances.
2. The developement of the doctrine of latent heat by Black, with
its train of important consequences, including the scientific
theory of the steam-engine.
3. The establishment of Wenzel’s law of definite proportions on his
own experiments, and those of Richter, a discovery
subsequently merged in the more general wording and better
development of Dalton’s atomic theory.
4. The precise determination of the atomic weights of the different
chemical elements, mainly due to the astonishing industry of
Berzelius, and his unrivalled command of chemical
resources, as well as to the researches of the other chemists
of the Swedish and German school.
5. The assimilation of gases and vapours, by which we are led to
regard the former, universally, as particular cases of the
latter, a generalization resulting chiefly from the experiments
of Faraday on the condensation of the gases, and those of
Gay-Lussac and Dalton, on the laws of their expansion by
heat compared with that of vapours.
6. The establishment of the laws of the combination of gases and
vapours by definite volumes, by Gay-Lussac.
7. The discovery of the chemical effects of electricity, and the
decomposing agency of the Voltaic pile, by Nicholson and
Carlisle; the investigation of the laws of such
decompositions, by Berzelius and Hisinger: the
decomposition of the alkalies by Davy, and the consequent
introduction into chemistry of new and powerful agents in
their metallic bases.
8. The application of chemical analysis to all the objects of
organized and unorganized nature, and the discovery of the
ultimate constituents of all, and the proximate ones of
organic matter, and the recognisance of the important

distinctions which appear to divide these great classes of
bodies from each other.
9. The applications of chemistry to innumerable processes in the
arts, and among other useful purposes to the discovery of the
essential medical principles in vegetables, and to important
medicaments in the mineral kingdom.
10. The establishment of the intimate connection between chemical
composition and crystalline form, by Haüy and Vauquelin,
with the successive rectifications the statement of that
connection has undergone in the hands of Mitscherlich,
Rose, and others, with the progress of chemical and
crystallographical knowledge.

(338.) To pursue these several heads into detail would lead us into a
treatise on chemistry; but a few remarks on one or two of them, as they bear
upon the general principles of all scientific enquiry, will not be irrelevant.
And first, then, with reference to the discovery of new elements, it will be
observed, that philosophical chemistry no more aims at determining the one
essential element out of which all matter is framed—the one ultimate
principle of the universe—than astronomy at discovering the origin of the
planetary movements in the application of a determinate projectile force in
a determinate direction, or geology at ascending to the creation of the earth.
There may be such an element. Some singular relations which have been
pointed out in the atomic weights of bodies seem to suggest to minds fond
of speculation that there is; but philosophical chemistry is content to wait
for some striking fact, which may either occur unexpectedly or be led to by
the slow progress of enlarged views, to disclose to us its existence. Still, the
multiplication of so-considered elementary bodies has been considered by
some as an inconvenience. We confess we do not coincide with this view.
Whatever they be, the obstinacy with which they resist decomposition
shows that they are ingredients of a very high and primary importance in
the economy of nature; and such as, in any state of science, it would be
indispensably necessary to be perfectly familiar with. Like particular
theorems in geometry, which, though not rising to the highest point of
generality, have yet their several scopes and ranges of extensive application,
they must be well and perfectly understood in all their bearings. Should we

ever arrive at an analysis of these bodies, the chemical properties of the new
elements which will then come into view will be known only by our
knowledge of these, or of other compounds of the same class, which they
may be capable of forming. Not but that such an analysis would be a most
important and indeed triumphant achievement, and change the face of
chemistry; but it would undo nothing that has been done, and render useless
no point of knowledge which we have yet arrived at.
(339.) The atomic theory, or the law of definite proportions, which is
the same thing presented in a form divested of all hypothesis, after the laws
of mechanics, is, perhaps, the most important which the study of nature has
yet disclosed. The extreme simplicity which characterizes it, and which is
itself an indication, not unequivocal, of its elevated rank in the scale of
physical truths, had the effect of causing it to be announced at once by Mr.
53
Dalton, in its most general terms, on the contemplation of a few instances ,
without passing through subordinate stages of painful inductive ascent by
the intermedium of subordinate laws, such as, had the contrary course been
pursued by him, would have been naturally preparatory to it, and such as
would have led others to it by the prosecution of Wenzel’s and Richter’s
researches, had they been duly attended to. This is, in fact, an example, and
a most remarkable one, of the effect of that natural propensity to generalize
and simplify (noticed in 171.), which, if it occasionally leads to over-hasty
conclusions, limited or disproved by further experience, is yet the legitimate
parent of many of our most valuable and soundest results. Instances like
this, where great and, indeed, immeasurable steps in our knowledge of
nature are made at once, and almost without intellectual effort, are well
calculated to raise our hopes of the future progress of science, and, by
pointing out the simplest and most obvious combinations as those which are
actually found to be agreeable to the harmony of creation, to hold out the
cheering prospect of difficulties diminishing as we advance, instead of
thickening around us in increasing complexity.
(340.) A consequence of this immediate presentation of the law of
definite proportions in its most general form is, that its subordinate laws—
those which limit its generality in particular cases, which diminish the
number of combinations abstractly possible, and restrain the indiscriminate
mixture of elements,—remain to be discovered. Some such limitations
have, in fact, been traced to a certain extent, but by no means so far as the

importance of the subject requires; and we have here abundant occupation
for chemists for some time.
(341.) The determination of the atomic weights of the chemical
elements, like that of other standard physical data, with the utmost
exactness, is in itself a branch of enquiry not only of the greatest
importance, but of extreme difficulty. Independent of the general reasons
for desiring accuracy in this respect, there is one peculiar to the subject. It
has been suggested (by Dr. Prout), and strongly insisted on (by Dr.
Thomson), that all the numbers representing these weights, constituting a
scale of great extent, in which the extremes already known are in proportion
to each other, as 1 to upwards of 200, are simple even multiples of the least
of them. If this be really the case, it opens views of such importance as to
justify any degree of labour and pains in the verification of the law as a
purely inductive one. But in the actual state of chemical analysis, with all
deference to such high authority, we confess it appears to us to stand in
great need of further confirmation, since it seems doubtful whether such
accuracy has yet been attained as to enable us to answer positively for a
fraction not exceeding the three or four hundredth part of the whole
quantity to be determined: at least the results of the first experimenters,
obtained with the greatest care, differ often by a greater amount; and this
degree of exactness, at least, would be required to verify the law
satisfactorily in the higher parts of the scale.
(342.) The mere agitation of such a question, however, points out a
class of phenomena in physical science of a remote and singular kind, and
of a very high and refined order, which could never become known but in
an advanced state of science, not only practical, but theoretical,—we mean,
such as consist in observed relations among the data of physics, which
show them to be quantities not arbitrarily assumed, but depending on laws
and causes which they may be the means of at length disclosing. A
remarkable instance of such a relation is the curious law which Bode
observed to obtain in the progression of the magnitudes of the several
planetary orbits. This law was interrupted between Mars and Jupiter, so as
to induce him to consider a planet as wanting in that interval;—a deficiency
long afterwards strangely supplied by the discovery of four new planets in
that very interval, all of whose orbits conform in dimension to the law in

question, within such moderate limits of error as may be due to causes
54
independent of those on which the law itself ultimately rests.
(343.) Neither is it irrelevant to our subject to remark, that the progress
which has been made in this department of chemistry, and the considerable
exactness actually attainable in chemical analysis, have been owing, in
great measure, to a circumstance which might at first have been hardly
considered likely to exercise much influence on the progress of a science,—
the discovery of platina. Without the resources placed at the ready disposal
of chemists by this invaluable metal, it is difficult to conceive that the
multitude of delicate analytical experiments which have been required to
construct the fabric of existing knowledge could have ever been performed.
This, among many such lessons, will teach us that the most important uses
of natural objects are not those which offer themselves to us most
obviously. The chief use of the moon for man’s immediate purposes
remained unknown to him for five thousand years from his creation. And,
since it cannot but be that innumerable and most important uses remain to
be discovered among the materials and objects already known to us, as well
as among those which the progress of science must hereafter disclose, we
may hence conceive a well-grounded expectation, not only of constant
increase in the physical resources of mankind, and the consequent
improvement of their condition, but of continual accessions to our power of
penetrating into the arcana of nature, and becoming acquainted with her
highest laws.

CHAP. V.
OF THE IMPONDERABLE FORMS OF MATTER.

Heat.

(344.) One of the chief agents in chemistry, on whose proper application
and management the success of a great number of its enquiries depends, and
many of whose most important laws are disclosed to us by phenomena of a
chemical nature, is HEAT. Although some of its effects are continually before
our eyes as matters of the most common occurrence, insomuch that there is
scarcely any process in the useful arts and manufactures which does not call
for its intervention, and although, independent of this high utility, and the
proportionate importance of a knowledge of its nature and laws, it presents
in itself a subject of the most curious speculation; yet there is scarcely any
physical agent of which we have so imperfect a knowledge, whose intimate
nature is more hidden, or whose laws are of such delicate and difficult
investigation.
(345.) The word heat generally implies the sensation which we
experience on approaching a fire; but, in the sense it carries in physics, it
denotes the cause, whatever it be, of that sensation, and of all the other
phenomena which arise on the application of fire, or of any other heating
cause. We should be greatly deceived if we referred only to sensation as an
indication of the presence of this cause. Many of those things which excite
in our organs, and especially of those of taste, a sensation of heat, owe this
property to chemical stimulants, and not at all to their being actually hot.
This error of judgment has produced a corresponding confusion of
55
language, and hence had actually at one period crept into physical
philosophy a great many illogical and absurd conclusions. Again, there are a

number of chemical agents, which, from their corroding, blackening, and
dissolving, or drying up the parts of some descriptions of bodies, and
producing on them effects not generally unlike (though intrinsically very
different from) those produced by heat, are said, in loose and vulgar
language, to burn them; and this error has even become rooted into a
prejudice, by the fact that some of these agents are capable of becoming
actually and truly hot during their action on moist substances, by reason of
their combination with the water the latter contain. Thus, quicklime and oil
of vitriol both exercise a powerful corrosive action on animal and vegetable
substances, and both become violently hot by their combination with water.
They are, therefore, set down in vulgar parlance as substances of a hot
nature; whereas, in their relations to the physical cause of heat, they agree
with the generality of bodies similarly constituted.
(346.) The nature of heat has hitherto been chiefly studied under the
general heads of—

1st, Its sources, or the phenomena which it usually accompanies.
2d, Its communication from its sources to substances capable of
receiving it, and from these to others, with a view to
discover the laws which regulate its distribution through
space or through the bodies which occupy it.
3d, Its effects, on our senses, and on the bodies to which it is
communicated in its various degrees of intensity, by which,
means are afforded us of measuring these degrees.
4th, Its intimate relations to the atoms of matter, as exhibited in its
capability of acquiring a latent state under certain
circumstances, and of entering into something like chemical
combinations.

(347.) The most obvious sources of heat are, the sun, fire, animal life,
fermentations, violent chemical actions of all kinds, friction, percussion,
lightning, or the electric discharge, in whatever manner produced, the
sudden condensation of air, and others, so numerous, and so varied, as to
show the extensive and important part it has to perform in the economy of
nature. The discoveries of chemists, however, have referred most of these to
the general head of chemical combination. Thus, fire, or the combustion of

inflammable bodies, is nothing more than a violent chemical action
attending the combination of their ingredients with the oxygen of the air.
Animal heat is, in like manner, referable to a process bearing no remote
analogy to a slow combustion, by which a portion of carbon, an
inflammable principle existing in the blood, is united with the oxygen of the
air in respiration; and thus carried off from the system: fermentation is
nothing more than a decomposition of chemical elements loosely united,
and their re-union in a more permanent state of combination. The analogy
between the sun and terrestrial fire is so natural as to have been chosen by
Newton to exemplify the irresistible force of an inference derived from that
principle. But the nature of the sun and the mode in which its wonderful
supply of light and heat is maintained are involved in a mystery which
every discovery that has been made either in chemistry or optics, so far
from elucidating, seems only to render more profound. Friction as a source
of heat is well known: we rub our hands to warm them, and we grease the
axles of carriage-wheels to prevent their setting fire to the wood; an
accident which, in spite of this precaution, does sometimes happen. But the
effect of friction, as a means of producing heat with little or no consumption
of materials, was not fully understood till made the subject of direct
experiment by count Rumford, whose results appear to have established the
extraordinary fact, that an unlimited supply of heat may be derived by
friction from the same materials. Condensation, whether of air by pressure,
or of metals by percussion, is another powerful source of heat. Thus, iron
may be so dexterously hammered as to become red-hot, and the rapid
condensation of a confined portion of air will set tinder on fire.
(348.) The most violent heats known are produced by the concentration
of the solar rays by burning glasses,—by the combustion of oxygen and
hydrogen gases mixed in the exact proportion in which they combine to
produce water,—and by the discharge of a continued and copious current of
electricity through a small conductor. As these three sources of heat are
independent of each other, and each capable of being brought into action in
a very confined space, there seems no reason why they might not all three
be applied at once at the same point, by which means, probably, effects
would be produced infinitely surpassing any hitherto witnessed.
(349.) Heat is communicated either by radiation between bodies at a
distance, or by conduction between bodies in contact, or between the

contiguous parts of one and the same body. The laws of the radiation of heat
have been studied with great attention, and have been found to present
strong analogies with that of light in some points, and singular differences
in others. Thus, the heat which accompanies the sun’s rays comports itself,
in all respects, like light; being subject to similar laws of reflection,
refraction, and even of polarization, as has been shown by Berard. Yet they
are not identical with each other; Sir William Herschel having shown, by
decisive experiments, verified by those of Sir H. Englefield, that there exist
in a solar beam both rays of heat which are not luminous, and rays of light
which have no heating power.
(350.) The heat, radiated by terrestrial fires, and by bodies obscurely
hot, by whatever means they have acquired their heat (even by exposure to
the sun’s rays), differs very materially from solar heat in their power of
penetrating transparent substances. This singular and important difference
was first noticed by Mariotte, and afterwards made the subject of many
curious and interesting experiments by Scheele, who found that terrestrial
heat, or that radiated from fires or heated bodies, is intercepted and detained
by glass or other transparent bodies, while solar heat is not; and that, being
so detained, it heats them: which the latter, as it passes freely through them,
is incapable of doing. The more recent researches of Delaroche, however,
have shown that this detention is complete only when the temperature of the
source of heat is low; but that, as that temperature is higher, a portion of the
heat radiated acquires a power of penetrating glass; and that the quantity
which does so bears continually a larger and larger proportion to the whole,
as the heat of the radiant body is more intense. This discovery is very
important, as it establishes a community of nature between solar and
terrestrial heat; while at the same time it leads us to regard the actual
temperature of the sun as far exceeding that of any earthly flame.
(351.) A variety of theories have been framed to account for these
curious phenomena; but the subject stands rather in need of further
elucidation from experiment, and is one which merits, and will probably
amply repay, the labours of those who may hereafter devote their attention
to it. The theory of the radiation of heat, in general, which seems to agree
best with the known phenomena, is that of M. Prevost, who considers all
bodies as constantly radiating out heat in all directions, and receiving it by a
similar means of communication from others, and thus tending, in any

space filled, wholly or in part, with bodies at various temperatures, to
establish an equilibrium or equality of heat in all parts. The application of
this idea to the explanation of the phenomenon of dew we have already seen
(see 167.). The laws of such radiation, under various circumstances, have
been lately investigated in a beautiful series of experiments on the cooling
of bodies by their own radiation in vacuo, by Messrs. Dulong and Petit,
which offer some of the best examples in science of the inductive
investigation of quantitative laws.
(352.) The communication of heat between bodies in contact, or
between the different parts of the same body, is performed by a process
called conduction. It is, in fact, only a particular case of radiation, as has
been explained above (217.); but a case so particular as to require a separate
and independent investigation of its laws. The most important consideration
introduced into the enquiry by this peculiarity is that of time. The
communication of heat by conduction is performed, for the most part, with
extreme slowness, while that performed by direct radiation is probably not
less rapid than the propagation of light itself. The analysis of the delicate
and difficult points which arise in the investigation of this subject in its
reduction to direct geometrical treatment has been executed with admirable
success by the late Baron Fourrier, whose recent lamented death has
deprived science of an ornament it could ill spare, thinned as its ranks have
been within the last few years. This acute philosopher and profound
mathematician has developed, in a series of elaborate memoirs presented to
the French Institute, the laws of the communication of heat through the
interior of solid masses, placed under the influence of any external heating
and cooling causes, and has in particular applied his results to the
conditions on which the maintenance of the actual observed temperature on
the earth’s surface depends; to the possible influence of a supposed central
heat on our climates; and to the determination of the actual amount of the
heat, derived to us from the sun, or at least that portion of it on which the
difference of the seasons depends.
(353.) The principal effects of heat are the sensations of warmth or cold
consequent on its entry or egress into or out of our bodies; the dilatation it
causes in the dimensions of all substances in which it is accumulated; the
changes of state it produces in the melting of solids, and the conversion of
them and of liquids into vapour; and the chemical changes it performs by

actual decompositions effected in the intimate molecules of various
substances, especially those of which vegetables and animals are composed;
to which we may add, the production of electric phenomena under certain
circumstances in the contact of metals, and the developement of electric
polarity in crystallised substances.
(354.) Cold has been considered by some as a positive quality, the
effect of a cause antagonist to that of heat; but this idea seems now (with
perhaps a single exception) to be universally abandoned. The sensation of
cold is as easily explicable by the passage of heat outwards through the
surface of the body as that of heat by its ingress from without; and the
experiments cited in proof of a radiation of cold are all perfectly explained
by Prevost’s theory of reciprocal interchange. It is remarkable, however,
how very limited our means of producing intense cold are, compared with
those we possess of effecting the accumulation of heat in bodies. This is one
of the strongest arguments adducible in favour of the doctrines of those who
maintain the possibility of exhausting the heat of a body altogether, and
leaving it in a state absolutely devoid of it. But we ought to consider, that
the known methods of generating heat chiefly turn on the production of
chemical combinations: we may easily conceive, therefore, that, to obtain
equally powerful corresponding frigorific effects, we ought to possess the
means of effecting a disunion equally extensive and rapid between such
elements, actually combined, as have already produced heat by their union.
This, however, we can only accomplish by engaging them in combinations
still more energetic, that is to say, in which we may reasonably expect more
heat to be produced by the new combination than would be destroyed or
abstracted by the proposed decomposition. Chemistry, however, (unaided
by electric agency,) affords no means of suddenly breaking the union of two
elements, and presenting both in an uncombined state. A certain analogy to
such disunion, however, and its consequences, may be traced in the sudden
expansion of condensed gases from a liquid state into vapour, which is the
most powerful source of cold known.
(355.) The dilatation of bodies by heat forms the subject of that branch
of science called pyrometry. There is no body but is capable of being
penetrated by heat, though some with greater, others with less rapidity; and
being so penetrated, all bodies (with a very few exceptions, and those
depending on very peculiar circumstances,) are dilated by it in bulk, though

with a great diversity in the amount of dilatation produced by the same
degree of heat. Of the several forms of natural bodies, gases and vapours
are observed to be most dilatable; liquids next, and solids least of all. The
dilatation of solids has been made a subject of repeated and careful
measurement by several experimenters; among whom, Smeaton, Lavoisier,
and Laplace, are the principal. The remarkable discovery of the unequal
dilatation of crystallised bodies by Mitscherlich has already been spoken of.
(266.) That of gases and vapours was examined about the same time by
Dalton and Gay-Lussac, who both arrived independently at the conclusion
of an equal dilatability subsisting in them all, which constitutes one of the
most remarkable points in their history.
(356.) The dilatation of air by heat affords, perhaps, the most
unexceptionable means known of measuring degrees of heat. The
thermometer, as originally constructed by Cornelius Drebell, was an air
thermometer. Those now in common use measure accessions of heat not by
the degree of dilatation of air but of mercury. It has been shown, by the
researches of Dulong and Petit, that its indications coincide exactly with
that of the air-thermometer in moderate temperatures; though at very
elevated ones they exhibit a sensible, and even considerable, deviation. By
this instrument, which owes its present convenience and utility to the happy
idea of Newton, who first thought of fixing determinate points on its scale,
we are enabled to estimate, or at least identify, the degrees of heat; and
thereby to investigate with accuracy the laws of its communication and its
other properties. Were we sure that equal additions of heat produced equal
increments of dimension in any substance, the indications of a thermometer
would afford a true and secure measure of the quantity present; but this is
so far from being the case, that we are nearly in total ignorance on this
important point; a circumstance which throws the greatest difficulty in the
way of all theoretical reasoning, and even of experimental enquiry. The
laws of the dilatation of liquids, in consequence of this deficiency of
necessary preliminary knowledge, are still involved in great obscurity,
notwithstanding the pains which have been bestowed on them by the
elaborate experiments and calculations of Gilpin, Blagden, Deluc, Dalton,
Gay-Lussac, and Biot.
(357.) The most striking and important of the effects of heat consist,
however, in the liquefaction of solid substances, and the conversion of the

liquids so produced into vapour. There is no solid substance known which,
by a sufficiently intense heat, may not be melted, and finally dissipated in
vapour; and this analogy is so extensive and cogent, that we cannot but
suppose that all those bodies which are liquid under ordinary circumstances,
owe their liquidity to heat, and would freeze or become solid if their heat
could be sufficiently reduced. In many we see this to be the case in ordinary
winters; for some, severe frosts are requisite; others freeze only with the
most intense artificial colds; and some have hitherto resisted all our
endeavours; yet the number of these last is few, and they will probably
cease to be exceptions as our means of producing cold become enlarged.
(358.) A similar analogy leads us to conclude that all aëriform fluids are
merely liquids kept in the state of vapour by heat. Many of them have been
actually condensed into the liquid state by cold accompanied with violent
pressure; and as our means of applying these causes of condensation have
improved, more and more refractory ones have successively yielded. Hence
we are fairly entitled to extend our conclusion to those which we have not
yet been able to succeed with; and thus we are led to regard it as a general
fact, that the liquid and aëriform or vaporous states are entirely dependent
on heat; that were it not for this cause, there would be nothing but solids in
nature; and that, on the other hand, nothing but a sufficient intensity of heat
is requisite to destroy the cohesion of every substance, and reduce all
bodies, first to liquids, and then into vapour.
(359.) But solids, themselves, by the abstraction of heat shrink in
dimension, and at the same time become harder, and more brittle; yielding
less to pressure, and permitting less separation between their parts by
tension. These facts, coupled with the greater compressibility of liquids, and
the still greater of gases, strongly induce us to believe that it is heat, and
heat alone, which holds the particles of all bodies at that distance from each
other which is necessary to allow of compression; which in fact gives them
their elasticity, and acts as the antagonist force to their mutual attraction,
which would otherwise draw them into actual contact, and retain them in a
state of absolute immobility and impenetrability. Thus we learn to regard
heat as one of the great maintaining powers of the universe, and to attach to
all its laws and relations a degree of importance which may justly entitle
them to the most assiduous enquiry.

(360.) It was first ascertained by Dr. Black that when heat produces the
liquefaction of a solid, or the conversion of a liquid into vapour, the liquid
or the vapour resulting is no hotter than the solid or liquid from which it
was produced, though a great deal of heat has been expended in producing
this effect, and has actually entered into the substance.
(361.) Hence he drew the conclusion that it has become latent, and
continues to exist in the product, maintaining it in its new state, without
increasing its temperature. He further proved, that when the vapour
condenses, or the liquid freezes, this latent heat is again given out from it.
This great discovery, with its natural and hardly less important concomitant,
that of the difference of specific heats in different bodies, or the different
quantities of heat they require to raise their temperature equally, are the
chief reasons for regarding heat as a material substance in a more decided
manner than light, with which in its radiant state it holds so close an
analogy.
(362.) The subject of latent heat has been far less attentively studied
than its great practical importance would appear to demand, when we
consider that it is to this part of physical science that the theory of the
steam-engine is mainly referable, and that material improvements may not
unreasonably be expected in that wonderful instrument, from a more
extended knowledge than we possess of the latent heats of different
vapours. This is not the case, however, with the subject of specific heat,
which was followed up immediately after its first promulgation with
diligence by Irvine; and, after a brief interval, by Lavoisier and Laplace, as
well as by our countryman Crawfurd, who determined the specific heats of
many substances, both solid and liquid. After a considerable period of
inactivity, the subject was again resumed by Delaroche and Berard, and
subsequently by Dulong and Petit: the result of whose investigations has
been the inductive establishment of one of those simple and elegant
physical laws which carry with them, if not their own evidence, at least
their own recommendation to our belief, as being in unison with every thing
we know of the harmony of nature. The law to which we allude is this:—
that the atoms of all the simple chemical elements have exactly the same
capacity for heat, or are all equally heated or cooled by equal accessions or
abstractions of heat. It is only among laws like this that we can expect to

find a clew capable of guiding us to a knowledge of the true nature of heat,
and its relations to ponderable matter.

Magnetism and Electricity.

(363.) These two subjects, which had long maintained a distinct
existence, and been studied as separate branches of science, are at length
effectually blended. This is, perhaps, the most satisfactory result which the
experimental sciences have ever yet attained. All the phenomena of
magnetic polarity, attraction, and repulsion, have at length been resolved
into one general fact, that two currents of electricity, moving in the same
direction repel, and in contrary directions attract, each other. The
phenomena of the communication of magnetism and what is called its
induced state, alone remain unaccounted for; but the interesting theory
which has been developed by M. Ampere, under the name of Electro-
dynamics, holds out a hope that this difficulty will also in its turn give way,
and the whole subject be at length completely merged, as far as the
consideration of the acting causes goes, in the more general one of
electricity. This, however, does not prevent magnetism from maintaining its
separate importance as a department of physical enquiry, having its own
peculiar laws and relations of the highest practical interest, which are
capable of being studied quite apart from all consideration of its electrical
origin. And not only so, but to study them with advantage, we must proceed
as if that origin were totally unknown, and, at least up to a certain point, and
that a considerably advanced one, conduct our enquiries into the subject on
the same inductive principles as if this branch of physics were absolutely
independent of all others.
(364.) Iron, and its oxides and alloys, were for a long time the only
substances considered susceptible of magnetism. The loadstone was even
one of the examples produced by Bacon of that class of physical instances
to which he applies the term “Instantiæ monodicæ”—singular instances.
And the history of magnetism affords a beautiful comment on his remark on
instances of this sort. “Nor should our enquiries,” he observes, “into their
nature be broken off, till the properties and qualities found in such things as
may be esteemed wonders in nature are reduced and comprehended under

some certain law; so that all irregularity or singularity may be found to
depend upon some common form, and the wonder only rest in the exact
differences, degrees, or extraordinary concurrence, and not in the species
itself.” The discovery of the magnetism of nickel, which though inferior to
that of iron, is still considerable; that of cobalt, yet feebler, and that of
titanium, which is only barely perceptible, have effectually broken down the
imaginary limit between iron and the other materials of the world, and
established the existence of that general law of continuity which it is one
chief business of philosophy to trace throughout nature. The more recent
discoveries of M. Arago (mentioned in 160.) have completed this
generalization, by showing that there is no substance but which, under
proper circumstances, is capable of exhibiting unequivocal signs of the
magnetic virtue. And to obliterate all traces of that line of separation which
was once so broad, we are now enabled, by the great discovery of Oërsted,
to communicate at and during pleasure to a coiled wire of any metal
indifferently all the properties of a magnet;—its attraction, repulsion, and
polarity; and that even in a more intense degree than was previously
thought to be possible in the best natural magnets. In short, in this case, and
in this case only, perhaps, in science, have we arrived at that point which
Bacon seems to have understood by the discovery of “forms.” “The form of
any nature,” says he, “is such, that where it is, the given nature must
infallibly be. The form, therefore, is perpetually present when that nature is
present; ascertains it universally, and accompanies it every where. Again,
this form is such, that when removed, the given nature infallibly vanishes.
Lastly, a true form is such as can deduce a given nature from some essential
property, which resides in many things.”
(365.) Magnetism is remarkable in another important point of view. It
offers a prominent, or “glaring instance” of that quality in nature which is
termed polarity (267.), and that under circumstances which peculiarly adapt
it for the study of this quality. It does not appear that the ancients had any
knowledge of this property of the magnet, though its attraction of iron was
well known to them. The first mention of it in modern times cannot be
traced earlier than 1180, though it was probably known to the Chinese
before that time. The polarity of the magnet consists in this, that if
suspended freely, one part of it will invariably direct itself towards a certain
point in the horizon, the other towards the opposite point; and that, if two
magnets, so suspended, be brought near each other, there will take place a

mutual action, in consequence of which, the positions of both will be
disturbed, in the same manner as would happen if the corresponding parts
of each repelled, and those oppositely directed attracted, each other; and by
properly varying the experiment, it is found that they really do so. If a small
magnet, freely suspended, be brought into the neighbourhood of a larger
one, it will take a position depending on the position of the poles of the
larger one, with respect to its point of suspension. And it has been
ascertained that these and all other phenomena exhibited by magnets in
their mutual attractions and repulsions are explicable on the supposition of
two forces or virtues lodged in the particles of the magnets, the one
predominating at one end, the other at the other; and such that each particle
shall attract those in which the opposite virtue to its own prevails, and repel
those in which a similar one resides with a force proportional to the inverse
square of their mutual distance.
(366.) The direction in which a magnetic bar, or needle of steel, freely
suspended, places itself, has been ascertained to be different at different
points of the earth’s surface. In some places it points exactly north and
south, in others it deviates from this direction more or less, and at some
actually stands at right angles to it. This remarkable phenomenon, which is
called the variation of the needle, and which was discovered by Sebastian
Cabot in the year 1500, is accompanied with another called the dip, noticed
by Robert Norman in 1576. It consists in a tendency of a needle, nicely
balanced on its centre, when unmagnetized, to dip or point downwards
when rendered magnetic, towards a point below the horizon, and situated
within the earth. By tracing the variation and dip over the whole surface of
the globe, it has been found that these phenomena take place as they would
do if the earth itself were a great magnet, having its poles deeply situated
below the surface,—and, what is very remarkable, possessing a slow
motion within it, in consequence of which neither the variation nor dip
remain constantly the same at the same place. The laws of this motion are at
present unknown; but the discovery of electro-magnetism, by rendering it
almost certain that the earth’s magnetism is merely an effect of the
continual circulation of great quantities of electricity round it, in a direction
generally corresponding with that of its rotation, have dissipated the greater
part of the mystery which hung over these phenomena; since a variety of
causes, both geological and others, may be imagined which may produce
considerable deviations in the intensity, and partial ones in the direction, of

such electric currents. The unequal distribution of land and sea in the two
hemispheres, by affecting the operation of the sun’s heat in producing
evaporation from the latter, which is probably one of the great sources of
terrestrial electricity, may easily be conceived to modify the general
tendency of such currents, and to produce irregularities in them, which may
render a satisfactory account of whatever still appears anomalous in the
phenomena of terrestrial magnetism. This branch of science thus becomes
connected, on a great scale, with that of meteorology, one of the most
complicated and difficult, but at the same time interesting, subjects of
physical research; one, however, which has of late begun to be studied with
a diligence which promises the speedy disclosure of relations and laws of
which at present we can form but a very imperfect notion.
(367.) The communication of magnetism from the earth to a magnetic
body, or from one magnetic body to another, is performed by a process to
which the name of induction has been given, and the laws and properties of
such induced magnetism have been studied with much perseverance and
success,—practically, by Gilbert, Boyle, Knight, Whiston, Cavallo, Canton,
Duhamel, Rittenhouse, Scoresby, and others; and theoretically, by Æpinus,
Coulomb, and Poisson, and in our own country by Messrs. Barlow and
Christie, who have investigated with great care the curious phenomena
which take place when masses of iron are presented successively, in
different positions, by rotation on an axis, to the influence of the earth’s
magnetism. The magnetism of crystallized bodies (partly from the extreme
rarity of such as are susceptible of any considerable magnetic virtue) has
not hitherto been at all examined, but would probably afford very curious
results.
(368.) To electricity the views of the physical enquirer now turn from
almost every quarter, as to one of those universal powers which Nature
seems to employ in her most important and secret operations. This
wonderful agent, which we see in intense activity in lightning, and in a
feebler and more diffused form traversing the upper regions of the
atmosphere in the northern lights, is present, probably in immense
abundance, in every form of matter which surrounds us, but becomes
sensible only when disturbed by excitements of peculiar kinds. The most
effectual of these is friction, which we have already observed to be a
powerful source of heat. Everybody is familiar with the crackling sparks

which fly from a cat’s back when stroked. These, by proper management,
may be accumulated in bodies suitably disposed to receive them, and,
although then no longer visible, give evidence of their existence by the
exhibition of a vast variety of extraordinary phenomena,—producing
attractions and repulsions in bodies at a distance,—admitting of being
transferred by contact, or by sudden and violent transilience of the interval
of separation, from one body to another, under the form of sparks and
flashes;—traversing with perfect facility the substance of the densest
metals, and a variety of other bodies called conductors, but being detained
by others, such as glass, and especially air, which are thence called non-
conductors,—producing painful shocks and convulsive motions, and even
death itself if in sufficient quantity, in animals through which they pass, and
finally imitating, on a small scale, all the effects of lightning.
(369.) The study of these phenomena and their laws until a
comparatively recent period occupied the entire attention of electricians,
and constituted the whole of the science of electricity. It appears, as the
result of their enquiries, that all the phenomena in question are explicable
on the supposition that electricity consists in a rare, subtle, and highly
elastic fluid, which in its tendency to expand and diffuse itself pervades
with more or less facility the substance of conductors, but is obstructed and
detained from expansion more or less completely by non-conductors. It is
supposed, moreover, that this electric fluid possesses a power of attraction
for the particles of all ponderable matter, together with that of a repulsion
for particles of its own kind. Whether it has weight, or is rather to be
regarded as a species of matter distinct from that of which ponderable
bodies consist, is a question of such delicacy, that no direct experiments
have yet enabled us to decide it; but at all events its inertia compared with
its elastic force must be conceived excessively small, so that it is to be
regarded as a fluid in the highest degree active, obeying every impulse,
internal or external, with the greatest promptitude; in short, a fluid whose
energies can only be compared with those of the ethereal medium by which,
in the undulatory doctrine, light is supposed to be conveyed. The properties
of hydrogen gas compared with those of the denser aëriform fluids will, in
some slight degree, aid our conception of the excessive mobility and
penetrating activity of a fluid so constituted. Electricity, however, must be
regarded as differing in some remarkable points from all those fluids to
which we have hitherto been accustomed to apply the epithet elastic, such

as air, gases, and vapours. In these, the repulsive force of the particles on
which their elasticity depends is considered as extending only to very small
distances, so as to affect only those in the immediate vicinity of each other,
while their attractive power, by which they obey the general gravitation of
all matter, extends to any distance. In electricity, on the other hand, the very
reverse must be admitted. The force by which its particles repel each other
extends to great distances, while its force of adhesion to ponderable matter
must be regarded as limited in its extent to such minute intervals as escape
observation.
(370.) The conception of a single fluid of this kind, which when
accumulated in excess in bodies tends constantly to escape, and seek a
restoration of equilibrium by communicating itself to any others where
there may be a deficiency, is that which occurs most naturally to the mind,
and was accordingly maintained by Franklin, to whom the science of
electricity is under great obligations for those decisive experiments which
informed us respecting the true nature of lightning. The same theory was
afterwards advocated by Æpinus, who first showed how the laws of
equilibrium of such a fluid might be reduced to strict mathematical
investigation. But there are phenomena accompanying its transfer from
body to body and the state of equilibrium it affects under various
circumstances, which appear to require the admission of two distinct fluids
antagonist to each other, each attracting the other, and repelling itself; but
each, alike, susceptible of adhesion to material substances, and of transfer
more or less rapid from particle to particle of them. These fluids in the
natural undisturbed state are conceived to exist in a state of combination
and mutual saturation; but this combination may be broken, and either of
them separately accumulated in a body to any amount without the other,
provided its escape be properly obstructed by surrounding it with non-
conductors. When so accumulated, its repulsion for its own kind and
attraction of the opposite species in neighbouring bodies tends to disturb the
natural equilibrium of the two fluids present in them, and to produce
phenomena of a peculiar description, which are termed induced electricity.
Curious and artificial as this theory may appear, there has hitherto been
produced no phenomenon of which it will not afford at least a plausible, and
in by far the majority of cases a very satisfactory, explanation. It has one
character which is extremely valuable in any theory, that of admitting the
application of strict mathematical reasoning to the conclusions we would

draw from it. Without this, indeed, it is scarcely possible that any theory
should ever be fairly brought to the test by a comparison with facts.
Accordingly, the mathematical theory of electrical equilibrium, and the laws
of the distribution of the electric fluids over the surfaces of bodies in which
they are accumulated, have been made the subject of elaborate geometrical
investigation by the most expert mathematicians, and have attained a degree
of extent and elegance which places this branch of science in a very high
rank in the scale of mathematico-physical enquiry. These researches are
grounded on the assumption of a law of attraction and repulsion similar to
those of gravity and magnetism, and which by the general accordance of the
results with facts, as well as by experiments instituted for the express
purpose of ascertaining the laws in question, are regarded as sufficiently
demonstrated.
(371.) The most obscure part of the subject is no doubt the original
mode of disturbance of electrical equilibrium, by which electricity is
excited in the first instance, either by friction or by any other of those
causes which have been ascertained to produce such an effect: analogies, it
56
is true, are not wanting ; but it must be allowed that hitherto nothing
decisive has been offered on the subject; and that conjectural modes of
action have in this instance too often usurped the place of those to which a
careful examination of facts alone can lead us.
(372.) Philosophers had long been familiar with the effects of
electricity above referred to, and with those which it produces in its sudden
and violent transfer from one body to another, in rending and shattering the
parts of the substances through which it passes, and where in great quantity,
producing all the effect of intense heat, igniting, fusing, and volatilizing
metals, and setting fire to inflammable bodies; even its occasional influence
in destroying or altering the polarity of the magnetic needle had been
noticed: but as heat was known to be produced by mechanical violence, and
as magnetism was also known to be greatly affected by the same cause,
these effects were referred rather to that cause than to any thing in the
peculiar nature of the electric matter, and regarded rather as an indirect
consequence of its mode of action than as connected with its intimate
nature. In short, electricity seemed destined to furnish another in addition to
many instances of subjects insulated from the rest of philosophy, and
capable of being studied only in its own internal relations, when the great

discoveries of Galvani and Volta placed a new power at the command of the
experimenter, by whose means those effects which had before been
crowded within an inappreciable instant could be developed in detail and
studied at leisure; and those forces which had previously exhibited
themselves only in a state of uncontrollable intensity were tamed down, as
it were, and made to distribute their efficacy over an indefinite time, and to
regulate their action at the will of the operator. It was then soon ascertained
that electricity in the act of its passage along conductors, produces a variety
of wonderful effects, which had never been previously suspected; and these
of such a nature, as to afford points of contact with several other branches
of physical enquiry, and to throw new and unexpected lights on some of the
most obscure operations of nature.
(373.) The history of this grand discovery affords a fine illustration of
the advantage to be derived in physical enquiry from a close and careful
attention to any phenomenon, however apparently trifling, which may at the
moment of observation appear inexplicable on received principles. The
convulsive motions of a dead frog in the neighbourhood of an electric
discharge, which originally drew Galvani’s attention to the subject, had
been noticed by others nearly a century before his time, but attracted no
further remark than as indicating a peculiar sensibility to electrical
excitement depending on that remnant of vitality which is not extinguished
in the organic frame of an animal by the deprivation of actual life. Galvani
was not so satisfied. He analysed the phenomenon; and in investigating all
the circumstances connected with it was led to the observation of a peculiar
electrical excitement which took place when a circuit was formed of three
distinct parts, a muscle, a nerve, and a metallic conductor, each placed in
contact with the other two, and which was manifested by a convulsive
motion produced in the muscle. To this phenomenon he gave the name of
animal electricity, an unfortunate epithet, since it tended to restrict enquiry
into its nature to the class of phenomena in which it first became apparent.
But this circumstance, which in a less enquiring age of science might have
exercised a fatal influence on the progress of knowledge, proved happily no
obstacle to the further developement of its principles, the subject being
immediately taken up with a kind of prophetic ardour by Volta, who at once
generalized the phenomena, rejecting the physiological considerations
introduced by Galvani, as foreign to the enquiry, and regarding the
contraction of the muscles as merely a delicate means of detecting the

production of electrical excitements too feeble to be rendered sensible by
any other means. It was thus that he arrived at the knowledge of a general
fact, that of the disturbance of electrical equilibrium by the mere contact of
different bodies, and the circulation of a current of electricity in one
constant direction, through a circuit composed of three different conductors.
To increase the intensity of the very minute and delicate effect thus
observed became his next aim, nor did his enquiry terminate till it had
placed him in possession of that most wonderful of all human inventions,
the pile which bears his name, through the medium of a series of well
conducted and logically combined experiments, which has rarely, if ever,
been surpassed in the annals of physical research.
(374.) Though the original pile of Volta was feeble compared to those
gigantic combinations which were afterwards produced, it sufficed,
however, to exhibit electricity under a very different aspect from any thing
which had gone before, and to bring into view those peculiar modifications
in its action which Dr. Wollaston was the first to render a satisfactory
account of, by referring them to an increase of quantity, accompanied with
a diminution of intensity in the supply afforded. The discovery had not long
been made public, and the instrument in the hands of chemists and
electricians, before it was ascertained that the electric current, transmitted
by it through conducting liquids, produces in them chemical
decompositions. This capital discovery appears to have been made, in the
first instance, by Messrs. Nicholson and Carlisle, who observed the
decomposition of water so produced. It was speedily followed up by the
still more important one of Berzelius and Hisinger, who ascertained it as a
general law, that, in all the decompositions so effected, the acids and
oxygen become transferred to, and accumulated around, the positive,—and
hydrogen, metals, and alkalies round the negative, pole of a Voltaic circuit;
being transferred in an invisible, and, as it were, a latent or torpid state, by
the action of the electric current, through considerable spaces, and even
through large quantities of water or other liquids, again to re-appear with all
their properties at their appropriate resting-places.
(375.) It was in this state of things that the subject was taken up by
Davy, who, seeing that the strongest chemical affinities were thus readily
subverted by the decomposing action of the pile, conceived the happy idea
of bringing to bear the intense power of the enormous batteries of the Royal

Institution on those substances which, though strongly suspected to be
compounds, had resisted all attempts to decompose them—the alkalies and
earths. They yielded to the force applied, and a total revolution was thus
effected in chemistry; not so much by the introduction of the new elements
thus brought to light, as by the mode of conceiving the nature of chemical
affinity, which from that time has been regarded (as Davy broadly laid it
down, in a theory which was readily adopted by the most eminent chemists,
and by none more readily than by Berzelius himself,) as entirely due to
electric attractions and repulsions, those bodies combining most intimately
whose particles are habitually in a state of the most powerful electrical
antagonism, and dispossessing each other, according to the amount of their
difference in this respect.
(376.) The connection of magnetism and electricity had long been
suspected, and innumerable fruitless trials had been made to determine, in
the affirmative or negative, the question of such connection. The
phenomena of many crystallized minerals which become electric by heat,
and develope opposite electric poles at their two extremities, offered an
analogy so striking to the polarity of the magnet, that it seemed hardly
possible to doubt a closer connection of the two powers. The developement
of a similar polarity in the Voltaic pile pointed strongly to the same
conclusion; and experiments had even been made with a view to ascertain
whether a pile in a state of excitement might not manifest a disposition to
place itself in the magnetic meridian; but the essential condition had been
omitted, that of allowing the pile to discharge itself freely, a condition
which assuredly never would have occurred of itself to any experimenter.
Of all the philosophers who had speculated on this subject, none had so
pertinaciously adhered to the idea of a necessary connection between the
phenomena as Oërsted. Baffled often, he returned to the attack; and his
perseverance was at length rewarded by the complete disclosure of the
wonderful phenomena of electro-magnetism. There is something in this
which reminds us of the obstinate adherence of Columbus to his notion of
the necessary existence of the New World; and the whole history of this
beautiful discovery may serve to teach us reliance on those general
analogies and parallels between great branches of science by which one
strongly reminds us of another, though no direct connection appears; as an
indication not to be neglected of a community of origin.