Fragments of Science_ A Series of Detached Essays_ Addresses_ and Reviews. V. 1-2 John Tyndall 1320 downloads.pdf

The Project Gutenberg eBook of Fragments of Science: A Series
of Detached Essays, Addresses, and Reviews. V. 1-2
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online at
www.gutenberg.org. If you are not located in the United States, you will
have to check the laws of the country where you are located before
using this eBook.

Title: Fragments of Science: A Series of Detached Essays, Addresses,
and Reviews. V. 1-2

Author: John Tyndall

Release date: February 6, 2008 [eBook #24527]
Most recently updated: July 22, 2017

Language: English

Other information and formats: www.gutenberg.org/ebooks/24527

Credits: Produced by Jon Richfield

*** START OF THE PROJECT GUTENBERG EBOOK FRAGMENTS OF
SCIENCE: A SERIES OF DETACHED ESSAYS, ADDRESSES, AND
REVIEWS. V. 1-2 ***

FRAGMENTS OF SCIENCE:
A Series of Detached

ESSAYS, ADDRESSES, AND REVIEWS.

BY

JOHN TYNDALL, F.R.S.
Printed By Spottiswoode AND CO.

NEW-STREET SQUARE

PARLIAMENT STREET

SIXTH EDITION,

VOL. 1.

LONDON: LONGMANS, GREEN, AND CO.

1879.

All rights reserved.

PREFACE TO THE SIXTH EDITION. *

VOL. I. INORGANIC NATURE *

I. THE CONSTITUTION OF NATURE. *

II. RADIATION. *

1. Visible and Invisible Radiation. *

2. Origin and Character of Radiation. The Aether. *

3. The Atomic Theory in reference to the Aether. *

4. Absorption of Radiant Heat by Gases. *

5. Formation of Invisible Foci. *

6. Visible and Invisible Rays of the Electric Light. *

Figure 1. Spectrum of Electric Light. *

7. Combustion by Invisible Rays. *

8. Transmutation of Rays: Calorescence. *

9. Deadness of the Optic Nerve to the Calorific Rays. *

10. Persistence of Rays. *

11. Absorption of Radiant Heat by Vapours and Odours. *

12. Aqueous Vapour in relation to the Terrestrial Temperatures. *

13. Liquids and their Vapours in relation to Radiant Heat. *

14. Reciprocity of Radiation and Absorption. *

15. Influence of Vibrating Period and Molecular Form. Physical Analysis of the Human Breath. *

16. Summary and Conclusion. *

III. ON RADIANT HEAT IN RELATION TO THE COLOUR AND CHEMICAL
CONSTITUTION OF BODIES. *

IV. NEW CHEMICAL REACTIONS PRODUCED BY LIGHT. *

1. DECOMPOSITION BY LIGHT. *

Physical Considerations. *

Production of Sky-blue by the Decomposition of Nitrite of Amyl. *

§ 2. ON THE BLUE COLOUR OF THE SKY, AND THE POLARISATION OF SKYLIGHT. *

§ 3. THE SKY OF THE ALPS. *

V. ON DUST AND DISEASE. *

Experiments on Dusty Air. *

The Germ Theory of Contagious Disease. *

Parasitic Diseases of Silkworms. Pasteur's Researches. *

Origin and Propagation of Contagious Matter. *

The Germ Theory applied to Surgery. *

The Luminous beam as a means of Research. *

The Floating Matter of the Air. *

Dr. Bennett's Experiments. *

Application of Luminous beams to Water. *

Chalk-water. Clark's Softening Process. *

Cotton-wool Respirator. *

Fireman's Respirator. *

Helmholtz on Hay Fever. *

VI. VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE. *

VII. NIAGARA. *

VIII. THE PARALLEL ROADS OF GLEN ROY. *

IX. ALPINE SCULPTURE. *

X. RECENT EXPERIMENTS ON FOG-SIGNALS. *

XI. ON THE STUDY OF PHYSICS. *

XII. ON CRYSTALLINE AND SLATY CLEAVAGE. *

XIII. ON PARAMAGNETIC AND DIAMAGNETIC FORCES. *

XIV. PHYSICAL BASIS OF SOLAR CHEMISTRY. *

XV. ELEMENTARY MAGNETISM. *

XVI. ON FORCE. *

XVII. CONTRIBUTIONS TO MOLECULAR PHYSICS. *

XVIII. LIFE, AND LETTERS OF FARADAY. *

XIX. THE COPLEY MEDALIST OF 1870. *

XX. THE COPLEY MEDALIST OF 1871. *

XXI. DEATH BY LIGHTNING. *

XXII. SCIENCE AND THE 'SPIRITS.' *

VOL. II. *

I. REFLECTIONS ON PRAYER AND NATURAL LAW. *

II. MIRACLES AND SPECIAL PROVIDENCES. *

ADDITIONAL REMARKS ON MIRACLES. *

III. ON PRAYER AS A FORM OF PHYSICAL ENERGY. *

IV. VITALITY. *

V. MATTER AND FORCE. *

VI. SCIENTIFIC MATERIALISM. *

VII. AN ADDRESS TO STUDENTS. *

VIII. SCIENTIFIC USE OF THE IMAGINATION. *

IX. THE BELFAST ADDRESS. *

X. APOLOGY FOR THE BELFAST ADDRESS. *

XI. THE REV. JAMES MARTINEAU AND THE BELFAST ADDRESS. *

XII. FERMENTATION, & ITS BEARINGS ON SURGERY & MEDICINE. *

XIII. SPONTANEOUS GENERATION. *

XIV. SCIENCE AND MAN. *

XV. PROFESSOR VIRCHOW AND EVOLUTION. *

XVI. THE ELECTRIC LIGHT. *

.

.

.

.

.

. PREFACE TO THE SIXTH EDITION.

TO AVOID unwieldiness of bulk this edition of the 'Fragments' is published
in two volumes, instead of, as heretofore, in one.

The first volume deals almost exclusively with the laws and phenomena of
matter. The second trenches upon questions in which the phenomena of
matter interlace more or less with those of mind.

New Essays have been added, while old ones have been revised, and in part
recast. To be clear, without being superficial, has been my aim throughout.

In neither volume have I aspired to sit in the seat of the scornful, but rather
to treat the questions touched upon with a tolerance, if not a reverence,

befitting their difficulty and weight.

Holding, as I do, the nebular hypothesis, I am logically bound to deduce the
life of the world from forces inherent in the nebula. With this view, which is
set forth in the second volume, it seemed but fair to associate the reasons
which cause me to conclude that every attempt made in our day to generate
life independently of antecedent life has utterly broken down.

A discourse on the Electric Light winds up the Second volume. The
incongruity of its position is to be referred to the lateness of its delivery.

.

.

.

--------------------

.

.

.

VOL. I. INORGANIC NATURE

I. THE CONSTITUTION OF NATURE.

[Footnote: 'Fortnightly Review,' 1865, vol. iii. p. 129.]

WE cannot think of space as finite, for wherever in imagination we erect a
boundary, we are compelled to think of space as existing beyond it. Thus by
the incessant dissolution of limits we arrive at a more or less adequate idea
of the infinity of space. But, though compelled to think of space as
unbounded, there is no mental necessity compelling us to think of it either as
filled or empty; whether it is so or not must be decided by experiment and
observation. That it is not entirely void, the starry heavens declare; but the
question still remains, Are the stars themselves hung in vacuo? Are the vast

regions which surround them, and across which their light is propagated,
absolutely empty? A century ago the answer to this question, founded on the
Newtonian theory, would have been, 'No, for particles of light are
incessantly shot through space.' The reply of modern science is also
negative, but on different grounds. It has the best possible reasons for
rejecting the idea of luminiferous particles; but, in support of the conclusion
that the celestial spaces are occupied by matter, it is able to offer proofs
almost as cogent as those which can be adduced of the existence of an
atmosphere round the earth. Men's minds, indeed, rose to a conception of the
celestial and universal atmosphere through the study of the terrestrial and
local one. From the phenomena of sound, as displayed in the air, they
ascended to the phenomena of light, as displayed in the aether; which is the
name given to the interstellar medium.

The notion of this medium must not be considered as a vague or fanciful
conception on the part of scientific men. Of its reality most of them are as
convinced as they are of the existence of the sun and moon. The
luminiferous aether has definite mechanical properties. It is almost infinitely
more attenuated than any known gas, but its properties are those of a solid
rather than of a gas. It resembles jelly rather than air. This was not the first
conception of the aether, but it is that forced upon us by a more complete
knowledge of its phenomena. A body thus constituted may have its
boundaries; but, although the aether may not be co-extensive with space, it
must at all events extend as far as the most distant visible stars. In fact it is
the vehicle of their light, and without it they could not be seen. This all-
pervading substance takes up their molecular tremors, and conveys them
with inconceivable rapidity to our organs of vision. It is the transported
shiver of bodies countless millions of miles distant, which translates itself in
human consciousness into the splendour of the firmament at night.

If the aether have a boundary, masses of ponderable matter might be
conceived to exist beyond it, but they could emit no light. Beyond the aether
dark suns might burn; there, under proper conditions, combustion might be
carried on; fuel might consume unseen, and metals be fused in invisible
fires. A body, moreover, once heated there, would continue for ever heated; a
sun or planet once molten, would continue for ever molten. For, the loss of
heat being simply the abstraction of molecular motion by the aether, where
this medium is absent no cooling could occur. A sentient being on

approaching a heated body in this region, would be conscious of no
augmentation of temperature. The gradations of warmth dependent on the
laws of radiation would not exist, and actual contact would first reveal the
heat of an extra ethereal sun.

Imagine a paddle-wheel placed in water and caused to rotate. From it, as a
centre, waves would issue in all directions, and a wader as he approached the
place of disturbance would be met by stronger and stronger waves. This
gradual augmentation of the impression made upon the wader is exactly
analogous to the augmentation of light when we approach a luminous
source. In the one case, however, the coarse common nerves of the body
suffice; for the other we must have the finer optic nerve. But suppose the
water withdrawn; the action at a distance would then cease, and, as far as the
sense of touch is concerned, the wader would be first rendered conscious of
the motion of the wheel by the blow of the paddles. The transference of
motion from the paddles to the water is mechanically similar to the
transference of molecular motion from the heated body to the aether; and the
propagation of waves through the liquid is mechanically similar to the
propagation of light and radiant heat.

As far as our knowledge of space extends, we are to conceive it as the holder
of the luminiferous aether, through which are interspersed, at enormous
distances apart, the ponderous nuclei of the stars. Associated with the star
that most concerns us we have a group of dark planetary masses revolving at
various distances round it, each again rotating on its own axis; and, finally,
associated with some of these planets we have dark bodies of minor note —
the moons. Whether the other fixed stars have similar planetary companions
or not is to us a matter of pure conjecture, which may or may not enter into
our conception of the universe. But probably every thoughtful person
believes, with regard to those distant suns, that there is, in space, something
besides our system on which they shine.

From this general view of the present condition of space, and of the bodies
contained in it, we pass to the enquiry whether things were so created at the
beginning. Was space furnished at once, by the fiat of Omnipotence, with
these burning orbs? In presence of the revelations of science this view is
fading more and more. Behind the orbs, we now discern the nebulae from
which they have been condensed. And without going so far back as the

nebulae, the man of science can prove that out of common non-luminous
matter this whole pomp of stars might have been evolved.

The law of gravitation enunciated by Newton is, that every particle of matter
in the universe attracts every other particle with a force which diminishes as
the square of the distance increases. Thus the sun and the earth mutually pull
each other; thus the earth and the moon are kept in company, the force which
holds every respective pair of masses together being the integrated force of
their component parts. Under the operation of this force a stone falls to the
ground and is warmed by the shock; under its operation meteors plunge into
our atmosphere and rise to incandescence. Showers of such meteors
doubtless fall incessantly upon the sun. Acted on by this force, the earth,
were it stopped in its orbit to-morrow, would rush towards, and finally
combine with, the sun. Heat would also be developed by this collision.
Mayer first, and Helmholtz and Thomson afterwards, have calculated its
amount. It would equal that produced by the combustion of more than 5,000
worlds of solid coal, all this heat being generated at the instant of collision.
In the attraction of gravity, therefore, acting upon non-luminous matter, we
have a source of heat more powerful than could be derived from any
terrestrial combustion. And were the matter of the universe thrown in cold
detached fragments into space, and there abandoned to the mutual
gravitation of its own parts, the collision of the fragments would in the end
produce the fires of the stars.

The action of gravity upon matter originally cold may, in fact, be the origin
of all light and heat, and also the proximate source of such other powers as
are generated by light and heat. But we have now to enquire what is the light
and what is the heat thus produced? This question has already been answered
in a general way. Both light and heat are modes of motion. Two planets clash
and come to rest; their motion, considered as that of masses, is destroyed,
but it is in great part continued as a motion of their ultimate particles. It is
this latter motion, taken up by the aether, and propagated through it with a
velocity of 186,000 miles a second, that comes to us as the light and heat of
suns and stars. The atoms of a hot body swing with inconceivable rapidity —
billions of times in a second — but this power of vibration necessarily
implies the operation of forces between the atoms themselves. It reveals to
us that while they are held together by one force, they are kept asunder by
another, their position at any moment depending on the equilibrium of

attraction and repulsion. The atoms behave as if connected by elastic
springs, which oppose at the same time their approach and their retreat, but
which tolerate the vibration called heat. The molecular vibration once set up
is instantly shared with the aether, and diffused by it throughout space.

We on the earth's surface live night and day in the midst of aethereal
commotion. The medium is never still. The cloud canopy above us may be
thick enough to shut out the light of the stars; but this canopy is itself a warm
body, which radiates its thermal motion through the aether. The earth also is
warm, and sends its heat-pulses incessantly forth. It is the waste of its
molecular motion in space that chills the earth upon a clear night; it is the
return of thermal motion from the clouds which prevents the earth's
temperature, on a cloudy night, from falling so low. To the conception of
space being filled, we must therefore add the conception of its being in a
state of incessant tremor.

The sources of this vibration are the ponderable masses of the universe. Let
us take a sample of these and examine it in detail. When we look to our
planet, we find it to be an aggregate of solids, liquids, and gases. Subjected
to a sufficiently low temperature, the two last, would also assume the solid
form. When we look at any one of these, we generally find it composed of
still more elementary parts. We learn, for example, that the water of our
rivers is formed by the union, in definite proportions, of two gases, oxygen
and hydrogen. We know how to bring these constituents together, so as to
form water: we also know how to analyse the water, and recover from it its
two constituents. So, likewise, as regards the solid portions of the earth. Our
chalk hills, for example, are formed by a combination of carbon, oxygen,
and calcium. These are the so-called elements the union of which, in definite
proportions, has resulted in the formation of chalk. The flints within the
chalk we know to be a compound of oxygen and silicium, called silica; and
our ordinary clay is, for the most part, formed by the union of silicium,
oxygen, and the well-known light metal, aluminium. By far the greater
portion of the earth's crust is compounded of the elementary substances
mentioned in these few lines.

The principle of gravitation has been already described as an attraction
which every particle of matter, however small, exerts on every other particle.
With gravity there is no selection; no particular atoms choose, by preference,

other particular atoms as objects of attraction; the attraction of gravitation is
proportional simply to the quantity of the attracting matter, regardless of its
quality. But in the molecular world which we have now entered matters are
otherwise arranged. Here we have atoms between which a strong attraction
is exercised, and also atoms between which a weak attraction is exercised.
One atom can jostle another out of its place, in virtue of a superior force of
attraction. But, though the amount of force exerted varies thus from atom to
atom, it is still an attraction of the same mechanical quality, if I may use the
term, as that of gravity itself. Its intensity might be measured in the same
way, namely by the amount of motion which it can generate in a certain
time. Thus the attraction of gravity at the earth's surface is expressed by the
number 32; because, when acting freely on a body for a second of time,
gravity imparts to the body a velocity of thirty-two feet a second. In like
manner the mutual attraction of oxygen and hydrogen might be measured by
the velocity imparted to the atoms in their rushing together. Of course such a
unit of time as a second is not here to be thought of, the whole interval
required by the atoms to cross the minute spaces which separate them
amounting only to an inconceivably small fraction of a second.

It has been stated that when a body falls to the earth it is warmed by the
shock. Here, to use the terminology of Mayer, we have a mechanical
combination of the earth and the body. Let us suffer the falling body and the
earth to dwindle in imagination to the size of atoms, and for the attraction of
gravity let us substitute that of chemical affinity; we have then what is called
a chemical combination. The effect of the union in this case also is the
development of heat, and from the amount of heat generated we can infer the
intensity of the atomic pull. Measured by ordinary mechanical standards, this
is enormous. Mix eight pounds of oxygen with one of hydrogen, and pass a
spark through the mixture; the gases instantly combine, their atoms rushing
over the little distances which separate them. Take a weight of 47,000
pounds to an elevation of 1,000 feet above the earth's surface, and let it fall;
the energy with which it will strike the earth will not exceed that of the eight
pounds of oxygen atoms, as they dash against one pound of hydrogen atoms
to form water.

It is sometimes stated that gravity is distinguished from all other forces by
the fact of its resisting conversion into other forms of force. Chemical
affinity, it is said, can be converted into heat and light, and these again into

magnetism and electricity: but gravity refuses to be so converted; being a
force maintaining itself under all circumstances, and not capable of
disappearing to give place to another. The statement arises from vagueness
of thought. If by it be meant that a particle of matter can never be deprived
of its weight, the assertion is correct; but the law which affirms the
convertibility of natural forces was never intended, in the minds of those
who understood it, to affirm that such a conversion as that here implied
occurs in any case whatever. As regards convertibility into heat, gravity and
chemical affinity stand on precisely the same footing. The attraction in the
one case is as indestructible as in the other. Nobody affirms that when a
stone rests upon the surface of the earth, the mutual attraction of the earth
and stone is abolished; nobody means to affirm that the mutual attraction of
oxygen for hydrogen ceases, after the atoms have combined to form water.
What is meant, in the case of chemical affinity, is, that the pull of that
affinity, acting through a certain space, imparts a motion of translation of the
one atom towards the other. This motion is not heat, nor is the force that
produces it heat. But when the atoms strike and recoil, the motion of
translation is converted into a motion of vibration, which is heat. The
vibration, however, so far from causing the extinction of the original
attraction, is in part carried on by that attraction. The atoms recoil, in virtue
of the elastic force which opposes actual contact, and in the recoil they are
driven too far back. The original attraction then triumphs over the force of
recoil, and urges the atoms once more together. Thus, like a pendulum, they
oscillate, until their motion is imparted to the surrounding aether; or, in other
words, until their heat becomes radiant heat.

In this sense, and in this sense only, is chemical affinity converted into heat.
There is, first of all, the attraction between the atoms; there is, secondly,
space between them. Across this space the attraction urges them. They
collide, they recoil, they oscillate. There is here a change in the form of the
motion, but there is no real loss. It is so with the attraction of gravity. To
produce motion by gravity space must also intervene between the attracting
bodies. When they strike together motion is apparently destroyed, but in
reality there is no destruction. Their atoms are suddenly urged together by
the shock; by their own perfect elasticity these atoms recoil; and thus is set
up the molecular oscillation which, when communicated to the proper
nerves, announces itself as heat.

It was formerly universally supposed that by the collision of unelastic bodies
force was destroyed. Men saw, for example, that when two spheres of clay,
painter's putty, or lead for example, were urged together, the motion
possessed by the masses, prior to impact, was more or less annihilated. They
believed in an absolute destruction of the force of impact. Until recent times,
indeed, no difficulty was experienced in believing this, whereas, at present,
the ideas of force and its destruction refuse to be united in most philosophic
minds. In the collision of elastic bodies, on the contrary, it was observed that
the motion with which they clashed together was in great part restored by the
resiliency of the masses, the more perfect the elasticity the more complete
being the restitution. This led to the idea of perfectly elastic bodies — bodies
competent to restore by their recoil the whole of the motion which they
possessed before impact — and this again to the idea of the conservation of
force, as opposed to that destruction of force which was supposed to occur
when unelastic bodies met in collision.

We now know that the principle of conservation holds equally good with
elastic and unelastic bodies. Perfectly elastic bodies would develop no heat
on collision. They would retain their motion afterwards, though its direction
might be changed; and it is only when sensible motion is wholly or partly
destroyed, that heat is generated. This always occurs in unelastic collision,
the heat developed being the exact equivalent of the sensible motion
extinguished. This heat virtually declares that the property of elasticity,
denied to the masses, exists among their atoms; by the recoil and oscillation
of which the principle of conservation is vindicated.

But ambiguity in the use of the term 'force' makes itself more and more felt
as we proceed. We have called the attraction of gravity a force, without any
reference to motion. A body resting on a shelf is as much pulled by gravity
as when, after having been pushed off the shelf, it falls towards the earth. We
applied the term force also to that molecular attraction which we called
chemical affinity. When, however, we spoke of the conservation of force, in
the case of elastic collision, we meant neither a pull nor a push, which, as
just indicated, might be exerted upon inert matter, but we meant force
invested in motion — the vis viva, as it is called, of the colliding masses.

Force in this form has a definite mechanical measure, in the amount of work
that it can perform. The simplest form of work is the raising of a weight. A

man walking up-hill, or up-stairs, with a pound weight in his hand, to an
elevation say of sixteen feet, performs a certain amount of work, over and
above the lifting of his own body. If he carries the pound to a height of
thirty-two feet, he does twice the work; if to a height of forty-eight feet, he
does three times the work; if to sixty-four feet, he does four times the work,
and so on. If, moreover, he carries up two pounds instead of one, other
things being equal, he does twice the work; if three, four, or five pounds, he
does three, four, or five times the work. In fact, it is plain that the work
performed depends on two factors, the weight raised and the height to which
it is raised. It is expressed by the product of these two factors.

But a body may be caused to reach a certain elevation in opposition to the
force of gravity, without being actually carried up. If a hodman, for example,
wished to land a brick at an elevation of sixteen feet above the place where
he stood, he would probably pitch it up to the bricklayer. He would thus
impart, by a sudden effort, a velocity to the brick sufficient to raise it to the
required height; the work accomplished by that effort being precisely the
same as if he had slowly carried up the brick. The initial velocity to be
imparted, in this case, is well known. To reach a height of sixteen feet, the
brick must quit the man's hand with a velocity of thirty-two feet a second. It
is needless to say, that a body starting with any velocity, would, if wholly
unopposed or unaided, continue to move for ever with the same velocity. But
when, as in the case before us, the body is thrown upwards, it moves in
opposition to gravity, which incessantly retards its motion, and finally brings
it to rest at an elevation of sixteen feet. If not here caught by the bricklayer,
it would return to the hodman with an accelerated motion, and reach his
hand with the precise velocity it possessed on quitting it.

An important relation between velocity and work is here to be pointed out.
Supposing the hodman competent to impart to the brick, at starting, a
velocity of sixty-four feet a second, or twice its former velocity, would the
amount of work performed be twice what it was in the first instance? No; it
would be four times that quantity; for a body starting with twice the velocity
of another, will rise to four times the height. In like manner, a three-fold
velocity will give a nine-fold elevation, a four-fold velocity will give a
sixteen-fold elevation, and so on. The height attained, then, is not
proportional to the initial velocity, but to the square of the velocity. As
before, the work is also proportional to the weight elevated. Hence the work

which any moving mass whatever is competent to perform, in virtue of the
motion which it at any moment possesses, is jointly proportional to its
weight and the square of its velocity. Here, then, we have a second measure
of work, in which we simply translate the idea of height into its equivalent
idea of motion.

In mechanics, the product of the mass of a moving body into the square of its
velocity, expresses what is called the vis viva, or living force. It is also
sometimes called the 'mechanical effect.' If, for example, a cannon pointed to
the zenith urge a ball upwards with twice the velocity imparted to a second
ball, the former will rise to four times the height attained by the latter. If
directed against a target, it will also do four times the execution. Hence the
importance of imparting a high velocity to projectiles in war. Having thus
cleared our way to a perfectly definite conception of the vis viva of moving
masses, we are prepared for the announcement that the heat generated by the
shock of a falling body against the earth is proportional to the vis viva
annihilated. The heat is proportional to the square of the velocity. In the case,
therefore, of two cannon-balls of equal weight, if one strike a target with
twice the velocity of the other, it will generate four times the heat, if with
three times the velocity, it will generate nine times the heat, and so on.

Mr. Joule has shown that a pound weight falling from a height of 772 feet, or
772 pounds falling through one foot, will generate by its collision with the
earth an amount of heat sufficient to raise a pound of water one degree
Fahrenheit in temperature. 772 "foot-pounds" constitute the mechanical
equivalent of heat. Now, a body falling from a height of 772 feet, has, upon
striking the earth, a velocity of 223 feet a second; and if this velocity were
imparted to the body, by any other means, the quantity of heat generated by
the stoppage of its motion would be that stated above. Six times that
velocity, or 1,338 feet, would not be an inordinate one for a cannon-ball as it
quits the gun. Hence, a cannon-ball moving with a velocity of 1,338 feet a
second, would, by collision, generate an amount of heat competent to raise
its own weight of water 36 degrees Fahrenheit in temperature. If composed
of iron, and if all the heat generated were concentrated in the ball itself, its
temperature would be raised about 360 degrees Fahrenheit; because one
degree in the case of water is equivalent to about ten degrees in the case of
iron. In artillery practice, the heat generated is usually concentrated upon the
front of the bolt, and on the portion of the target first struck. By this

concentration the heat developed becomes sufficiently intense to raise the
dust of the metal to incandescence, a flash of light often accompanying
collision with the target.

Let us now fix our attention for a moment on the gunpowder which urges the
cannon-ball. This is composed of combustible matter, which if burnt in the
open air would yield a certain amount of heat. It will not yield this amount if
it perform the work of urging a ball. The heat then generated by the
gunpowder will fall short of that produced in the open air, by an amount
equivalent to the vis viva of the ball; and this exact amount is restored by the
ball on its collision with the target. In this perfect way are heat and
mechanical motion connected.

Broadly enunciated, the principle of the conservation of force asserts, that
the quantity of force in the universe is as unalterable as the quantity of
matter; that it is alike impossible to create force and to annihilate it. But in
what sense are we to understand this assertion? It would be manifestly
inapplicable to the force of gravity as defined by Newton; for this is a force
varying inversely as the square of the distance; and to affirm the constancy
of a varying force would be self-contradictory. Yet, when the question is
properly understood, gravity forms no exception to the law of conservation.
Following the method pursued by Helmholtz, I will here attempt an
elementary exposition of this law. Though destined in its applications to
produce momentous changes in human thought, it is not difficult of
comprehension.

For the sake of simplicity we will consider a particle of matter, which we
may call F, to be perfectly fixed, and a second movable particle, D, placed at
a distance from F. We will assume that these two particles attract each other
according to the Newtonian law. At a certain distance, the attraction is of a
certain definite amount, which might be determined by means of a spring
balance. At half this distance the attraction would be augmented four times;
at a third of the distance, nine times; at one-fourth of the distance, sixteen
times, and so on. In every case, the attraction might be measured by
determining, with the spring balance, the amount of tension just sufficient to
prevent D from moving towards F. Thus far we have nothing whatever to do
with motion; we deal with statics, not with dynamics. We simply take into

account the distance of D from F, and the pull exerted by gravity at that
distance.

It is customary in mechanics to represent the magnitude of a force by a line
of a certain length, a force of double magnitude being represented by a line
of double length, and so on. Placing then the particle D at a distance from F,
we can, in imagination, draw a straight line from D to F, and at D erect a
perpendicular to this line, which shall represent the amount of the attraction
exerted on D. If D be at a very great distance from F, the attraction will be
very small, and the perpendicular consequently very short. If the distance be
practically infinite, the attraction is practically nil. Let us now suppose at
every point in the line joining F and D a perpendicular to be erected,
proportional in length to the attraction exerted at that point; we thus obtain
an infinite number of perpendiculars, of gradually increasing length, as D
approaches F. Uniting the ends of all these perpendiculars, we obtain a
curve, and between this curve and the straight line joining F and D we have
an area containing all the perpendiculars placed side by side. Each one of
this infinite series of perpendiculars representing an attraction, or tension, as
it is sometimes called, the area just referred to represents the sum of the
tensions exerted upon the particle D, during its passage from its first position
to F.

Up to the present point we have been dealing with tensions, not with motion.
Thus far vis viva has been entirely foreign to our contemplation of D and F.
Let us now suppose D placed at a practically infinite distance from F; here,
as stated, the pull of gravity would be infinitely small, and the perpendicular
representing it would dwindle almost to a point. In this position the sum of
the tensions capable of being exerted on D would be a maximum. Let D now
begin to move in obedience to the infinitesimal attraction exerted upon it.
Motion being once set up, the idea of vis viva arises. In moving towards F
the particle D consumes, as it were, the tensions. Let us fix our attention on
D, at any point of the path over which it is moving. Between that point and F
there is a quantity of unused tensions; beyond that point the tensions have
been all consumed, but we have in their place an equivalent quantity of vis
viva. After D has passed any point, the tension previously in store at that
point disappears, but not without having added, during the infinitely small
duration of its action, a due amount of motion to that previously possessed
by D. The nearer D approaches to F, the smaller is the sum of the tensions

remaining, but the greater is the vis viva; the farther D is from F, the greater
is the sum of the unconsumed tensions, and the less is the living force. Now
the principle of conservation affirms not the constancy of the value of the
tensions of gravity, nor yet the constancy of the vis viva, taken separately,
but the absolute constancy of the value of both taken together. At the
beginning the vis viva was zero, and the tension area was a maximum; close
to F the vis viva is a maximum, while the tension area is zero. At every other
point, the work-producing power of the particle D consists in part of vis viva,
and in part of tensions.

If gravity, instead of being attraction, were repulsion, then, with the particles
in contact, the sum of the tensions between D and F would be a maximum,
and the vis viva zero. If, in obedience to the repulsion, D moved away from
F, vis viva would be generated; and the farther D retreated from F the greater
would be its vis viva, and the less the amount of tension still available for
producing motion. Taking repulsion as well as attraction into account, the
principle of the conservation of force affirms that the mechanical value of
the tensions and vires vivae of the material universe, so far as we know it, is
a constant quantity. The universe, in short, possesses two kinds of property
which are mutually convertible. The diminution of either carries with it the
enhancement of the other, the total value of the property remaining
unchanged.

The considerations here applied to gravity apply equally to chemical affinity.
Ina mixture of oxygen and hydrogen the atoms exist apart, but by the
application of proper means they may be caused to rush together across that
space that separates them. While this space exists, and as long as the atoms
have not begun to move towards each other, we have tensions and nothing
else. During their motion towards each other the tensions, as in the case of
gravity, are converted into vis viva. After they clash we have still vis viva,
but in another form. It was translation, it is vibration. It was molecular
transfer, it is heat.

It is possible to reverse these processes, to unlock the combined atoms and
replace them in their first positions. But, to accomplish this, as much heat
would be required as was generated by their union. Such reversals occur
daily and hourly in nature. By the solar waves, the oxygen of water is
divorced from its hydrogen in the leaves of plants. As molecular vis viva the

waves disappear, but in so doing they re-endow the atoms of oxygen and
hydrogen with tension. The atoms are thus enabled to recombine, and when
they do so they restore the precise amount of heat consumed in their
separation. The same remarks apply to the compound of carbon and oxygen,
called carbonic acid, which is exhaled from our lungs, produced by our fires,
and found sparingly diffused everywhere throughout the air. In the leaves of
plants the sunbeams also wrench the atoms of carbonic acid asunder, and
sacrifice themselves in the act; but when the plants are burnt, the amount of
heat consumed in their production is restored.

This, then, is the rhythmic play of Nature as regards her forces. Throughout
all her regions she oscillates from tension to vis viva, from vis viva to
tension. We have the same play in the planetary system. The earth's orbit is
an ellipse, one of the foci of which is occupied by the sun. Imagine the earth
at the most distant part of the orbit. Her motion, and consequently her vis
viva, is then a minimum. The planet rounds the curve, and begins its
approach to the sun. In front it has a store of tensions, which are gradually
consumed, an equivalent amount of vis viva being generated. When nearest
to the sun the motion, and consequently the vis viva, reach a maximum. But
here the available tensions have been used up. The earth rounds this portion
of the curve and retreats from the sun. Tensions are now stored up, but vis
viva is lost, to be again restored at the expense of the complementary force
on the opposite side of the curve. Thus beats the heart of the universe, but
without increase or diminution of its total stock of force.

I have thus far tried to steer clear amid confusion, by fixing the mind of the
reader upon things rather than upon names. But good names are essential;
and here, as yet, we are not provided with such. We have had the force of
gravity and living force — two utterly distinct things. We have had pulls and
tensions; and we might have had the force of heat, the force of light, the
force of magnetism, or the force of electricity — all of which terms have
been employed more or less loosely by writers on physics. This confusion is
happily avoided by the introduction of the term 'energy,' which embraces
both tension and vis viva. Energy is possessed by bodies already in motion; it
is then actual, and we agree to call it actual or dynamic energy. It is our old
vis viva. On the other hand, energy is possible to bodies not in motion, but
which, in virtue of attraction or repulsion, possess a power of motion which
would realise itself if all hindrances were removed. Looking, for example, at

gravity; a body on the earth's surface in a position from which it cannot fall
to a lower one possesses no energy. It has neither motion nor power of
motion. But the same body suspended at a height above the earth has a
power of motion, though it may not have exercised it. Energy is possible to
such a body, and we agree to call this potential energy. It consists of our old
tensions. We, moreover, speak of the conservation of energy, instead of the
conservation of force; and say that the sum of the potential and dynamic
energies of the material universe is a constant quantity.

A body cast upwards consumes the actual energy of projection, and lays up
potential energy. When it reaches its utmost height all its actual energy is
consumed, its potential energy being then a maximum. When it returns, there
is a reconversion of the potential into the actual. A pendulum at the limit of
its swing possesses potential energy; at the lowest point of its arc its energy
is all actual. A patch of snow resting on a mountain slope has potential
energy; loosened, and shooting down as an avalanche, it possesses dynamic
energy. The pine-trees growing on the Alps have potential energy; but
rushing down the Holzrinne of the woodcutters they possess actual energy.
The same is true of the mountains themselves. As long as the rocks which
compose them can fall to a lower level, they possess potential energy, which
is converted into actual when the frost ruptures their cohesion and hands
them over to the action of gravity. The stone avalanches of the Matterhorn
and Weisshorn are illustrations in point. The hammer of the great bell of
Westminster, when raised before striking, possesses potential energy; when it
falls, the energy becomes dynamic; and after the stroke, we have the
rhythmic play of potential and dynamic in the vibrations of the bell. The
same holds good for the molecular oscillations of a heated body. An atom is
driven against its neighbour, and recoils. The ultimate amplitude of the recoil
being attained, the motion of the atom in that direction is checked, and for an
instant its energy is all potential. It is then drawn towards its neighbour with
accelerated speed; thus, by attraction, converting its potential into dynamic
energy. Its motion in this direction is also finally checked, and again, for an
instant, its energy is all potential. It once more retreats, converting, by
repulsion, its potential into dynamic energy, till the latter attains a maximum,
after which it is again changed into potential energy. Thus, what is true of
the earth, as she swings to and fro in her yearly journey round the sun, is also
true of her minutest atom. We have wheels within wheels, and rhythm within
rhythm.

When a body is heated, a change of molecular arrangement always occurs,
and to produce this change heat is consumed. Hence, a portion only of the
heat communicated to the body remains as dynamic energy. Looking back
on some of the statements made at the beginning of this article, now that our
knowledge is more extensive, we see the necessity of qualifying them.
When, for example, two bodies clash, heat is generated; but the heat, or
molecular dynamic energy, developed at the moment of collision, is not the
exact equivalent of the sensible dynamic energy destroyed. The true
equivalent is this heat, plus the potential energy conferred upon the
molecules by the placing of greater distances between them. This molecular
potential energy is afterwards, on the cooling of the body, converted into
heat.

Wherever two atoms capable of uniting together by their mutual attractions
exist separately, they form a store of potential energy. Thus our woods,
forests, and coal-fields on the one hand, and our atmospheric oxygen on the
other, constitute a vast store of energy of this kind — vast, but far from
infinite. We have, besides our coal-fields, metallic bodies more or less
sparsely distributed through the earth's crust. These bodies can be oxydised;
and hence they are, so far as they go, stores of energy. But the attractions of
the great mass of the earth's crust are already satisfied, and from them no
further energy can possibly be obtained. Ages ago the elementary
constituents of our rocks clashed together and produced the motion of heat,
which was taken up by the aether and carried away through stellar space. It
is lost for ever as far as we are concerned. In those ages the hot conflict of
carbon, oxygen, and calcium produced the chalk and limestone hills which
are now cold; and from this carbon, oxygen, and calcium no further energy
can be derived. So it is with almost all the other constituents of the earth's
crust. They took their present form in obedience to molecular force; they
turned their potential energy into dynamic, and yielded it as radiant heat to
the universe, ages before man appeared upon this planet. For him a residue
of potential energy remains, vast, truly, in relation to the life and wants of an
individual, but exceedingly minute in comparison with the earth's primitive
store.

To sum up. The whole stock of energy or working-power in the world
consists of attractions, repulsions, and motions. If the attractions and
repulsions be so circumstanced as to be able to produce motion, they are

sources of working-power, but not otherwise. As stated a moment ago, the
attraction exerted between the earth and a body at a distance from the earth's
surface, is a source of working-power; because the body can be moved by
the attraction, and in falling can perform work. When it rests at its lowest
level it is not a source of power or energy, because it can fall no farther. But
though it has ceased to be a source of energy, the attraction of gravity still
acts as a force, which holds the earth and weight together.

The same remarks apply to attracting atoms and molecules. As long as
distance separates them, they can move across it in obedience to the
attraction; and the motion thus produced may, by proper appliances, be
caused to perform mechanical work. When, for example, two atoms of
hydrogen unite with one of oxygen, to form water, the atoms are first drawn
towards each other — they move, they clash, and then by virtue of their
resiliency, they recoil and quiver. To this quivering motion we give the name
of heat. This atomic vibration is merely the redistribution of the motion
produced by the chemical affinity; and this is the only sense in which
chemical affinity can be said to be converted into heat. We must not imagine
the chemical attraction destroyed, or converted into anything else. For the
atoms, when mutually clasped to form a molecule of water, are held together
by the very attraction which first drew them towards each other. That which
has really been expended is the pull exerted through the space by which the
distance between the atoms has been diminished.

If this be understood, it will be at once seen that gravity, as before insisted
on, may, in this sense, be said to be convertible into heat; that it is in reality
no more an outstanding and inconvertible agent, as it is sometimes stated to
be, than is chemical affinity. By the exertion of a certain pull through a
certain space, a body is caused to clash with a certain definite velocity
against the earth. Heat is thereby developed, and this is the only sense in
which gravity can be said to be converted into heat. In no case is the force,
which produces the motion annihilated or changed into anything else. The
mutual attraction of the earth and weight exists when they are in contact, as
when they were separate but the ability of that attraction to employ itself in
the production of motion does not exist.

The transformation, in this case, is easily followed by the mind's eye. First,
the weight as a whole is set in motion by the attraction of gravity. This

motion of the mass is arrested by collision with the earth, being broken up
into molecular tremors, to which we give the name of heat.

And when we reverse the process, and employ those tremors of heat to raise
a weight — which is done through the intermediation of an elastic fluid in
the steam-engine — a certain definite portion of the molecular motion is
consumed. In this sense, and in this sense only, can the heat be said to be
converted into gravity; or, more correctly, into potential energy of gravity.
Here the destruction of the heat has created no new attraction; but the old
attraction has conferred upon it a power of exerting a certain definite pull,
between the starting-point of the falling weight and the earth.

When, therefore, writers on the conservation of energy speak of tensions
being 'consumed' and 'generated,' they do not mean thereby that old
attractions have been annihilated, and new ones brought into existence, but
that, in the one case, the power of the attraction to produce motion has been
diminished by the shortening of the distance between the attracting bodies,
while, in the other case, the power of producing motion has been augmented
by the increase of the distance. These remarks apply to all bodies, whether
they be sensible masses or molecules.

Of the inner quality that enables matter to attract matter we know nothing;
and the law of conservation makes no statement regarding that quality. It
takes the facts of attraction as they stand, and affirms only the constancy of
working-power. That power may exist in the form of MOTION; or it may
exist in the form of FORCE, with distance to act through. The former is
dynamic energy, the latter is potential energy, the constancy of the sum of
both being affirmed by the law of conservation. The convertibility of natural
forces consists solely in transformations of dynamic into potential, and of
potential into dynamic energy. In no other sense has the convertibility of
force any scientific meaning.

.

Grave errors have been entertained as to what is really intended to be
conserved by the doctrine of conservation. This exposition I hope will tend
to remove them.

.

.

.

.

.

.

II. RADIATION.

[Footnote: The Rede Lecture delivered in the Senate House before the
University of Cambridge, May 16, 1865.]

1. Visible and Invisible Radiation.

BETWEEN the mind of man and the outer world are interposed the nerves
of the human body, which translate, or enable the mind to translate, the
impressions of that world into facts of consciousness and thought.

Different nerves are suited to the perception of different impressions. We do
not see with the ear, nor hear with the eye, nor are we rendered sensible of
sound by the nerves of the tongue. Out of the general assemblage of physical
actions, each nerve, or group of nerves, selects and responds to those for the
perception of which it is specially organised.

The optic nerve passes from the brain to the back of the eyeball and there
spreads out, to form the retina, a web of nerve filaments, on which the
images of external objects are projected by the optical portion of the eye.
This nerve is limited to the apprehension of the phenomena of radiation, and,
notwithstanding its marvellous sensibility to certain impressions of this
class, it is singularly obtuse to other impressions.

Nor does the optic nerve embrace the entire range even of radiation. Some
rays, when they reach it, are incompetent to evoke its power, while others
never reach it at all, being absorbed by the humours of the eye. To all rays

which, whether they reach the retina or not, fail to excite vision, we give the
name of invisible or obscure rays. All non-luminous bodies emit such rays.
There is no body in nature absolutely cold, and every body not absolutely
cold emits rays of heat. But to render radiant heat fit to affect the optic nerve
a certain temperature is necessary. A cool poker thrust into a fire remains
dark for a time, but when its temperature has become equal to that of the
surrounding coals, it glows like them. In like manner, if a current of
electricity, of gradually increasing strength, be sent through a wire of the
refractory metal platinum, the wire first becomes sensibly warm to the touch;
for a time its heat augments, still however remaining obscure; at length we
can no longer touch the metal with impunity; and at a certain definite
temperature it emits a feeble red light. As the current augments in power the
light augments in brilliancy, until finally the wire appears of a dazzling
white. The light which it now emits is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the texture of solar light,
and by the same simple instrument we can investigate the luminous changes
of our platinum wire. In passing through the prism all its rays (and they are
infinite in variety) are bent or refracted from their straight course; and, as
different rays are differently refracted by the prism, we are by it enabled to
separate one class of rays from another. By such prismatic analysis Dr.
Draper has shown, that when the platinum wire first begins to glow, the light
emitted is sensibly red. As the glow augments the red becomes more
brilliant, but at the same time orange rays are added to the emission.
Augmenting the temperature still further, yellow rays appear beside the
orange; after the yellow, green rays are emitted; and after the green come, in
succession, blue, indigo, and violet rays. To display all these colours at the
same time the platinum wire must be white-hot: the impression of whiteness
being in fact produced by the simultaneous action of all these colours on the
optic nerve.

In the experiment just described we began with a platinum wire at an
ordinary temperature, and gradually raised it to a white heat. At the
beginning, and even before the electric current had acted at all upon the
wire, it emitted invisible rays. For some time after the action of the current
had commenced, and even for a time after the wire had become intolerable
to the touch, its radiation was still invisible. The question now arises, What
becomes of these invisible rays when the visible ones make their

appearance? It will be proved in the sequel that they maintain themselves in
the radiation; that a ray once emitted continues to be emitted when the
temperature is increased, and hence the emission from our platinum wire,
even when it has attained its maximum brilliancy, consists of a mixture of
visible and invisible rays. If, instead of the platinum wire, the earth itself
were raised to incandescence, the obscure radiation which it now emits
would continue to be emitted. To reach incandescence the planet would have
to pass through all the stages of non-luminous radiation, and the final
emission would embrace the rays of all these stages. There can hardly be a
doubt that from the sun itself, rays proceed similar in kind to those which the
dark earth pours nightly into space. In fact, the various kind of obscure rays
emitted by all the planets of our system are included in the present radiation
of the sun.

The great pioneer in this domain of science was Sir William Herschel.
Causing a beam of solar light to pass through a prism, he resolved it into its
coloured constituents; he formed what is technically called the solar
spectrum. Exposing thermometers to the successive colours he determined
their heating power, and found it to augment from the violet or most
refracted end, to the red or least refracted end of the spectrum. But he did not
stop here. Pushing his thermometers into the dark space beyond the red he
found that, though the light had disappeared, the radiant heat falling on the
instruments was more intense than that at any visible part of the spectrum. In
fact, Sir William Herschel showed, and his results have been verified by
various philosophers since his time, that, besides its luminous rays, the sun
pours forth a multitude of other rays, more powerfully calorific than the
luminous ones, but entirely unsuited to the purposes of vision.

At the less refrangible end of the solar spectrum, then, the range of the sun's
radiation is not limited by that of the eye. The same statement applies to the
more refrangible end. Ritter discovered the extension of the spectrum into
the invisible region beyond the violet; and, in recent times, this ultra-violet
emission has had peculiar interest conferred upon it by the admirable
researches of Professor Stokes. The complete spectrum of the sun consists,
therefore, of three distinct parts :— first, of ultra-red rays of high heating
power, but unsuited to the purposes of vision; secondly, of luminous rays
which display the succession of colours, red, orange, yellow, green, blue,
indigo, violet; thirdly, of ultra-violet rays which, like the ultra-red ones, are

incompetent to excite vision, but which, unlike the ultra-red rays, possess a
very feeble heating power. In consequence, however, of their chemical
energy these ultra-violet rays are of the utmost importance to the organic
world.

.

.

2. Origin and Character of Radiation. The Aether.

When we see a platinum wire raised gradually to a white heat, and emitting
in succession all the colours of the spectrum, we are simply conscious of a
series of changes in the condition of our own eyes. We do not see the actions
in which these successive colours originate, but the mind irresistibly infers
that the appearance of the colours corresponds to certain contemporaneous
changes in the wire. What is the nature of these changes? In virtue of what
condition does the wire radiate at all? We must now look from the wire, as a
whole, to its constituent atoms. Could we see those atoms, even before the
electric current has begun to act upon them, we should find them in a state of
vibration. In this vibration, indeed, consists such warmth as the wire then
possesses. Locke enunciated this idea with great precision, and it has been
placed beyond the pale of doubt by the excellent quantitative researches of
Mr. Joule. ' Heat,' says Locke, 'is a very brisk agitation of the insensible parts
of the object, which produce in us that sensation from which we denominate
the object hot: so what in our sensations is heat in the object is nothing but
motion.' When the electric current, still feeble, begins to pass through the
wire, its first act is to intensify the vibrations already existing, by causing the
atoms to swing through wider ranges. Technically speaking, the amplitudes
of the oscillations are increased. The current does this, however, without
altering the periods of the old vibrations, or the times in which they were
executed. But besides intensifying the old vibrations the current generates
new and more rapid ones, and when a certain definite rapidity has been
attained, the wire begins to glow. The colour first exhibited is red, which
corresponds to the lowest rate of vibration of which the eye is able to take
cognisance. By augmenting the strength of the electric current more rapid
vibrations are introduced, and orange rays appear. A quicker rate of vibration
produces yellow, a still quicker green; and by further augmenting the

rapidity, we pass through blue, indigo, and violet, to the extreme ultra-violet
rays.

Such are the changes recognised by the mind in the wire itself, as concurrent
with the visual changes taking place in the eye. But what connects the wire
with this organ? By what means does it send such intelligence of its varying
condition to the optic nerve? Heat being as defined by Locke, 'a very brisk
agitation of the insensible parts of an object,' it is readily conceivable that on
touching a heated body the agitation may communicate itself to the adjacent
nerves, and announce itself to them as light or heat. But the optic nerve does
not touch the hot platinum, and hence the pertinence of the question, By
what agency are the vibrations of the wire transmitted to the eye?

The answer to this question involves one of the most important physical
conceptions that the mind of man has yet achieved: the conception of a
medium filling space and fitted mechanically for the transmission of the
vibrations of light and heat, as air is fitted for the transmission of sound. This
medium is called the luminiferous aether. Every vibration of every atom of
our platinum wire raises in this aether a wave, which speeds through it at the
rate of 186,000 miles a second.

The aether suffers no rupture of continuity at the surface of the eye, the inter-
molecular spaces of the various humours are filled with it; hence the waves
generated by the glowing platinum can cross these humours and impinge on
the optic nerve at the back of the eye. [Footnote: The action here described
is analogous to the passage of sound-waves through thick felt whose
interstices are occupied by air.] Thus the sensation of light reduces itself to
the acceptance of motion. Up to this point we deal with pure mechanics; but
the subsequent translation of the shock of the aethereal waves into
consciousness eludes mechanical science. As an oar dipping into the Cam
generates systems of waves, which, speeding from the centre of disturbance,
finally stir the sedges on the river's bank, so do the vibrating atoms generate
in the surrounding aether undulations, which finally stir the filaments of the
retina. The motion thus imparted is transmitted with measurable, and not
very great velocity to the brain, where, by a process which the science of
mechanics does not even tend to unravel, the tremor of the nervous matter is
converted into the conscious impression of light.

Darkness might then be defined as aether at rest; light as aether in motion.
But in reality the aether is never at rest, for in the absence of light-waves we
have heat-waves always speeding through it. In the spaces of the universe
both classes of undulations incessantly commingle. Here the waves issuing
from uncounted centres cross, coincide, oppose, and pass through each other,
without confusion or ultimate extinction. Every star is seen across the
entanglement of wave-motions produced by all other stars. It is the ceaseless
thrill caused by those distant orbs collectively in the aether, that constitutes
what we call the 'temperature of space.' As the air of a room accommodates
itself to the requirements of an orchestra, transmitting each vibration of
every pipe and string, so does the inter-stellar aether accommodate itself to
the requirements of light and heat. Its waves mingle in space without
disorder, each being endowed with an individuality as indestructible as if it
alone had disturbed the universal repose.

All vagueness with regard to the use of the terms 'radiation' and 'absorption'
will now disappear. Radiation is the communication of vibratory motion to
the aether; and when a body is said to be chilled by radiation, as for example
the grass of a meadow on a starlight night, the meaning is, that the molecules
of the grass have lost a portion of their motion, by imparting it to the
medium in which they vibrate. On the other hand, the waves of aether may
so strike against the molecules of a body exposed to their action as to yield
up their motion to the latter; and in this transfer of the motion from the
aether to the molecules consists the absorption of radiant heat. All the
phenomena of heat are in this way reducible to interchanges of motion; and
it is purely as the recipients or the donors of this motion, that we ourselves
become conscious of the action of heat and cold.

.

.

3. The Atomic Theory in reference to the Aether.

The word 'atoms' has been more than once employed in this discourse.
Chemists have taught us that all matter is reducible to certain elementary
forms to which they give this name. These atoms are endowed with powers
of mutual attraction, and under suitable circumstances they coalesce to form

compounds. Thus oxygen and hydrogen are elements when separate, or
merely mixed, but they may be made to combine so as to form molecules,
each consisting of two atoms of hydrogen and one of oxygen. In this
condition they constitute water. So also chlorine and sodium are elements,
the former a pungent gas, the latter a soft metal; and they unite together to
form chloride of sodium or common salt. In the same way the element
nitrogen combines with hydrogen, in the proportion of one atom of the
former to three of the latter, to form ammonia. Picturing in imagination the
atoms of elementary bodies as little spheres, the molecules of compound
bodies must be pictured as groups of such spheres. This is the atomic theory
as Dalton conceived it. Now if this theory have any foundation in fact, and if
the theory of an aether pervading space, and constituting the vehicle of
atomic motion, be founded in fact, it is surely of interest to examine whether
the vibrations of elementary bodies are modified by the act of combination
— whether as regards radiation and absorption, or, in other words, whether
as regards the communication of motion to the aether, and the acceptance of
motion from it, the deportment of the uncombined atoms will be different
from that of the combined.

.

.

.

4. Absorption of Radiant Heat by Gases.

We have now to submit these considerations to the only test by which they
can be tried, namely, that of experiment. An experiment is well defined as a
question put to Nature; but, to avoid the risk of asking amiss, we ought to
purify the question from all adjuncts which do not necessarily belong to it.
Matter has been shown to be composed of elementary constituents, by the
compounding of which all its varieties are produced. But, besides the
chemical unions which they form, both elementary and compound bodies
can unite in another and less intimate way. Gases and vapours aggregate to
liquids and solids, without any change of their chemical nature. We do not
yet know how the transmission of radiant heat may be affected by the
entanglement due to cohesion; and, as our object now is to examine the

influence of chemical union alone, we shall render our experiments more
pure by liberating the atoms and molecules entirely from the bonds of
cohesion, and employing them in the gaseous or vaporous form.

Let us endeavour to obtain a perfectly clear mental image of the problem
now before us. Limiting in the first place our enquiries to the phenomena of
absorption, we have to picture a succession of waves issuing from a radiant
source and passing through a gas; some of them striking against the gaseous
molecules and yielding up their motion to the latter; others gliding round the
molecules, or passing through the intermolecular spaces without apparent
hindrance. The problem before us is to determine whether such free
molecules have any power whatever to stop the waves of heat; and if so,
whether different molecules possess this power in different degrees.

In examining the problem let us fall back upon an actual piece of work,
choosing as the source of our heat-waves a plate of copper, against the back
of which a steady sheet of flame is permitted to play. On emerging from the
copper, the waves, in the first instance, pass through a space devoid of air,
and then enter a hollow glass cylinder, three feet long and three inches wide.
The two ends of this cylinder are stopped by two plates of rock-salt, a solid
substance which offers a scarcely sensible obstacle to the passage of the
calorific waves. After passing through the tube, the radiant heat falls upon
the anterior face of a thermo-electric pile, [Footnote: In the Appendix to the
first chapter of 'Heat as a Mode of Motion,' the construction of the thermo-
electric pile is fully explained.] which instantly converts the heat into an
electric current. This current conducted round a magnetic needle deflects it,
and the magnitude of the deflection is a measure of the heat falling upon the
pile. This famous instrument, and not an ordinary thermometer, is what we
shall use in these enquiries, but we shall use it in a somewhat novel way. As
long as the two opposite faces of the thermo-electric pile are kept at the same
temperature, no matter how high that may be, there is no current generated.
The current is a consequence of a difference of temperature between the two
opposite faces of the pile. Hence, if after the anterior face has received the
heat from our radiating source, a second source, which we may call the
compensating source, be permitted to radiate against the posterior face, this
latter radiation will tend to neutralise the former. When the neutralisation is
perfect, the magnetic needle connected with the pile is no longer deflected,
but points to the zero of the graduated circle over which it hangs.

And now let us suppose the glass tube, through which the waves from the
heated plate of copper are passing, to be exhausted by an air-pump, the two
sources of heat acting at the same time on the two opposite faces of the pile.
When by means of an adjusting screen, perfectly equal quantities of heat are
imparted to the two faces, the needle points to zero. Let any gas be now
permitted to enter the exhausted tube; if its molecules possess any power of
intercepting the calorific waves, the equilibrium previously existing will be
destroyed, the compensating source will triumph, and a deflection of the
magnetic needle will be the immediate consequence. From the deflections
thus produced by different gases, we can readily deduce the relative amounts
of wave-motion which their molecules intercept.

In this way the substances mentioned in the following table were examined,
a small portion only of each being admitted into the glass tube. The quantity
admitted in each case was just sufficient to depress a column of mercury
associated with the tube one inch: in other words, the gases were examined
at a pressure of one-thirtieth of an atmosphere. The numbers in the table
express the relative amounts of wave-motion absorbed by the respective
gases, the quantity intercepted by air being taken as unity.

.

Radiation through Gases.

Name of gas Relative absorption

Air 1

Oxygen 1

Nitrogen 1

Hydrogen 1

Carbonic oxide 750

Carbonic acid 972

Hydrochloric acid. 1,005

Nitric oxide 1,590

Nitrous oxide 1,860

Sulphide of hydrogen 2,100

Ammonia 5,460

Olefiant gas 6,030

Sulphurous acid 6,480

Every gas in this table is perfectly transparent to light, that is to say, all
waves within the limits of the visible spectrum pass through it without
obstruction; but for the waves of slower period, emanating from our heated
plate of copper, enormous differences of absorptive power are manifested.
These differences illustrate in the most unexpected manner the influence of
chemical combination. Thus the elementary gases, oxygen, hydrogen, and
nitrogen, and the mixture atmospheric air, prove to be practical vacua to the

rays of heat; for every ray, or, more strictly speaking, for every unit of wave-
motion, which any one of them intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while sulphurous acid gas
absorbs 6,480 units. What, becomes of the wave-motion thus intercepted? It
is applied to the heating of the absorbing gas. Through air, oxygen,
hydrogen, and nitrogen, the waves of aether pass without absorption, and
these gases are not sensibly changed in temperature by the most powerful
calorific rays. The position of nitrous oxide in the foregoing table is worthy
of particular notice. In this gas we have the same atoms in a state of
chemical union, that exist uncombined in the atmosphere; but the absorption
of the compound is 1,800 times that of air.

.

.

.

5. Formation of Invisible Foci.

This extraordinary deportment of the elementary gases naturally directed
attention to elementary bodies in other states of aggregation. Some of
Melloni's results now attained a new significance. This celebrated
experimenter had found crystals of sulphur to be highly pervious to radiant
heat; he had also proved that lamp-black, and black glass, (which owes its
blackness to the element carbon) were to a considerable extent transparent to
calorific rays of low refrangibility. These facts, harmonising so strikingly
with the deportment of the simple gases, suggested further enquiry. Sulphur
dissolved in bisulphide of carbon was found almost perfectly diathermic.
The dense and deeply-coloured element bromine was examined, and found
competent to cut off the light of our most brilliant flames, while it
transmitted the invisible calorific rays with extreme freedom. Iodine, the
companion element of bromine, was next thought of, but it was found
impracticable to examine the substance in its usual solid condition. It
however dissolves freely in bisulphide of carbon. There is no chemical union
between the liquid and the iodine; it is simply a case of solution, in which
the uncombined atoms of the element can act upon the radiant heat. When
permitted to do so, it was found that a layer of dissolved iodine, sufficiently

opaque to cut off the light of the midday sun, was almost absolutely
transparent to the invisible calorific rays. [Footnote: Professor Dewar has
recently succeeded in producing a medium highly opaque to light, and highly
transparent to obscure heat, by fusing together sulphur and iodine.]

By prismatic analysis Sir William Herschel separated the luminous from the
non-luminous rays of the sun, and he also sought to render the obscure rays
visible by concentration. Intercepting the luminous portion of his spectrum
he brought, by a converging lens, the ultra-red rays to a focus, but by this
condensation he obtained no light. The solution of iodine offers a means of
filtering the solar beam, or failing it, the beam of the electric lamp, which
renders attainable far more powerful foci of invisible rays than could
possibly be obtained by the method of Sir William Herschel. For to form his
spectrum he was obliged to operate upon solar light which had passed
through a narrow slit or through a small aperture, the amount of the obscure
heat being limited by this circumstance. But with our opaque solution we
may employ the entire surface of the largest lens, and having thus converged
the rays, luminous and non-luminous, we can intercept the former by the
iodine, and do what we please with the latter. Experiments of this character,
not only with the iodine solution, but also with black glass and layers of
lampblack, were publicly performed at the Royal Institution in the early part
of 1862, and the effects at the foci of invisible rays, then obtained, were such
as had never been witnessed previously.

In the experiments here referred to, glass lenses were employed to
concentrate the rays. But glass, though highly transparent to the luminous, is
in a high degree opaque to the invisible, heat-rays of the electric lamp, and
hence a large portion of those rays was intercepted by the glass. The obvious
remedy here is to employ rock-salt lenses instead of glass ones, or to
abandon the use of lenses wholly, and to concentrate the rays by a metallic
mirror. Both of these improvements have been introduced, and, as
anticipated, the invisible foci have been thereby rendered more intense. The
mode of operating remains however the same, in principle, as that made
known in 1862. It was then found that an instant's exposure of the face of the
thermoelectric pile to the focus of invisible rays, dashed the needles of a
coarse galvanometer violently aside. It is now found that on substituting for
the face of the thermo-electric pile a combustible body, the invisible rays are
competent to set that body on fire.

.

.

6. Visible and Invisible Rays of the Electric Light.

We have next to examine what proportion the non-luminous rays of the
electric light bear to the luminous ones. This the opaque solution of iodine
enables us to do with an extremely close approximation to the truth.

The pure bisulphide of carbon, which is the solvent of the iodine, is perfectly
transparent to the luminous, and almost perfectly transparent to the dark,
rays of the electric lamp. Supposing the total radiation of the lamp to pass
through the transparent bisulphide, while through the solution of iodine only
the dark rays are transmitted. If we determine, by means of a thermoelectric
pile, the total radiation, and deduct from it the purely obscure, we obtain the
value of the purely luminous emission. Experiments, performed in this way,
prove that if all the visible rays of the electric light were converged to a
focus of dazzling brilliancy, its heat would only be one-eighth of that
produced at the unseen focus of the invisible rays.

Exposing his thermometers to the successive colours of the solar spectrum,
Sir William Herschel determined the heating power of each, and also that of
the region beyond the extreme red. Then drawing a straight line to represent
the length of the spectrum, he erected, at various points, perpendiculars to
represent the calorific intensity existing at those points. Uniting the ends of
all his perpendiculars, he obtained a curve which showed at a glance the
manner in which the heat was distributed in the solar spectrum. Professor
Müller of Freiburg, with improved instruments, afterwards made similar
experiments, and constructed a more accurate diagram of the same kind. We
have now to examine the distribution of heat in the spectrum of the electric
light; and for this purpose we shall employ a particular form of the thermo-
electric pile, devised by Melloni. Its face is a rectangle, which by means of
movable side-pieces can be rendered as narrow as desired. We can, for
example, have the face of the pile the tenth, the hundredth, or even the
thousandth of an inch in breadth. By means of an endless screw, this linear
thermo-electric pile may be moved through the entire spectrum, from the

violet to the red, the amount of heat falling upon the pile at every point of its
march, being declared by a magnetic needle associated with the pile.

When this instrument is brought up to the violet end of the spectrum of the
electric light, the heat is found to be insensible. As the pile is gradually
moved from the violet end towards the red, heat soon manifests itself,
augmenting as we approach the red. Of all the colours of the visible
spectrum the red possesses the highest heating power. On pushing the pile
into the dark region beyond the red, the heat, instead of vanishing, rises
suddenly and enormously in intensity, until at some distance beyond the red
it attains a maximum. Moving the pile still forward, the thermal power falls,
somewhat more rapidly than it rose. It then gradually shades away, but, for a
distance beyond the red greater than the length of the whole visible
spectrum, signs of heat may be detected.

Drawing a datum line, and erecting along it perpendiculars, proportional in
length to the thermal intensity at the respective points, we obtain the
extraordinary curve, shown on the opposite page, which exhibits the
distribution of heat in the spectrum of the electric light. In the region of dark
rays, beyond the red, the curve shoots up to B, in a steep and massive peak
— a kind of Matterhorn of heat, which dwarfs the portion of the diagram C
D E, representing the luminous radiation. Indeed the idea forced upon the
mind by this diagram is that the light rays are a mere insignificant appendage
to the heat-rays represented by the area A B C D, thrown in as it were by
nature for the purpose of vision.

.

.

Figure 1. Spectrum of Electric Light.

The diagram drawn by Professor Müller to represent the distribution of heat
in the solar spectrum is not by any means so striking as that just described,
and the reason, doubtless, is that prior to reaching the earth the solar rays
have to traverse our atmosphere. By the aqueous vapour there diffused, the
summit of the peak representing the sun's invisible radiation is cut off. A
similar lowering of the mountain of invisible heat is observed when the rays
from the electric light are permitted to pass through a film of water, which
acts upon them as the atmospheric vapour acts upon the rays of the sun.

.

.

7. Combustion by Invisible Rays.

The sun's invisible rays far transcend the visible ones in heating power, so
that if the alleged performances of Archimedes during the siege of Syracuse

had any foundation in fact, the dark solar rays would have been the
philosopher's chief agents of combustion. On a small scale we can readily
produce, with the purely invisible rays of the electric light, all that
Archimedes is said to have performed with the sun's total radiation. Placing
behind the electric light a small concave mirror, the rays are converged, the
cone of reflected rays and their point of convergence being rendered clearly
visible by the dust always floating in the air. Placing between the luminous
focus and the source of rays our solution of iodine, the light of the cone is
entirely cut away; but the intolerable heat experienced when the band is
placed, even for a moment, at the dark focus, shows that the calorific rays
pass unimpeded through the opaque solution.

Almost anything that ordinary fire can effect may be accomplished at the
focus of invisible rays; the air at the focus remaining at the same time
perfectly cold, on account of its transparency to the heat-rays. An air
thermometer, with a hollow rack-salt bulb, would be unaffected by the heat
of the focus: there would be no expansion, and in the open air there is no
convection. The aether at the focus, and not the air, is the substance in which
the heat is embodied. A block of wood, placed at the focus, absorbs the heat,
and dense volumes of smoke rise swiftly upwards, showing the manner in
which the air itself would rise, if the invisible rays were competent to heat it.
At the perfectly dark focus dry paper is instantly inflamed: chips of wood are
speedily burnt up: lead, tin, and zinc are fused: and disks of charred paper
are raised to vivid incandescence. It might be supposed that the obscure rays
would show no preference for black over white; but they do show a
preference, and to obtain rapid combustion, the body, if not already black,
ought to be blackened. When metals are to be burned, it is necessary to
blacken or otherwise tarnish them, so as to diminish their reflective power.
Blackened zinc foil, when brought into the focus of invisible rays, is
instantly caused to blaze, and burns with its peculiar purple light.
Magnesium wire flattened, or tarnished magnesium ribbon, also bursts into
flame. Pieces of charcoal suspended in a receiver full of oxygen are also set
on fire when the invisible focus falls upon them; the dark rays after having
passed through the receiver, still possessing sufficient power to ignite the
charcoal, and thus initiate the attack of the oxygen. If, instead of being
plunged in oxygen, the charcoal be suspended in vacuo, it immediately
glows at the place where the focus falls.

.

.

8. Transmutation of Rays: Calorescence.

[Footnote: I borrow this term from Professor Challis, 'Philosophical
Magazine,' vol. xii. p. 521.]

Eminent experimenters were long occupied in demonstrating the substantial
identity of light and radiant heat, and we have now the means of offering a
new and striking proof of this identity. A concave mirror produces, beyond
the object which it reflects, an inverted and magnified image of the object.
Withdrawing, for example, our iodine solution, an intensely luminous
inverted image of the carbon points of the electric light is formed at the
focus of the mirror employed in the foregoing experiments. When the
solution is interposed, and the light is cut away, what becomes of this image?
It disappears from sight; but an invisible thermograph remains, and it is only
the peculiar constitution of our eyes that disqualifies us from seeing the
picture formed by the calorific rays. Falling on white paper, the image chars
itself out: falling on black paper, two holes are pierced in it, corresponding to
the images of the two coke points: but falling on a thin plate of carbon in
vacuo, or upon a thin sheet of platinised platinum, either in vacuo or in air,
radiant heat is converted into light, and the image stamps itself in vivid
incandescence upon both the carbon and the metal. Results similar to those
obtained with the electric light have also been obtained with the invisible
rays of the lime-light and of the sun.

Before a Cambridge audience it is hardly necessary to refer to the excellent
researches of Professor Stokes at the opposite end of the spectrum. The
above results constitute a kind of complement to his discoveries. Professor
Stokes named the phenomena which he has discovered and investigated
Fluorescence; for the new phenomena here described I have proposed the
term Calorescence. He, by the interposition of a proper medium, so lowered
the refrangibility of the ultraviolet rays of the spectrum as to render them
visible. Here, by the interposition of the platinum foil, the refrangibility of
the ultra-red rays is so exalted as to render them visible. Looking through a
prism at the incandescent image of the carbon points, the light of the image

is decomposed, and a complete spectrum is obtained. The invisible rays of
the electric light, remoulded by the atoms of the platinum, shine thus visibly
forth; ultra-red rays being converted into red, orange, yellow, green, blue,
indigo, violet, and ultraviolet ones. Could we, moreover, raise the original
source of rays to a sufficiently high temperature, we might not only obtain
from the dark rays of such a source a single incandescent image, but from
the dark rays of this image we might obtain a second one, from the dark rays
of the second a third, and so on — a series of complete images and spectra
being thus extracted from the invisible emission of the primitive source.
[Footnote: On investigating the calorescence produced by rays transmitted
through glasses of various colours, it was found that in the case of certain
specimens of blue glass, the platinum foil glowed with a pink or purplish
light. The effect was not subjective, and considerations of obvious interest
are suggested by it. Different kinds of black glass differ notably as to their
power of transmitting radiant heat. When thin, some descriptions tint the sun
with a greenish hue: others make it appear a glowing red without any trace
of green. The latter are far more diathermic than the former. In fact, carbon
when perfectly dissolved and incorporated with a good white glass, is highly
transparent to the calorific rays, and by employing it as an absorbent the
phenomena of 'calorescence' may be obtained, though in a less striking form
than with the iodine. The black glass chosen for thermometers, and intended
to absorb completely the solar heat, may entirely fail in this object, if the
glass in which the carbon is incorporated be colourless. To render the bulb
of a thermometer a perfect absorbent, the glass ought in the first instance to
be green. Soon after the discovery of fluorescence the late Dr. William Allen
Miller pointed to the lime-light as an illustration of exalted refrangibility.
Direct experiments have since entirely confirmed the view expressed at page
210 of his work on 'Chemistry,' published in 1855.]

.

.

9. Deadness of the Optic Nerve to the Calorific Rays.

The layer of iodine used in the foregoing experiments intercepted the rays of
the noonday sun. No trace of light from the electric lamp was visible in the
darkest room, even when a white screen was placed at the focus of the

mirror employed to concentrate the light. It was thought, however, that if the
retina itself were brought into the focus the sensation of light might be
experienced. The danger of this experiment was twofold. If the dark rays
were absorbed in a high degree by the humours of the eye the albumen of the
humours might coagulate along the line of the rays. If, on the contrary, no
such high absorption took place, the rays might reach the retina with a force
sufficient to destroy it. To test the likelihood of these results, experiments
were made on water and on a solution of alum, and they showed it to be very
improbable that in the brief time requisite for an experiment any serious
damage could be done. The eye was therefore caused to approach the dark
focus, no defence, in the first instance, being provided; but the heat, acting
upon the parts surrounding the pupil, could not be borne. An aperture was
therefore pierced in a plate of metal, and the eye, placed behind the aperture,
was caused to approach the point of convergence of invisible rays. The focus
was attained, first by the pupil and afterwards by the retina. Removing the
eye, but permitting the plate of metal to remain, a sheet of platinum foil was
placed in the position occupied by the retina a moment before. The platinum
became red-hot. No sensible damage was done to the eye by this experiment;
no impression of light was produced; the optic nerve was not even conscious
of heat.

But the humours of the eye are known to be highly impervious to the
invisible calorific rays, and the question therefore arises, 'Did the radiation in
the foregoing experiment reach the retina at all?' The answer is, that the rays
were in part transmitted to the retina, and in part absorbed by the humours.
Experiments on the eye of an ox showed that the proportion of obscure rays
which reached the retina amounted to 18 per cent. of the total radiation;
while the luminous emission from the electric light amounts to no more than
10 per cent. of the same total. Were the purely luminous rays of the electric
lamp converged by our mirror to a focus, there can be no doubt as to the fate
of a retina placed there. Its ruin would be inevitable; and yet this would be
accomplished by an amount of wave-motion but little more than half of that
which the retina, without exciting consciousness, bears at the focus of
invisible rays.

This subject will repay a moment's further attention. At a common distance
of a foot the visible radiation of the electric light employed in these
experiments is 800 times the light of a candle. At the same distance, the

portion of the radiation of the electric light which reaches the retina, but fails
to excite vision, is about 1,500 times the luminous radiation of the candle.
[Footnote: It will be borne in mind that the heat which any ray, luminous or
non-luminous, is competent to generate is the true measure of the energy of
the ray.] But a candle on a clear night can readily be seen at a distance of a
mile, its light at this distance being less than 1/20,000,000 of its light at the
distance of a foot.

Hence, to make the candle-light a mile off equal in power to the non-
luminous radiation received from the electric light at a foot distance, its
intensity would have to be multiplied by 1,500 x 20,000,000, or by thirty
thousand millions. Thus the thirty thousand millionth part of the invisible
radiation from the electric light, received by the retina at the distance of a
foot, would, if slightly changed in character, be amply sufficient to provoke
vision. Nothing could more forcibly illustrate that special relationship
supposed by Melloni and others to subsist between the optic nerve and the
oscillating periods of luminous bodies. The optic nerve responds, as it were,
to the waves with which it is in consonance, while it refuses to be excited by
others of almost infinitely greater energy, whose periods of recurrence are
not in unison with its own.

.

.

.

10. Persistence of Rays.

At an early part of this lecture it was affirmed, that when a platinum wire
was, gradually raised to a state of high incandescence, new rays were
constantly added, while the intensity of the old ones was increased. Thus, in
Dr. Draper's experiments, the rise of temperature that generated the orange,
yellow, green, and blue augmented the intensity of the red. What is true of
the red is true of every other ray of the spectrum, visible and invisible. We
cannot indeed see the augmentation of intensity in the region beyond the red,
but we can measure it and express it numerically. With this view the
following experiment was performed: A spiral of platinum wire was

surrounded by a small glass globe to protect it from currents of air; through
an orifice in the globe the rays could pass from the spiral and fall afterwards
upon a thermo-electric pile. Placing in front of the orifice an opaque solution
of iodine, the platinum was gradually raised from a low dark heat to the
fullest incandescence, with the following results :—

Appearance of spiral Energy of

obscure radiation

Dark 1

Dark, but hotter 3

Dark, but still hotter 5

Dark, but still hotter 10

Feeble red 19

Dull red 25

Red 37

Full red. 62

Orange 89

Bright orange 144

Yellow 202

White 276

Intense white 440

Thus the augmentation of the electric current, which raises the wire from its
primitive dark condition to an intense white heat, exalts at the same time the
energy of the obscure radiation, until at the end it is fully 440 times what it
was at the beginning.

What has been here proved true of the totality of the ultra-red rays is true for
each of them singly. Placing our linear thermo-electric pile in any part of the
ultra-red spectrum, it may be proved that a ray once emitted continues to be
emitted with increased energy as the temperature is augmented. The
platinum spiral, so often referred to, being raised to whiteness by an electric
current, a brilliant spectrum was formed from its light. A linear thermo-
electric pile was placed in the region of obscure rays beyond the red, and by
diminishing the current the spiral was reduced to a low temperature. It was
then caused to pass through various degrees of darkness and incandescence,
with the following results :—

Appearance Energy of

of spiral obscure rays

Dark 1

Dark 6

Faint red 10

Dull red 13

Red 18

Full red. 27

Orange 60

Yellow 93

White 122

Here, as in the former case, the dark and bright radiations reached their
maximum together; as the one augmented, the other augmented, until at last
the energy of the obscure rays of the particular refrangibility here chosen,
became 122 times what it was at first. To reach a white heat the wire has to
pass through all the stages of invisible radiation, but in its most brilliant
condition it embraces, in an intensified form, the rays of all those stages.

And thus it is with all other kinds of matter, as far as they have hitherto been
examined. Coke, whether brought to a white heat by the electric current, or
by the oxyhydrogen jet, pours out invisible rays with augmented energy, as
its light is increased. The same is true of lime, bricks, and other substances.
It is true of all metals which are capable of being heated to incandescence. It
also holds good for phosphorus burning in oxygen. Every gush of dazzling

light has associated with it a gush of invisible radiant heat, which far
transcends the light in energy. This condition of things applies to all bodies
capable of being raised to a white heat, either in the solid or the molten
condition. It would doubtless also apply to the luminous fogs formed by the
condensation of incandescent vapours. In such cases when the curve
representing the radiant energy of the body is constructed, the obscure
radiation towers upwards like a mountain, the luminous radiation resembling
a mere 'spur' at its base. From the very brightness of the light of some of the
fixed stars we may infer the intensity of that dark radiation, which is the
precursor and inseparable associate of their luminous rays.

We thus find the luminous radiation appearing when the radiant body has
attained a certain temperature; or, in other words, when the vibrating atoms
of the body have attained a certain width of swing. In solid and molten
bodies a certain amplitude cannot be surpassed without the introduction of
periods of vibration, which provoke the sense of vision. How are we to
figure this? If permitted to speculate, we might ask, are not these more rapid
vibrations the progeny of the slower? Is it not really the mutual action of the
atoms, when they swing through very wide spaces, and thus encroach upon
each other, that causes them to tremble in quicker periods? If so, whatever be
the agency by which the large swinging space is obtained, we shall have
light-giving vibrations associated with it. It matters not whether the large
amplitudes be produced by the strokes of a hammer, or by the blows of the
molecules of a non-luminous gas, like air at some height above a gas-flame;
or by the shock of the aether particles when transmitting radiant heat. The
result in all cases will be incandescence. Thus, the invisible waves of our
filtered electric beam may be regarded as generating synchronous vibrations
among the atoms of the platinum on which they impinge; but, once these
vibrations have attained a certain amplitude, the mutual jostling of the atoms
produces quicker tremors, and the light-giving waves follow as the necessary
product of the heat-giving ones.

.

.

11. Absorption of Radiant Heat by Vapours and Odours.

We commenced the demonstrations brought forward in this lecture by
experiments on permanent gases, and we have now to turn our attention to
the vapours of volatile liquids. Here, as in the case of the gases, vast
differences have been proved to exist between various kinds of molecules, as
regards their power of intercepting the calorific waves. While some vapours
allow the waves a comparatively free passage, the faintest mixture of other
vapours causes a deflection of the magnetic needle. Assuming the absorption
effected by air, at a pressure of one atmosphere, to be unity, the following
are the absorptions effected by a series of vapours at a pressure of 1/60th of
an atmosphere :—

Name of vapour Absorption

Bisulphide of carbon 47

Iodide of methyl 115

Benzol 136

Amylene 321

Sulphuric ether 440

Formic ether 548

Acetic ether 612

Bisulphide of carbon is the most transparent vapour in this list; and acetic
ether the most opaque; 1/60th of an atmosphere of the former, however,
produces 47 times the effect of a whole atmosphere of air, while 1/60th of an
atmosphere of the latter produces 612 times the effect of a whole atmosphere
of air. Reducing dry air to the pressure of the acetic ether here employed, and
comparing them then together, the quantity of wave-motion intercepted by
the ether would be many thousand times that intercepted by the air.

Any one of these vapours discharged into the free atmosphere, in front of a
body emitting obscure rays, intercepts more or less of the radiation. A similar
effect is. produced by perfumes diffused in the air, though their attenuation is
known to be almost infinite. Carrying, for example, a current of dry air over
bibulous paper, moistened by patchouli, the scent taken up by the current
absorbs 30 times the quantity of heat intercepted by the air which carries it;
and yet patchouli acts more feebly on radiant heat than any other perfume
yet examined.

Here follow the results obtained with various essential oils, the odour, in
each case, being carried by a current of dry air into the tube already
employed for gases and vapours:—

Name of perfume Absorption

Patchouli 30

Sandal wood 32

Geranium 33

Oil of cloves 34

Otto of roses 37

Bergamot 44

Neroli 47

Lavender 60

Lemon 65

Portugal 67

Thyme 68

Rosemary 74

Oil of laurel 80

Camomile flowers 87

Cassia 109

Spikenard 355

Aniseed 372

Thus the absorption by a tube full of dry air being 1, that of the odour of
patchouli diffused in it is 30, at of lavender 60, that of rosemary 74, whilst
that of aniseed amounts to 372. It would be idle to speculate the quantities of
matter concerned in these actions.

.

.

12. Aqueous Vapour in relation to the Terrestrial Temperatures.

We are now fully prepared for a result which, without such preparation,
might appear. incredible. Water is, to some extent, a volatile body, and our
atmosphere, resting as it does upon the surface of the ocean, receives from it
a continual supply of aqueous vapour. It would be an error to confound
clouds or fog or any visible mist with the vapour of water, which is a
perfectly impalpable gas, diffused, even on the clearest days, throughout the
atmosphere. Compared with the great body of the air, the aqueous vapour it
contains is of almost infinitesimal amount, 99.5 out of every 100 parts of the
atmosphere being composed of oxygen and nitrogen. In the absence of
experiment, we should never think of ascribing to this scant and varying
constituent any important influence on terrestrial radiation; and yet its
influence is far more potent than that of the great body of the air. To say that
on a day of average humidity in England, the atmospheric vapour exerts 100
times the action of the air itself, would certainly be an understatement of the
fact. Comparing a single molecule of aqueous vapour with an atom of either
of the main constituents of our atmosphere, I am not prepared to say how
many thousand times the action of the former exceeds that of the latter.

But it must be borne in mind that these large numbers depend, in part, on the
extreme feebleness of the air; the power of aqueous vapour seems vast,
because that of the air with which it is compared is infinitesimal. Absolutely
considered, however, this substance, notwithstanding its small specific
gravity, exercises a very potent action. Probably from 10 to 15 per cent. of

the heat radiated from the earth is absorbed within 10 or 20 feet of the earth's
surface. This must evidently be of the utmost consequence to the life of the
world. Imagine the superficial molecules of the earth agitated with the
motion of heat, and imparting it to the surrounding aether; this motion would
be carried rapidly away, and lost for ever to our planet, if the waves of aether
had nothing but the air to contend with in their outward course. But the
aqueous vapour takes up the motion, and becomes hereby heated, thus
wrapping the earth like a warm garment, and protecting its surface from the
deadly chill which it would otherwise sustain. Various philosophers have
speculated on the influence of an atmospheric envelope. De Saussure,
Fourier, M. Pouillet, and Mr. Hopkins have, one and all, enriched scientific
literature with contributions on this subject, but the considerations which
these eminent men have applied to atmospheric air, have, if my experiments
be correct, to be transferred to the aqueous vapour.

The observations of meteorologists furnish important, though hitherto
unconscious evidence of the influence of this agent. Wherever the air is dry
we are liable to daily extremes of temperature. By day, such places, the sun's
heat reaches the earth unimpeded, and renders the maximum high; by night,
on the other hand, the earth's heat escapes unhindered to space, and renders
the minimum low. Hence the difference between the maximum and
minimum is greatest where the air is driest. In the plains of India, the heights
of the Himalaya, in central Asia, in Australia — wherever drought reigns, we
have the heat of day forcibly contrasted with the chill of night. In the Sahara
itself, when the sun's rays cease to impinge on the burning soil, the
temperature runs rapidly down to freezing, because there is no vapour
overhead to check the calorific drain. And here another instance might be
added to the numbers already known, in which nature tends as it were to
check her own excess. By nocturnal refrigeration, the aqueous vapour of the
air is condensed to water on the surface of the earth; and, as only the
superficial portions radiate, the act of condensation makes water the
radiating body. Now experiment proves that to the rays emitted by water,
aqueous vapour is especially opaque. Hence the very act of condensation,
consequent on terrestrial cooling, becomes a safeguard to the earth,
imparting to its radiation that particular character which renders it most
liable to be prevented from escaping into space.

It might however be urged that, inasmuch as we derive all our heat from the
sun, the selfsame covering which protects the earth from chill must also shut
out the solar radiation. This is partially true, but only partially; the sun's rays
are different in quality from the earth's rays, and it does not at all follow that
the substance which absorbs the one must necessarily absorb the other.
Through a layer of water, for example, one tenth of an inch in thickness, the
sun's rays are transmitted with comparative freedom; but through a layer half
this thickness, as Melloni has proved, no single ray from the warmed earth
could pass. In like manner, the sun's rays pass with comparative freedom
through the aqueous vapour of the air: the absorbing power of this substance
being mainly exerted upon the invisible heat that endeavours to escape from
the earth. In consequence of this differential action upon solar and terrestrial
heat, the mean temperature of our planet is higher than is due to its distance
from the sun.

.

.

13. Liquids and their Vapours in relation to Radiant Heat.

The deportment here assigned to atmospheric vapour has been established by
direct experiments on it taken from the streets and parks of London, from the
downs of Epsom, from the hills and sea-beach of the Isle of Wight, and also
by experiments on air in the first instance dried, and afterwards rendered
artificially humid by pure distilled water. It has also been established in the
following way: Ten volatile liquids were taken at random and the power of
these ;liquids, at a common thickness, to intercept the waves of heat, was
carefully determined. The vapours of the liquids were next taken, in
quantities proportional to the quantities of liquid, and the power of the
vapours intercept the waves of heat was also determined.

Commencing with the substance which exerted the least absorptive power,
and proceeding onwards to the most energetic, the following order of
absorption was observed :—

Liquids Vapours

Bisulphide of carbon Bisulphide of carbon

Chloroform Chloroform

Iodide of methyl Iodide of methyl

Iodide of ethyl Iodide of ethyl

Benzol Benzol

Amylene Amylene

Sulphuric aether Sulphuric aether

Acetic aether Acetic aether

Formic aether Formic aether

Alcohol Alcohol

Water

We here find the order of absorption in both cases be the same. We have
liberated the molecules from the bonds which trammel them more or less in
a liquid condition; but this change in their state of aggregation does not
change their relative powers of absorption. Nothing could more clearly
prove that the act of absorption depends upon the individual molecule, which
equally asserts its power in the liquid and the gaseous state. We may safely
conclude from the above table that the position of a vapour is determined by
that of its liquid. Now at the very foot of the list of liquids stands water,
signalising itself above all others by its enormous power of absorption. And
from this fact, even if no direct experiment on the vapour of water had ever
been made, we should be entitled to rank that vapour as our most powerful
absorber of radiant heat. Its attenuation, however, diminishes its action. I
have proved that a shell of air two inches in thickness surrounding our
planet, and saturated with the vapour of sulphuric aether, would intercept 35
per cent. of the earth's radiation. And though the quantity of aqueous vapour
necessary to saturate air is much less than the amount of sulphuric aether
vapour which it can sustain, it is still extremely probable that the estimate
already made of the action of atmospheric vapour within 10 feet of the
earth's surface, is under the mark; and that we are indebted to this wonderful
substance, to an extent not accurately determined, but certainly far beyond
what has hitherto been imagined, for the temperature now existing at the
surface of the globe.

.

.

14. Reciprocity of Radiation and Absorption.

Throughout the reflections which have hitherto occupied us, the image
before the mind has been that of a radiant source sending forth calorific
waves, which on passing among the molecules of a gas or vapour were
intercepted by those molecules in various degrees. In all cases it was the
transference of motion from the aether to the comparatively quiescent
molecules of the gas or vapour that occupied our thoughts. We have now to
change the form of our conception, and to figure these molecules not as
absorbers but as radiators, not as the recipients but as the originators of
wave-motion. That is to say, we must figure them vibrating, and generating

in the surrounding aether undulations which speed through it with the
velocity of light. Our object now is to enquire whether the act of chemical
combination, which proves so potent as regards the phenomena of
absorption, does not also manifest its power in the phenomena of radiation.
For the examination of this question it is necessary, in the first place, to heat
our gases and vapours to the same temperature, and then examine their
power of discharging the motion thus imparted to them upon the aether in
which they swing.

A heated copper ball was placed above a ring gas-burner possessing a great
number of small apertures, the burner being connected by a tube with vessels
containing the various gases to be examined. By gentle pressure the gases
were forced through the orifices of the burner against the copper ball, where
each of them, being heated, rose in an ascending column. A thermoelectric
pile, entirely screened from the hot ball, was exposed to the radiation of the
warm gas, while the deflection of a magnetic needle connected with the pile
declared the energy of the radiation.

By this mode of experiment it was proved that the selfsame molecular
arrangement which renders a gas a powerful absorber, renders it a powerful
radiator — that the atom or molecule which is competent to intercept the
calorific waves is, in the same degree, competent to send them forth. Thus,
while the atoms of elementary gases proved themselves unable to emit any
sensible amount of radiant heat, the molecules of compound gases were
shown to be capable of powerfully disturbing the surrounding aether. By
special modes of experiment the same was proved to hold good for the
vapours of volatile liquids, the radiative power of every vapour being found
proportional to its absorptive power.

The method of experiment here pursued, though not of the simplest
character, is still easy to grasp. When air is permitted to rush into an
exhausted tube, the temperature of the air is raised to a degree equivalent to
the vis viva extinguished. [Footnote: See above for a definition of vis viva.]
Such air is said to be dynamically heated, and, if pure, it shows itself
incompetent to radiate, even when a rock-salt window is provided for the
passage of its rays. But if instead of being empty the tube contain a small
quantity of vapour, the warmed air communicates its heat by contact to the
vapour, the molecules of which convert into the radiant form the heat

imparted to them by the atoms of the air. By this process also, which I have
called Dynamic Radiation, the reciprocity of radiation and absorption has
been conclusively proved.[Footnote: When heated air imparts its motion to
another gas or vapour, the transference of heat is accompanied by a change
of vibrating period. The Dynamic Radiation of vapours is rendered possible
by this transmutation of vibrations.]

In the excellent researches of Leslie, De la Provostaye and Detains, and
Balfour Stewart, the same reciprocity, as regards solid bodies, has been
variously illustrated; while the labours, theoretical and experimental, of
Kirchhoff have given this subject a wonderful expansion, and enriched it by
applications of the highest kind. To their results are now to be added the
foregoing, whereby gases and vapours, which have been hitherto thought
inaccessible to experiments with the thermo-electric pile, are proved by it to
exhibit the indissoluble duality of radiation and absorption, the influence of
chemical combination on both being exhibited in the most decisive and
extraordinary way.

.

.

15. Influence of Vibrating Period and Molecular Form. Physical
Analysis of the Human Breath.

In the foregoing experiments with gases and vapours have employed
throughout invisible rays, and found some of these bodies so impervious to
radiant heat, that lengths of a few feet they intercept every ray as actually as
a layer of pitch. The substances, however, which show themselves thus
opaque to radiant heat perfectly transparent to light. Now the rays of light
differ from those of invisible heat merely in point of period, the former
failing to affect the retina because their periods of recurrence are too slow.
Hence, in one way or other, the transparency of our gases and vapours
depends upon the periods of the waves which impinge upon them. What is
the nature of this dependence? The admirable researches of Kirchhoff help
us an answer. The atoms and molecules of every gas have certain definite
rates of oscillation, and those waves aether are most copiously absorbed
whose periods recurrence synchronise with those of the atomic groups

amongst which they pass. Thus, when we find invisible rays absorbed and
the visible ones transmitted by a layer of gas, we conclude that the
oscillating periods of the atoms constituting the gaseous molecules coincide
with those of the invisible, and not with those of the visible spectrum.

It requires some discipline of the imagination to form a clear picture of this
process. Such a picture is, however, possible, and ought to be obtained.
When the waves of aether impinge upon molecules whose periods of
vibration coincide with the recurrence of the undulations, the timed strokes
of the waves augment the vibration of the molecules, as a heavy pendulum is
set in motion by well-timed puffs of breath. Millions of millions of shocks
are received every second from the calorific waves; and it is not difficult to
see that as every wave arrives just in time to repeat the action of its
predecessor, the molecules must finally be caused to swing through wider
spaces than if the arrivals were not so timed. In fact, it is not difficult to see
that an assemblage of molecules, operated upon by contending waves, might
remain practically quiescent. This is actually the case when the waves of the
visible spectrum pass through a transparent gas or vapour. There is here no
sensible transference of motion from the aether to the molecules; in other
words, there is no sensible absorption of heat.

One striking example of the influence of period may be here recorded.
Carbonic acid gas is one of the feeblest absorbers of the radiant heat emitted
by solid bodies. It is, for example, to a great extent transparent to the rays
emitted by the heated copper plate already referred to. There are, however,
certain rays, comparatively few in number, emitted by the copper, to which
the carbonic acid is impervious; and could we obtain a source of heat
emitting such rays only, we should find carbonic acid more opaque to the
radiation from that source, than any other gas. Such a source is actually
found in the flame of carbonic oxide, where hot carbonic acid constitutes the
main radiating body. Of the rays emitted by our heated plate of copper,
olefiant gas absorbs ten times the quantity absorbed by carbonic acid. Of the
rays emitted by a carbonic oxide flame, carbonic acid absorbs twice as much
as olefiant gas. This wonderful change in the power of the former, as an
absorber, is simply due to the fact, that the periods of the hot and cold
carbonic acid are identical, and that the waves from the flame freely transfer
their motion to the molecules which synchronise with them. Thus it is that
the tenth an atmosphere of carbonic acid, enclosed in a tube four feet long,

absorbs 60 per cent. of the radiation from carbonic oxide flame, while one-
thirtieth of an atmosphere absorbs 48 per cent. of the heat from the same
source.

In fact, the presence of the minutest quantity of carbonic acid may be
detected by its action on the rays from the carbonic oxide flame. Carrying,
for example, the dried human breath into a tube four feet long, the
absorption there effected by the carbonic acid of the breath amounts to 50
per cent. of the entire radiation. Radiant heat may indeed be employed as a
means of determining practically the amount of carbonic acid expired from
the lungs. My late assistant, Mr. Barrett, while under my direction, made this
determination. The absorption produced by the breath freed from its
moisture, but retaining its carbonic acid, was first determined. Carbonic acid,
artificially prepared, was then mixed with dry air in such proportions that the
action of the mixture upon the rays of heat was the same as that of the dried
breath. The percentage of the former being known, immediately gave that of
the latter. The same breath, analysed chemically by Dr. Frankland, and
physically by Mr. Barrett, gave the following results :—

Percentage of Carbonic Acid in the Human Breath.

Chemical analysis Physical analysis

4.66 4.56

5.33 5.22

It is thus proved that in the quantity of aethereal motion which it is
competent to take up, we have a practical measure of the carbonic acid of the
breath, and hence of the combustion going on in the human lungs.

Still this question of period, though of the utmost importance, is not
competent to account for the whole of the observed facts. The aether, as far
as we know, accepts vibrations of all periods with the same readiness. To it

the oscillations of an atom of free oxygen are just as acceptable as those of
the atoms in a molecule of olefiant gas; that the vibrating oxygen then stands
so far below the olefiant gas in radiant power must be referred not to period,
but to some other peculiarity. The atomic group which constitutes the
molecule of olefiant gas, produces many thousand times the disturbance
caused by the oxygen, it may be because the group is able to lay a vastly
more powerful hold upon the aether than the single atoms can. Another, and
probably very potent cause of the difference may be, that the vibrations,
being those of the constituent atoms of the molecule, [Footnote: See
'Physical Considerations,' Art. iv.] are generated in highly condensed aether,
which acts like condensed air upon sound. But whatever may be the fate of
these attempts to visualise the physics of the process, it will still remain true,
that to account for the phenomena of radiation and absorption we must take
into consideration the shape, size, and condition of the aether within the
molecules, by which the external aether is disturbed.

.

.

16. Summary and Conclusion.

Let us now cast a momentary glance over the ground that we have left
behind. The general nature of light and heat was first briefly described: the
compounding of matter from elementary atoms, and the influence of the act
of combination on radiation and absorption, were considered and
experimentally illustrated. Through the transparent elementary gases radiant
heat was found to pass as through a vacuum, while many of the compound
gases presented almost impassable obstacles to the calorific-waves. This
deportment of the simple gases directed our attention to other elementary
bodies, the examination of which led to the discovery that the element
iodine, dissolved in bisulphide of carbon, possesses the power of detaching,
with extraordinary sharpness, the light of the spectrum from its heat,
intercepting all luminous rays up to the extreme red, and permitting the
calorific rays beyond the red to pass freely through it. This substance was
then employed to filter the beams of the electric light, and to form foci of
invisible rays so intense as to produce almost all the effects obtainable in
ordinary fire. Combustible bodies were burnt, and refractory ones were

raised to a white heat, by the concentrated invisible rays. Thus, by exalting
their refrangibility, the invisible rays of the electric light were rendered
visible, and all the colours of the solar spectrum were extracted from utter
darkness. The extreme richness of the electric light in invisible rays of low
refrangibility was demonstrated, one-eighth only of its radiation consisting
of luminous rays. The deadness of the optic nerve to those invisible rays was
proved, and experiments were then added to show that the bright and the
dark rays of a solid body, raised gradually to incandescence, are
strengthened together; intense dark heat being an invariable accompaniment
of intense white heat. A sun could not be formed, or a meteorite rendered
luminous, on any other condition. The light-giving rays constituting only a
small fraction of the total radiation, their unspeakable importance to us is
due to the fact, that their periods are attuned to the special requirements of
the eye.

Among the vapours of volatile liquids vast differences were also found to
exist, as regards their powers of absorption. We followed various molecules
from a state of liquid to a state of gas, and found, in both states of
aggregation, the power of the individual molecules equally asserted. The
position of a vapour as an absorber of radiant heat was shown to be
determined by that of the liquid from which it is derived. Reversing our
conceptions, and regarding the molecules of gases and vapours not as the
recipients but as the originators of wave-motion; not as absorbers but as
radiators; it was proved that the powers of absorption and radiation went
hand in hand, the self-same chemical act which rendered a body competent
to intercept the waves of aether, rendering it competent, in the same degree,
to generate them. Perfumes were next subjected to examination, and,
notwithstanding their extraordinary tenuity, they were found vastly superior,
in point of absorptive power, to the body of the air in which they were
diffused. We were led thus slowly up to the examination of the most widely
diffused and most important of all vapours — the aqueous vapour of our
atmosphere, and we found in it a potent absorber of the purely calorific rays.
The power of this substance to influence climate, and its general influence
on the temperature of the earth, were then briefly dwelt upon. A cobweb
spread above a blossom is sufficient to protect it from nightly chill; and thus
the aqueous vapour of our air, attenuated as it is, checks the drain of
terrestrial heat, and saves the surface of our planet from the refrigeration
which would assuredly accrue, were no such substance interposed between it

and the voids of space. We considered the influence of vibrating period, and
molecular form, on absorption and radiation, and finally deduced, from its
action upon radiant heat, the exact amount of carbonic acid expired by the
human lungs.

Thus, in brief outline, were placed before you some of the results of recent
enquiries in the domain of Radiation, and my aim throughout has been to
raise in your minds distinct physical images of the various processes
involved in our researches. It is thought by some that natural science has a
deadening influence on the imagination, and a doubt might fairly be raised
as to the value of any study which would necessarily have this effect. But the
experience of the last hour must, I think, have convinced you, that the study
of natural science goes hand in hand with the culture of the imagination.
Throughout the greater part of this discourse we have been sustained by this
faculty. We have been picturing atoms, and molecules, and vibrations, and
waves, which eye has never seen nor ear heard, and which can only be
discerned by the exercise of imagination. This, in fact, is the faculty which
enables us transcend the boundaries of sense, and connect the phenomena of
our visible world with those of an invisible one. Without imagination we
never could have risen to the conceptions which have occupied us here
today; and in proportion to your power of exercising this faculty aright, and
of associating definite mental images with the terms employed, will be the
pleasure and the profit which you will derive from this lecture.

The outward facts of nature are insufficient to satisfy the mind. We cannot be
content with knowing that the light and heat of the sun illuminate and warm
the world. We are led irresistibly to enquire, 'What is light, and what is heat?'
and this question leads us at once out of the region of sense into that of
imagination. [Footnote: This line of thought was pursued further five years
subsequently. See 'Scientific Use of the Imagination' in Vol. II.]

Thus pondering, and questioning, and striving to supplement that which is
felt and seen, but which is incomplete, by something unfelt and unseen
which is necessary to its completeness, men of genius have in part discerned,
not only the nature of light and heat, but also, through them, the general
relationship of natural phenomena. The working power of Nature consists of
actual or potential motion, of which all its phenomena are but special forms.
This motion manifests itself in tangible and in intangible matter, being

incessantly transferred from the one to the other, and incessantly transformed
by the change. It is as real in the waves of the aether as in the waves of the
sea; the latter — derived as they are from winds, which in their turn are
derived from the sun — are, indeed, nothing more than the heaped-up
motion of the aether waves. It is the calorific waves emitted by the sun
which heat our air, produce our winds, and hence agitate our ocean. And
whether they break in foam upon the shore, or rub silently against the
ocean's bed, or subside by the mutual friction of their own parts, the sea
waves, which cannot subside without producing heat, finally resolve
themselves into waves of aether, thus regenerating the motion from which
their temporary existence was derived. This connection is typical. Nature is
not an aggregate of independent parts, but an organic whole. If you open a
piano and sing into it, a certain string will respond. Change the pitch of our
voice; the first string ceases to vibrate, but another replies. Change again the
pitch; the first two strings are silent, while another resounds. Thus is sentient
man acted on by Nature, the optic, the auditory, and other nerves of the
human body being so many strings differently tuned, and responsive to
different forms of the universal power.

.

.

.

.

.

.

III ON RADIANT HEAT IN RELATION TO THE COLOUR AND
CHEMICAL CONSTITUTION OF BODIES.

[Footnote: A discourse delivered in the Royal Institution of Great Britain,
Jan. 19, 1866.]

ONE of the most important functions of physical science, considered as a
discipline of the mind, is to enable us by means of the sensible processes of

Nature to apprehend the insensible. The sensible processes give direction to
the line of thought; but this once given, the length of the line is not limited
by the boundaries of the senses. Indeed, the domain of the senses, in Nature,
is almost infinitely small in comparison with the vast region accessible to
thought which lies beyond them. From a few observations of a comet, when
it comes within the range of his telescope, an astronomer can calculate its
path in regions which no telescope can reach: and in like manner, by means
of data furnished in the narrow world of the senses, we make ourselves at
home in other and wider worlds, which are traversed by the intellect alone.

From the earliest ages the questions, 'What is light?' and 'What is heat?' have
occurred to the minds of men; but these questions never would have been
answered had they not been preceded by the question, 'What is sound?' Amid
the grosser phenomena of acoustics the mind was first disciplined,
conceptions being thus obtained from direct observation, which were
afterwards applied to phenomena of a character far too subtle to be observed
directly. Sound we know to be due to vibratory motion. A vibrating tuning-
fork, for example, moulds the air around it into undulations or waves, which
speed away on all sides with a certain measured velocity, impinge upon the
drum of the ear, shake the auditory nerve, and awake in the brain the
sensation of sound. When sufficiently near a sounding body we can feel the
vibrations of the air. A deaf man, for example, plunging his hand into a bell
when it is sounded, feels through the common nerves of his body those
tremors which, when imparted to the nerves of healthy ears, are translated
into sound. There are various ways of rendering those sonorous vibrations
not only tangible but visible; and it was not until numberless experiments of
this kind had been executed, that the scientific investigator abandoned
himself wholly, and without a shadow of misgiving, to the conviction that
what is sound within us is, outside of us, a motion of the air.

But once having established this fact — once having proved beyond all
doubt that the sensation of sound is produced by an agitation of the auditory
nerve — the thought soon suggested itself that light might be due to an
agitation of the optic nerve. This was a great step in advance of that ancient
notion which regarded light as something emitted by the eye, and not as
anything imparted to it. But if light be produced by an agitation of the retina,
what is it that produces the agitation? Newton, you know, supposed minute
particles to be shot through the humours of the eye against the retina, which

he supposed to hang like a target at the back of the eye. The impact of these
particles against the target, Newton believed to be the cause of light. But
Newton's notion has not held its ground, being entirely driven from the field
by the more wonderful and far more philosophical notion that light, like
sound, is a product of wave-motion.

The domain in which this motion of light is carried on lies entirely beyond
the reach of our senses. The waves of light require a medium for their
formation and propagation; but we cannot see, or feel, or taste, or smell this
medium. How, then, has its existence been established? By showing, that by
the assumption of this wonderful intangible aether, all the phenomena of
optics are accounted for, with a fulness, and clearness, and conclusiveness,
which leave no desire of the intellect unsatisfied. When the law of
gravitation first suggested itself to the mind of Newton, what did he do? He
set himself to examine whether it accounted for all the facts. He determined
the courses of the planets; he calculated the rapidity of the moon's fall
towards the earth; he considered the precession of the equinoxes, the ebb and
flow of the tides, and found all explained by the law of gravitation. He
therefore regarded this law as established, and the verdict of science
subsequently confirmed his conclusion. On similar, and, if possible, on
stronger grounds, we found our belief in the existence of the universal
aether. It explains facts far more various and complicated than those on
which Newton based his law. If a single phenomenon could be pointed out
which the aether is proved incompetent to explain, we should have to give it
up; but no such phenomenon has ever been pointed out. It is, therefore, at
least as certain that space is filled with a medium, by means of which suns
and stars diffuse their radiant power, as that it is traversed by that force
which holds in its grasp, not only our planetary system, but the
immeasurable heavens themselves.

There is no more wonderful instance than this of the production of a line of
thought, from the world of the senses into the region of pure imagination. I
mean by imagination here, not that play of fancy which can give to airy
nothings a local habitation and a name, but that power which enables the
mind to conceive realities which lie beyond the range of the senses — to
present to itself distinct images of processes which, though mighty in the
aggregate beyond all conception, are so minute individually as to elude all
observation. It is the waves of air excited by a tuning-fork which render its

vibrations audible. It is the waves of aether sent forth from those lamps
overhead which render them luminous to us; but so minute are these waves,
that it would take from 30,000 to 60,000 of them placed end to end to cover
a single inch. Their number, however, compensates for their minuteness.
Trillions of them have entered your eyes, and hit the retina at the backs of
your eyes, in the time consumed in the utterance of the shortest sentence of
this discourse. This is the steadfast result of modern research; but we never
could have reached it without previous discipline. We never could have
measured the waves of light, nor even imagined them to exist, had we not
previously exercised ourselves among the waves of sound. Sound and light
are now mutually helpful, the conceptions of each being expanded,
strengthened, and defined by the conceptions of the other.

The aether which conveys the pulses of light and heat not only fills celestial
space, swathing suns, and planets, and moons, but it also encircles the atoms
of which these bodies are composed. It is the motion of these atoms, and not
that of any sensible parts of bodies, that the aether conveys. This motion is
the objective cause of what, in our sensations, are light and heat. An atom,
then, sending its pulses through the aether, resembles a tuning-fork sending
its pulses through the air. Let us look for a moment at this thrilling medium,
and briefly consider its relation to the bodies whose vibrations it conveys.
Different bodies, when heated to the same temperature, possess very
different powers of agitating the aether: some are good radiators, others are
bad radiators; which means that some are so constituted as to communicate
their atomic motion freely to the aether, producing therein powerful
undulations; while the atoms of others are unable thus to communicate their
motions, but glide through the medium without materially disturbing its
repose. Recent experiments have proved that elementary bodies, except
under certain anomalous conditions, belong to the class of bad radiators. An
atom, vibrating in the aether, resembles a naked tuning-fork vibrating in the
air. The amount of motion communicated to the air by the thin prongs is too
small to evoke at any distance the sensation of sound. But if we permit the
atoms to combine chemically and form molecules, the result, in many cases,
is an enormous change in the power of radiation. The amount of aethereal
disturbance, produced by the combined atoms of a body, may be many
thousand times that produced by the same atoms when uncombined.

The pitch of a musical note depends upon the rapidity of its vibrations, or, in
other words, on the length of its waves. Now, the pitch of a note answers to
the colour of light. Taking a slice of white light from the sun, or from an
electric lamp, and causing the light to pass through an arrangement of
prisms, it is decomposed. We have the effect obtained by Newton, who first
unrolled the solar beam into the splendours of the solar spectrum. At one end
of this spectrum we have red light, at the other, violet; and between those
extremes lie the other prismatic colours. As we advance along the spectrum
from the red to the violet, the pitch of the light — if I may use the expression
— heightens, the sensation of violet being produced by a more rapid
succession of impulses than that which produces the impression of red. The
vibrations of the violet are about twice as rapid as those of the red; in other
words, the range of the visible spectrum is about an octave.

There is no solution of continuity in this spectrum one colour changes into
another by insensible gradations. It is as if an infinite number of tuning-
forks, of gradually augmenting pitch, were vibrating at the same time. But
turning to another spectrum — that, namely, obtained from the incandescent
vapour of silver — you observe that it consists of two narrow and intensely
luminous green bands. Here it is as if two forks only, of slightly different
pitch, were vibrating. The length of the waves which produce this first band
is such that 47,460 of them, placed end to end, would fill an inch. The waves
which produce the second band are a little shorter; it would take of these
47,920 to fill an inch. In the case of the first band, the number of impulses
imparted, in one second, to every eye which sees it, is 677 millions of
millions; while the number of impulses imparted, in the same time, by the
second band is 600 millions of millions. We may project upon a white screen
the beautiful stream of green light from which these bands were derived.
This luminous stream is the incandescent vapour of silver. The rates of
vibration of the atoms of that vapour are as rigidly fixed as those of two
tuning-forks; and to whatever height the temperature of the vapour may be
raised, the rapidity of its vibrations, and consequently its colour, which
wholly depends upon that rapidity, remain unchanged.

The vapour of water, as well as the vapour of silver, has its definite periods
of vibration, and these are such as to disqualify the vapour, when acting
freely as such, from being raised to a white heat. The oxyhydrogen flame,
for example, consists of hot aqueous vapour. It is scarcely visible in the air

of this room, and it would be still less visible if we could burn the gas in a
clean atmosphere. But the atmosphere, even at the summit of Mont Blanc, is
dirty; in London it is more than dirty; and the burning dirt gives to this flame
the greater portion of its present light. But the heat of the flame is enormous.
Cast iron fuses at a temperature of 2,000° Fahr.; while the temperature of the
oxyhydrogen flame is 6,000° Fahr. A piece of platinum is heated to vivid
redness, at a distance of two inches beyond the visible termination of the
flame. The vapour which produces incandescence is here absolutely dark. In
the flame itself the platinum is raised to dazzling whiteness, and is even
pierced by the flame. When this flame impinges on a piece of lime, we have
the dazzling Drummond light. But the light is here due to the fact that when
it impinges upon the solid body, the vibrations excited in that body by the
flame are of periods different from its own.

Thus far we have fixed our attention on atoms and molecules in a state of
vibration, and surrounded by a medium which accepts their vibrations, and
transmits them through space. But suppose the waves generated by one
system of molecules to impinge upon another system, how will the waves be
affected? Will they be stopped, or will they be permitted to pass? Will they
transfer their motion to the molecules on which they impinge, or will they
glide round the molecules, through the intermolecular spaces, and thus
escape?

The answer to this question depends upon a condition which may be
beautifully exemplified by an experiment on sound. These two tuning-forks
are tuned absolutely alike. They vibrate with the same rapidity, and, mounted
thus upon their resonant cases, you hear them loudly sounding the same
musical note. Stopping one of the forks, I throw the other into strong
vibration, and bring that other near the silent fork, but not into contact with
it. Allowing them to continue in this position for four or five seconds, and
then stopping the vibrating fork, the sound does not cease. The second fork
has taken up the vibrations of its neighbour, and is now sounding in its turn.
Dismounting one of the forks, and permitting the other to remain upon its
stand, I throw the dismounted fork into strong vibration. You cannot hear it
sound. Detached from its case, the amount of motion which it can
communicate to the air is too small to be sensible at any distance. When the
dismounted fork is brought close to the mounted one, but not into actual
contact with it, out of the silence rises a mellow sound. Whence comes it?

From the vibrations which have been transferred from the dismounted fork
to the mounted one.

That the motion should thus transfer itself through the air it is necessary that
the two forks should be in perfect unison. If a morsel of wax not larger than
a pea be placed on one of the forks, it is rendered thereby powerless to
affect, or to be affected by, the other. It is easy to understand this experiment.
The pulses of the one fork can affect the other, because they are perfectly
timed. A single pulse causes the prong of the silent fork to vibrate through an
infinitesimal space. But just as it has completed this small vibration another
pulse is ready to strike it. Thus, the impulses add themselves together. In the
five seconds during which the forks were held near each other, the vibrating
fork sent 1,280 waves against its neighbour and those 1,280 shocks, all
delivered at the proper moment, all, as I have said, perfectly timed, have
given such strength to the vibrations of the mounted fork as to render them
audible to all.

Another curious illustration of the influence of synchronism on musical
vibrations, is this: Three small gas-flames are inserted into three glass tubes
of different lengths. Each of these flames can be caused to emit a musical
note, the pitch of which is determined by the length of the tube surrounding
the flame. The shorter the tube the higher is the pitch. The flames are now
silent within their respective tubes, but each of them can be caused to
respond to a proper note sounded anywhere in this room. With an instrument
called a syren, a powerful musical note, of gradually increasing pitch, can be
produced. Beginning with a low note, and ascending gradually to a higher
one, we finally attain the pitch of the flame in the longest tube. The moment
it is reached, the flame bursts into song. The other flames are still silent
within their tubes. But by urging the instrument on to higher notes, the
second flame is started, and the third alone remains. A still higher note starts
it also. Thus, as the sound of the syren rises gradually in pitch, it awakens
every flame in passing, by striking it with a series of waves whose periods of
recurrence are similar to its own.

Now the wave-motion from the syren is in part taken up by the flame which
synchronises with the waves; and were these waves to impinge upon a
multitude of flames, instead of upon one flame only, the transference might
be so great as to absorb the whole of the original wave motion. Let us apply

these facts to radiant heat. This blue flame is the flame of carbonic oxide;
this transparent gas is carbonic acid gas. In the blue flame we have carbonic
acid intensely heated, or, in other words, in a state of intense vibration. It
thus resembles the sounding fork, while this cold carbonic acid resembles
the silent one. What is the consequence? Through the synchronism of the hot
and cold gas, the waves emitted by the former are intercepted by the latter,
the transmission of the radiant heat being thus prevented. The cold gas is
intensely opaque to the radiation from this particular flame, though highly
transparent to heat of every other kind. We are here manifestly dealing with
that great principle which lies at the basis of spectrum analysis, and which
has enabled scientific men to determine the substances of which the sun, the
stars, and even the nebulae are composed; the principle, namely, that a body
which is competent to emit any ray, whether of heat or light, is competent in
the same degree to absorb that ray. The absorption depends on the
synchronism existing between the vibrations of the atoms from which the
rays, or more correctly the waves, issue, and those of the atoms on which
they impinge.

To its almost total incompetence to emit white light, aqueous vapour adds a
similar incompetence to absorb white light. It cannot, for example, absorb
the luminous rays of the sun, though it can absorb the non-luminous rays of
the earth. This incompetence of the vapour to absorb luminous rays is shared
by water and ice — in fact, by all really transparent substances. Their
transparency is due to their inability to absorb luminous rays. The molecules
of such substances are in dissonance with luminous waves; and hence such
waves pass through transparent bodies without disturbing the molecular rest.
A purely luminous beam, however intense may be its heat, is sensibly
incompetent to melt ice. We can, for example, converge a powerful luminous
beam upon a surface covered with hoar frost, without melting a single
spicula of the crystals. How then, it may be asked, are the snows of the Alps
swept away by the sunshine of summer? I answer, they are not swept away
by sunshine at all, but by rays which have no sunshine whatever in them.
The luminous rays of the sun fall upon the snow-fields and are flashed in
echoes from crystal to crystal, but they find next to no lodgment within the
crystals. They are hardly at all absorbed, and hence they cannot produce
fusion. But a body of powerful dark rays is emitted by the sun; and it is these
that cause the glaciers to shrink and the snows to disappear; it is they that fill

the banks of the Arve and Arveyron, and liberate from their frozen captivity
the Rhone and the Rhine.

Placing a concave silvered mirror behind the electric light its rays are
converged to a focus of dazzling brilliancy. Placing in the path of the rays,
between the light and the focus, a vessel of water, and introducing at the
focus a piece of ice, the ice is not melted by the concentrated beam.
Matches, at the same place, are ignited, and wood is set on fire. The
powerful heat, then, of this luminous beam is incompetent to melt the ice.
On withdrawing the cell of water, the ice immediately liquefies, and the
water trickles from it in drops. Reintroducing the cell of water, the fusion is
arrested, and the drops cease to fall. The transparent water of the cell exerts
no sensible absorption on the luminous rays, still it withdraws something
from the beam, which, when permitted to act, is competent to melt the ice.
This something is the dark radiation of the electric light. Again, I place a slab
of pure ice in front of the electric lamp; send a luminous beam first through
our cell of water and then through the ice. By means of a lens an image of
the slab is cast upon a white screen. The beam, sifted by the water, has little
power upon the ice. But observe what occurs when the water is removed; we
have here a star and there a star, each star resembling a flower of six petals,
and growing visibly larger before our eyes. As the leaves enlarge, their edges
become serrated, but there is no deviation from the six-rayed type. We have
here, in fact, the crystallisation of the ice reversed by the invisible rays of the
electric beam. They take the molecules down in this wonderful way, and
reveal to us the exquisite atomic structure of the substance with which
Nature every winter roofs our ponds and lakes.

Numberless effects, apparently anomalous, might be adduced in illustration
of the action of these lightless rays. These two powders, for example, are
both white, and undistinguishable from each other by the eye. The luminous
rays of the sun are unabsorbed by both — from such rays these powders
acquire no heat; still one of them, sugar, is heated so highly by the
concentrated beam of the electric lamp, that it first smokes and then
violently inflames, while the other substance, salt, is barely warmed at the
focus. Placing two perfectly transparent liquids in test-tubes at the focus, one
of them boils in a couple of seconds, while the other, in a similar position, is
hardly warmed. The boiling-point of the first liquid is 78°C., which is
speedily reached; that of the second liquid is only 48°C., which is never

reached at all. These anomalies are entirely due to the unseen element which
mingles with the luminous rays of the electric beam, and indeed constitutes
90 per cent. of its calorific power.

A substance, as many of you know, has been discovered, by which these
dark rays may be detached from the total emission of the electric lamp. This
ray-filter is a liquid, black as pitch to the luminous, but bright as a diamond
to the non-luminous, radiation. It mercilessly cuts off the former, but allows
the latter free transmission. When these invisible rays are brought to a focus,
at a distance of several feet from the electric lamp, the dark rays form an
invisible image of their source. By proper means, this image may be
transformed into a visible one of dazzling brightness. It might, moreover, be
shown, if time permitted, how, out of those perfectly dark rays, could be
extracted, by a process of transmutation, all the colours of the solar
spectrum. It might also be proved that those rays, powerful as they are, and
sufficient to fuse many metals, can be permitted to enter the eye, and to
break upon the retina, without producing the least luminous impression.

The dark rays being thus collected, you see nothing at their place of
convergence. With a proper thermometer it could be proved that even the air
at the focus is just as cold as the surrounding air. And mark the conclusion to
which this leads. It proves the aether at the focus to be practically detached
from the air, — that the most violent aethereal motion may there exist,
without the least aerial motion. But, though you see it not, there is sufficient
heat at that focus to set London on fire. The heat there is competent to raise
iron to a temperature at which it throws off brilliant scintillations. It can heat
platinum to whiteness, and almost fuse that refractory metal. It actually can
fuse gold, silver, copper, and aluminium. The moment, moreover, that wood
is placed at the focus it bursts into a blaze.

It has been already affirmed that, whether as regards radiation or absorption,
the elementary atoms possess but little power. This might be illustrated by a
long array of facts; and one of the most singular of these is furnished by the
deportment of that extremely combustible substance, phosphorus, when
placed at the dark focus. It is impossible to ignite there a fragment of
amorphous phosphorus. But ordinary phosphorus is a far quicker
combustible, and its deportment towards radiant heat is still more
impressive. It may be exposed to the intense radiation of an ordinary fire

without bursting into flame. It may also be exposed for twenty or thirty
seconds at an obscure focus, of sufficient power to raise platinum to a red
heat, without ignition. Notwithstanding the energy of the aethereal waves
here concentrated, notwithstanding the extremely inflammable character of
the elementary body exposed to their action, the atoms of that body refuse to
partake of the motion of the powerful waves of low refrangibility, and
consequently cannot be affected by their heat.

The knowledge we now possess will enable us to analyse with profit a
practical question. White dresses are worn in summer, because they are
found to be cooler than dark ones. The celebrated Benjamin Franklin placed
bits of cloth of various colours upon snow, exposed them to direct sunshine,
and found that they sank to different depths in the snow. The black cloth
sank deepest, the white did not sink at all. Franklin inferred from this
experiment that black bodies are the best absorbers, and white ones the worst
absorbers, of radiant heat. Let us test the generality of this conclusion. One
of these two cards is coated with a very dark powder, and the other with a
perfectly white one. I place the powdered surfaces before a fire, and leave
them there until they have acquired as high a temperature as they can attain
in this position. Which of the cards is then most highly heated? It requires no
thermometer to answer this question. Simply pressing the back of the card,
on which the white powder is strewn, against the cheek or forehead, it is
found intolerably hot. Placing the dark card in the same position, it is found
cool. The white powder has absorbed far more heat than the dark one. This
simple result abolishes a hundred conclusions which have been hastily
drawn from the experiments of Franklin. Again, here are suspended two
delicate mercurial thermometers at the same distance from a gas-flame. The
bulb of one of them is covered by a dark substance, the bulb of the other by a
white one. Both bulbs have received the radiation from the flame, but the
white bulb has absorbed most, and its mercury stands much higher than that
of the other thermometer. This experiment might be varied in a hundred
ways: it proves that from the darkness of a body you can draw no certain
conclusion regarding its power of absorption.

The reason of this simply is, that colour gives us intelligence of only one
portion, and that the smallest one, of the rays impinging on the coloured
body. Were the rays all luminous, we might with certainty infer from the
colour of a body its power of absorption; but the great mass of the radiation

from our fire, our gas-flame, and even from the sun itself, consists of
invisible calorific rays, regarding which colour teaches us nothing. A body
may be highly transparent to the one class of rays, and highly opaque to the
other. Thus the white powder, which has shown itself so powerful an
absorber, has been specially selected on account of its extreme perviousness
to the visible rays, and its extreme imperviousness to the invisible ones;
while the dark powder was chosen on account of its extreme transparency to
the invisible, and its extreme opacity to the visible, rays. In the case of the
radiation from our fire, about 98 per cent of the whole emission consists of
invisible rays; the body, therefore, which was most opaque to these
triumphed as an absorber, though that body was a white one.

And here it is worth while to consider the manner in which we obtain from
natural facts what may be called their intellectual value. Throughout the
processes of Nature we have interdependence and harmony; and the main
value of physics, considered as a mental discipline, consists in the tracing
out of this interdependence, and the demonstration of this harmony. The
outward and visible phenomena are the counters of the intellect; and our
science would not be worthy of its name and fame if it halted at facts,
however practically useful, and neglected the laws which accompany and
rule the phenomena. Let us endeavour, then, to extract from the experiment
of Franklin all that it can yield, calling to our aid the knowledge which our
predecessors have already stored. Let us imagine two pieces of cloth of the
same texture, the one black and the other white, placed upon sunned snow.
Fixing our attention on the white piece, let us enquire whether there is any
reason to expect that it will sink in the snow at all. There is knowledge at
hand which enables us to reply at once in the negative. There is, on the
contrary, reason to expect that, after a sufficient exposure, the bit of cloth
will be found on an eminence instead of in a hollow; that instead of a
depression, we shall have a relative elevation of the bit of cloth. For, as
regards the luminous rays of the sun, the cloth and the snow are alike
powerless; the one cannot be warmed, nor the other melted, by such rays.
The cloth is white and the snow is white, because their confusedly mingled
fibres and particles are incompetent to absorb the luminous rays. Whether,
then, the cloth will sink or not depends entirely upon the dark rays of the
sun. Now the substance which absorbs these dark rays with the greatest
avidity is ice, — or snow, which is merely ice in powder. Hence, a less
amounts of heat will be lodged in the cloth than in the surrounding snow.

The cloth must therefore act as a shield to the snow on which it rests; and, in
consequence of the more rapid fusion of the exposed snow, its shield must,
in due time, be left behind, perched upon an eminence like a glacier-table.

But though the snow transcends the cloth, both as a radiator and absorber, it
does not much transcend it. Cloth is very powerful in both these respects.
Let us now turn our attention to the piece of black cloth, the texture and
fabric of which I assume to be the same as that of the white. For our object
being to compare the effects of colour, we must, in order to study this effect
in its purity, preserve all the other conditions constant. Let us then suppose
the black cloth to be obtained from the dyeing of the white. The cloth itself,
without reference to the dye, is nearly as good an absorber of heat as the
snow around it. But to the absorption of the dark solar rays by the undyed
cloth, is now added the absorption of the whole of the luminous rays, and
this great additional influx of heat is far more than sufficient to turn the
balance in favour of the black cloth. The sum of its actions on the dark and
luminous rays, exceeds the action of the snow on the dark rays alone. Hence
the cloth will sink in the snow, and this is the complete analysis of Franklin's
experiments.

Throughout this discourse the main stress has been laid on chemical
constitution, as influencing most powerfully the phenomena of radiation and
absorption.

With regard to gases and vapours, and to the liquids from which these
vapours are derived, it has been proved by the most varied and conclusive
experiments that the acts of radiation and absorption are molecular — that
they depend upon chemical, and not upon mechanical, condition. In
attempting to extend this principle to solids I was met by a multitude of
facts, obtained by celebrated experimenters, which seemed flatly to forbid
such an extension. Mellon, for example, had found the same radiant and
absorbent power for chalk and lamp-black. MM. Masson and Courtépée had
performed a most elaborate series of experiments on chemical precipitates of
various kinds, and found that they one and all manifested the same power of
radiation. They concluded from their researches, that when bodies are
reduced to an extremely fine state of division, the influence of this state is so
powerful as entirely to mask and override whatever influence may be due to
chemical constitution.

But it appears to me that through the whole of these researches an oversight
has run, the mere mention of which will show what caution is essential in the
operations of experimental philosophy; while an experiments or two will
make clear wherein the oversight consists. Filling a brightly polished metal
cube with boiling water, I determine the quantity of heat emitted by two of
the bright surfaces. As a radiator of heat one of them far transcends the other.
Both surfaces appear to be metallic; what, then, is the cause of the observed
difference in their radiative power? Simply this: one of the surfaces is coated
with transparent gum, through which, of course, is seen the metallic lustre
behind; and this varnish, though so perfectly transparent to luminous rays, is
as opaque as pitch, or lamp-black, to non-luminous ones. It is a powerful
emitter of dark rays; it is also a powerful absorber. While, therefore, at the
present moment, it is copiously pouring forth radiant heat itself, it does not
allow a single ray from the metal behind to pass through it. The varnish then,
and not the metal, is the real radiator.

Now Melloni, and Masson, and Courtépée experimented thus: they mixed
their powders and precipitates with gum-water, and laid them, by means of a
brush, upon the surfaces of a cube like this. True, they saw their red powders
red, their white ones white, and their black ones black, but they saw these
colours through the coat of varnish which surrounded every particle. When,
therefore, it was concluded that colour had no influence on radiation, no
chance had been given to it of asserting its influence; when it was found that
all chemical precipitates radiated alike, it was the radiation from a varnish,
common to them all, which showed the observed constancy. Hundreds,
perhaps thousands, of experiments on radiant heat have been performed in
this way, by various enquirers, but the work will, I fear, have to be done over
again. I am not, indeed, acquainted with an instance in which an oversight of
so trivial a character has been committed by so many able men in
succession, vitiating so large an amounts of otherwise excellent work.
Basing our reasonings thus on demonstrated facts, we arrive at the extremely
probable conclusion that the envelope of the particles, and not the particles
themselves, was the real radiator in the experiments just referred to. To
reason thus, and deduce their more or less probable consequences from
experimental facts, is an incessant exercise of the student of physical
science. But having thus followed, for a time, the light of reason alone
through a series of phenomena, and emerged from them with a purely

intellectual conclusion, our duty is to bring that conclusion to an
experimental test. In this way we fortify our science.

For the purpose of testing our conclusion regarding the influence of the gum,
I take two powders presenting the same physical appearance; one of them is
a compound of mercury, and the other a compound of lead. On two surfaces
of a cube are spread these bright red powders, without varnish of any kind.
Filling the cube with boiling water, and determining the radiation from the
two surfaces, one of them is found to emit thirty-nine units of heat, while the
other emits seventy-four. This, surely, is a great difference. Here, however, is
a second cube, having two of its surfaces coated with the same powders, the
only difference being that the powders are laid on by means of a transparent
gum. Both surfaces are now absolutely alike in radiative power. Both of
them emit somewhat more than was emitted by either of the unvarnished
powders, simply because the gum employed is a better radiator than either of
them. Excluding all varnish, and comparing white with white, vast
differences are found; comparing black with black, they are also different;
and when black and white are compared, in some cases the black radiates far
more than the white, while in other cases the white radiates far more than the
black. Determining, moreover, the absorptive power of those powders, it is
found to go hand-in-hand with their radiative power. The good radiator is a
good absorber, and the bad radiator is a bad absorber. From all this it is
evident that as regards the radiation and absorption of non-luminous heat,
colour teaches us nothing; and that even as regards the radiation of the sun,
consisting as it does mainly of non-luminous rays, conclusions as to the
influence of colour may be altogether delusive. This is the strict scientific
upshot of our researches. But it is not the less true that in the case of wearing
apparel — and this for reasons which I have given in analysing the
experiments of Franklin — black dresses are more potent than white ones as
absorbers of solar heat.

Thus, in brief outline, have been brought before you a few of the results of
recent enquiry. If you ask me what is the use of them, I can hardly answer
you, unless you define the term use. If you meant to ask whether those dark
rays which clear away the Alpine snows, will ever be applied to the roasting
of turkeys, or the driving of steam-engines — while affirming their power to
do both, I would frankly confess that they are not at present capable of
competing profitably with coal in these particulars. Still they may have great

uses unknown to me; and when our coal-fields are exhausted, it is possible
that a more aethereal race than we are may cook their victuals, and perform
their work, in this transcendental way. But is it necessary that the student of
science should have his labours tested by their possible practical
applications? What is the practical value of Homer's Iliad? You smile, and
possibly think that Homer's Iliad is good as a means of culture. There's the
rub. The people who demand of science practical uses, forget, or do not
know, that it also is great as a means of culture — that the knowledge of this
wonderful universe is a thing profitable in itself, and requiring no practical
application to justify its pursuit.

But while the student of Nature distinctly refuses to have his labours judged
by their practical issues, unless the term practical be made to include mental
as well as material good, he knows full well that the greatest practical
triumphs have been episodes in the search after pure natural truth. The
electric telegraph is the standing wonder of this age, and the men whose
scientific knowledge, and mechanical skill, have made the telegraph what it
is, are deserving of all honour. In fact, they have had their reward, both in
reputation and in those more substantial benefits which the direct service of
the public always carries in its train. But who, I would ask, put the soul into
this telegraphic body? Who snatched from heaven the fire that flashes along
the line? This, I am bound to say, was done by two men, the one a dweller in
Italy, [Footnote: Volta] the other a dweller in England, [Footnote: Faraday]
who never in their enquiries consciously set a practical object before them
— whose only stimulus was the fascination which draws the climber to a
never-trodden peak, and would have made Caesar quit his victories for the
sources of the Nile. That the knowledge brought to us by those prophets,
priests, and kings of science is what the world calls 'useful knowledge,' the
triumphant application of their discoveries proves. But science has another
function to fulfil, in the storing and the training of the human mind; and I
would base my appeal to you on the specimen which has this evening been
brought before you, whether any system of education at the present day can
be deemed even approximately complete, in which the knowledge of Nature
is neglected or ignored.

.

.

.

.

--------------------

.

.

IV. NEW CHEMICAL REACTIONS PRODUCED BY LIGHT.

1868-69.

1. DECOMPOSITION BY LIGHT.

MEASURED by their power, not to excite vision, but to produce heat — in
other words, measured by their absolute energy — the ultra-red waves of the
sun and of the electric light, as shown in the preceding articles, far transcend
the visible. In the domain of chemistry, however, there are numerous cases in
which the more powerful waves are ineffectual, while the more minute
waves, through what may be called their timeliness of application, are able
to produce great effects. A series of these, of a novel and beautiful character,
discovered in 1868, and further illustrated in subsequent years, may be
exhibited by subjecting the vapours of volatile liquids to the action of
concentrated sunlight, or to the concentrated beam of the electric light. Their
investigation led up to the discourse on 'Dust and Disease' which follows in
this volume; and for this reason some account of them is introduced here.

-----

A glass tube 3 feet long and 3 inches wide, which had been frequently
employed in my researches on radiant heat, was supported horizontally on
two stands. At one end of the tube was placed an electric lamp, the height
and position of both being so arranged, that the axis of the tube, and that of
the beam issuing from the lamp, were coincident. In the first experiments the
two ends of the tube were closed by plates of rock-salt, and subsequently by
plates of glass. For the sake of distinction, I call this tube the experimental

tube. It was connected with an air-pump, and also with a series of drying and
other tubes used for the purification of the air.

A number of test-tubes, like F, fig. 2 (I have used at least fifty of them), were
converted into Woulf's flasks. Each of them was stopped by a cork, through
which passed two glass tubes: one of these tubes (a) ended immediately
below the cork, while the other (b) descended to the bottom of the flask,
being drawn out at its lower end to an orifice about 0.03 of an inch in
diameter. It was found necessary to coat the cork carefully with cement. In
the later experiments corks of vulcanised India-rubber were invariably
employed.

The little flask, thus formed, being partially filled with the liquid whose
vapour was to be examined, was introduced into the path of the purified
current of air. The experimental tube being exhausted, and the cock which
cut off the supply of purified air being cautiously turned on, the air entered
the flask through the tube b, and escaped by the small orifice at the lower
end of b into the liquid. Through this it bubbled, loading itself with vapour,
after which the mixed air and vapour, passing from the flask by the tube a,
entered the experimental tube, where they were subjected to the action of
light.

The whole arrangement is shown in fig. 3, where L represents the electric
lamp, s s' the experimental tube, pp' the pipe leading to the air-pump, and F
the test-tube containing the volatile liquid. The tube t t' is plugged with
cotton-wool intended to intercept the floating matter of the air; the bent tube
T' contains caustic potash, the tube T sulphuric acid, the one intended to
remove the carbonic acid and the other the aqueous vapour of the air.

The power of the electric beam to reveal the existence of anything within the
experimental tube, or the impurities of the tube itself, is extraordinary. When
the experiments is made in a darkened room, a tube which in ordinary
daylight appears absolutely clean, is often shown by the present mode of
examination to be exceedingly filthy.

The following are some of the results obtained with this arrangement :—

Nitrite of amyl. — The vapour of this liquid was in the first instance
permitted to enter the experimental tube, while the beam from the electric
lamp was passing through it. Curious clouds, the cause of which was then
unknown, were observed to form near the place of entry, being afterwards
whirled through the tube.

The tube being again exhausted, the mixed air and vapour were allowed to
enter it in the dark. The slightly convergent beam of the electric light was
then sent through the mixture. For a moment the tube was optically empty,
nothing whatever being seen within it; but before a second had elapsed a
shower of particles was precipitated on the beam. The cloud thus generated
became denser as the light continued to act, slowing at some places vivid
iridescence.

The lens of the electric lamp was now placed so as to form within the tube a
strongly convergent cone of rays. the tube was cleansed and again filled in
darkness. When the light was sent through it, the precipitation upon the
beam was so rapid and intense that the cone, which a moment before was
invisible, flashed suddenly forth like a solid luminous spear. The effect was
the same when the air and vapour were allowed to enter the tube in diffuse
daylight. The cloud, however, which shone with such extraordinary radiance
under the electric beam, was invisible in the ordinary light of the laboratory.

The quantity of mixed air and vapour within the experimental tube could of
course be regulated at pleasure. The rapidity of the action diminished with
the attenuation of the vapour. When, for example, the mercurial column
associated with the experimental tube was depressed only five inches, the
action was not nearly so rapid as when the tube was full. In such cases,
however, it was exceedingly interesting to observe, after some seconds of
waiting, a thin streamer of delicate bluish-white cloud slowly forming along
the axis of the tube, and finally swelling so as to fill it.

.

Fig. 2.

g
Fig. 3.

When dry oxygen was employed to carry in the vapour the effect was the
same as that obtained with air.

When dry hydrogen was used as a vehicle, the effect was also the same.

The effect, therefore, is not due to any interaction between the vapour of the
nitrite and its vehicle.

This was further demonstrated by the deportment of the vapour itself. When
it was permitted to enter the experimental tube unmixed with air or any
other gas, the effect was substantially the same. Hence the seat of the
observed action is the vapour.

This action is not to be ascribed to heat. As regards the glass of the
experimental tube, and the air within the tube, the beam employed in these
experiments was perfectly cold. It had been sifted by passing it through a
solution of alum, and through the thick double-convex lens of the lamp.
When the unsifted beam of the lamp was employed, the effect was still the
same; the obscure calorific rays did not appear to interfere with the result.

My object here being simply to point out to chemists a method of
experiments which reveals a new and beautiful series of reactions, I left to
them the examination of the products of decomposition. The group of atoms
forming the molecule of nitrite of amyl is obviously shaken asunder by
certain specific waves of the electric beam, nitric oxide and other products,
of which the nitrate of amyl is probably one, being the result of the
decomposition. The brown fumes of nitrous acid were seen mingling with
the cloud within the experimental tube. The nitrate of amyl, being less
volatile than the nitrite, and not being able to maintain itself in the condition
of vapour, would be precipitated as a visible cloud along the track of the
beam.

In the anterior portions of the tube a powerful sifting of the beam by the
vapour occurs, which diminishes the chemical action in the posterior
portions. In some experiments the precipitated cloud only extended halfway
down the tube. When, under these circumstances, the lamp was shifted so as
to send the beam through the other end of the tube, copious precipitation
occurred there also.

Solar light also effects the decomposition of the nitrite-of-amyl vapour. On
October 10, 1868, I partially darkened a small room in the Royal Institution,
into which the sun shone, permitting the light to enter through an open
portion of the window-shutter. In the track of the beam was placed a large
plano-convex lens, which formed a fine convergent cone in the dust of the
room behind it. The experimental tube was filled in the laboratory, covered
with a black cloth, and carried into the partially darkened room. On
thrusting one end of the tube into the cone of rays behind the lens,
precipitation within the cone was copious and immediate. The vapour at the
distant end of the tube was in part shielded by that in front, and was also
more feebly acted on through the divergence of the rays. On reversing the
tube, a second and similar cone was precipitated.

Physical Considerations.

I sought to determine the particular portion of the light which produced the
foregoing effects. When, previous to entering the experimental tube, the
beam was caused to pass through a red glass, the effect was greatly
weakened, but not extinguished. This was also the case with various
samples of yellow glass. A blue glass being introduced before the removal
of the yellow or the red, on taking the latter away prompt precipitation
occurred along the track of the blue beam. Hence, in this case, the more
refrangible rays are the most chemically active. The colour of the liquid
nitrite of amyl indicates that this must be the case; it is a feeble but distinct
yellow: in other words, the yellow portion of the beam is most freely
transmitted. It is not, however, the transmitted portion of any beam which
produces chemical action, but the absorbed portion. Blue, as the
complementary colour to yellow, is here absorbed, and hence the more
energetic action of the blue rays.

This reasoning, however, assumes that the same rays are absorbed by the
liquid and its vapour. The assumption is worth testing. A solution of the
yellow chromate of potash, the colour of which may be made almost, if not
altogether, identical with that of the liquid nitrite of amyl, was found far
more effective in stopping the chemical rays than either the red or the
yellow glass. But of all substances the liquid nitrite itself is most potent in
arresting the rays which act upon its vapour. A layer one-eighth of an inch in
thickness, which scarcely perceptibly affected the luminous intensity,

absorbed the entire chemical energy of the concentrated beam of the electric
light.

The close relation subsisting between a liquid and its vapour, as regards
their action upon radiant heat, has been already amply demonstrated.
[Footnote: 'Phil. Trans.' 1864; 'Heat, a Mode of Motion,' chap, xii.; and P.
61 of this volume.] As regards the nitrite of amyl, this relation is more
specific than in the cases hitherto adduced; for here the special constituent
of the beam, which provokes the decomposition of the vapour, is shown to
be arrested by the liquid.

A question of extreme importance in molecular physics here arises: What is
the real mechanism of this absorption, and where is its seat? [Footnote: My
attention was very forcibly directed to this subject some years ago by a
conversation with my excellent friend Professor Clausius.]

I figure, as others do, a molecule as a group of atoms, held together by their
mutual forces, but still capable of motion among themselves. The vapour of
the nitrite of amyl is to be regarded as an assemblage of such molecules.
The question now before us is this: In the act of absorption, is it the
molecules that are effective, or is it their constituent atoms? Is the vis viva
of the intercepted light-waves transferred to the molecule as a whole, or to
its constituent parts?

The molecule, as a whole, can only vibrate in virtue of the forces exerted
between it and its neighbour molecules. The intensity of these forces, and
consequently the rate of vibration, would, in this case, be a function of the
distance between the molecules. Now the identical absorption of the liquid
and of the vaporous nitrite of amyl indicates an identical vibrating period on
the part of liquid and vapour, and this, to my mind, amounts to an
experimental proof that the absorption occurs in the main within the
molecule. For it can hardly be supposed, if the absorption were the act of
the molecule as a whole, that it could continue to affect waves of the same
period after the substance had passed from the vaporous to the liquid state.

In point of fact, the decomposition of the nitrite of amyl is itself to some
extent an illustration of this internal molecular absorption; for were the
absorption the act of the molecule as a whole, the relative motions of its

constituent atoms would remain unchanged, and there would be no
mechanical cause for their separation. It is probably the synchronism of the
vibrations of one portion of the molecule with the incident waves, that
enables the amplitude of those vibrations to augment, until the chain which
binds the parts of the molecule together is snapped asunder.

I anticipate wide, if not entire, generality for the fact that a liquid and its
vapour absorb the same rays. A cell of liquid chlorine would, I imagine,
deprive light more effectually of its power of causing chlorine and hydrogen
to combine than any other filter of the luminous rays. The rays which give
chlorine its colour have nothing to do with this combination, those that are
absorbed by the chlorine being the really effective rays. A highly sensitive
bulb, containing chlorine and hydrogen, in the exact proportions necessary
for the formation of hydrochloric acid, was placed at one end of an
experimental tube, the beam of the electric lamp being sent through it from
the other. The bulb did not explode when the tube was filled with chlorine,
while the explosion was violent and immediate when the tube was filled
with air. I anticipate for the liquid chlorine an action similar to, but still
more energetic than, that exhibited by the gas. If this should prove to be the
case, it will favour the view that chlorine itself is molecular and not
monatomic.

Production of Sky-blue by the Decomposition of Nitrite of Amyl.

When the quantity of nitrite vapour is considerable, and the light intense,
the chemical action is exceedingly rapid, the particles precipitated being so
large as to whiten the luminous beam. Not so, however, when a well-mixed
and highly attenuated vapour fills the experimental tube. The effect now to
be described was first obtained when the vapour of the nitrite was derived
from a portion of its liquid which had been accidentally introduced into the
passage through which the dry air flowed into the experimental tube.

In this case, the electric beam traversed the tube for several seconds before
any action was visible. Decomposition then visibly commenced, and
advanced slowly. _When the light was very strong, the cloud appeared of a
milky blue. When, on the contrary, the intensity was moderate, the blue was
pure and deep. In Brücke's important experiments on the blue of the sky and
the morning and evening red, pure mastic is dissolved in alcohol, and then

dropped into water well stirred. When the proportion of mastic to alcohol is
correct, the resin is precipitated so finely as to elude the highest microscopic
power. By reflected light, such a medium appears bluish, by transmitted
light yellowish, which latter colour, by augmenting the quantity of the
precipitate, can be caused to pass into orange or red.

But the development of colour in the attenuated nitrite-of-amyl vapour is
doubtless more similar to what takes place in our atmosphere. The blue,
moreover, is far purer and more sky-like than that obtained from Bruecke's
turbid medium. Never, even in the skies of the Alps, have I seen a richer or
a purer blue than that attainable by a suitable disposition of the light falling
upon the precipitated vapour.

Iodide of Allyl. — Among the liquids hitherto subjected to the concentrated
electric light, iodide of allyl, in point of rapidity and intensity of action,
comes next to the nitrite of amyl. With the iodide I have employed both
oxygen and hydrogen, as well as air, as a vehicle, and found the effect in all
cases substantially the same. The cloud-column here was exquisitely
beautiful. It revolved round the axis of the decomposing beam; it was
nipped at certain places like an hour-glass, and round the two bells of the
glass delicate cloud-filaments twisted themselves in spirals. It also folded
itself into convolutions resembling those of shells. In certain conditions of
the atmosphere in the Alps I have often observed clouds of a special pearly
lustre; when hydrogen was made the vehicle of the iodide-of allyl vapour a
similar lustre was most exquisitely shown. With a suitable disposition of the
light, the purple hue of iodine-vapour came out very strongly in the tube.

The remark already made, as to the bearing of the decomposition of nitrite
of amyl by light on the question of molecular absorption, applies here also;
for were the absorption the work of the molecule as a whole, the iodine
would not be dislodged from the allyl with which it is combined. The non-
synchronism of iodine with the waves of obscure heat is illustrated by its
marvellous transparency to such heat. May not its synchronism with the
waves of light in the present instance be the cause of its divorce from the
allyl?

Iodide of Isopropyl. — The action of light upon the vapour of this liquid is,
at first, more languid than upon iodide of allyl; indeed many beautiful

reactions may be overlooked, in consequence of this languor at the
commencement. After some minutes' exposure, however, clouds begin to
form, which grow in density and in beauty as the light continues to act. In
every experiment hitherto made with this substance the column of cloud
filling the experimental tube, was divided into two distinct parts near the
middle of the tube. In one experiments a globe of cloud formed at the
centre, from which, right and left, issued an axis uniting the globe with two
adjacent cylinders. Both globe and cylinders were animated by a common
motion of rotation. As the action continued, paroxysms of motion were
manifested; the various parts of the cloud would rush through each other
with sudden violence. During these motions beautiful and grotesque cloud-
forms were developed. At some places the nebulous mass would become
ribbed so as to resemble the graining of wood; a longitudinal motion would
at times generate in it a series of curved, transverse bands, the retarding
influence of the sides the tube causing an appearance resembling, on a small
scale, the dirt-bands of the Mer de Glace. In the anterior portion of the tube
those sudden commotion were most intense; here buds of cloud would
sprout forth, and grow in a few seconds into perfect flower-like forms. The
cloud of iodide of isopropyl had a character Of its own, and differed
materially from all others that I had seen. A gorgeous mauve colour was
observed in the last twelve inches of the tube; the vapour of iodine was
present, and it may have been the sky-blue scattered by the precipitated
particles which, mingling with the purple of the iodine, produced the
mauve. As in all other cases here adduced, the effects were proved to be due
to the light; they never occurred in darkness.

The forms assumed by some of those actinic clouds, as I propose to call
them, in consequence of rotations and other motions, due to differences of
temperature, are perfectly astounding. I content myself here with a meagre
description of one more of them.

The tube being filled with the sensitive mixture, the beam was sent through
it, the lens at the same time being so placed as to produce a cone of very
intense light. Two minutes elapsed before anything was visible; but at the
end of this time a faint bluish cloud appeared to hang itself on the most
concentrated portion of the beam.

Soon afterwards a second cloud was formed five inches farther down the
experimental tube. Both clouds were united by a slender cord of the same
bluish tint as themselves.

As the action of the light continued, the first cloud gradually resolved itself
into a series of parallel disks of exquisite delicacy, which rotated round an
axis perpendicular to their surfaces, and finally blended to a screw surface
with an inclined generatrix. This gradually changed into a filmy funnel,
from the narrow end of which the 'cord' extended to the cloud in advance.

The latter also underwent slow but incessant modification. It first resolved
itself into a series of strata resembling those of the electric discharge. After
a little time, and through changes which it was difficult to follow, both
clouds presented the appearance of a series of concentric funnels set one
within the other, the interior ones being seen through the outer ones. Those
of the distant cloud resembled claret-glasses in shape. As many as six
funnels were thus concentrically set together, the two series being united by
the delicate cord of cloud already referred to. Other cords and Blender tubes
were afterwards formed, which coiled themselves in delicate spirals around
the funnels.

Rendering the light along the connecting-cord more intense, it diminished in
thickness and became whiter; this was a consequence of the enlargement of
its particles. The cord finally disappeared, while the funnels melted into two
ghost-like films, shaped like parasols. They were barely visible, being of an
exceedingly delicate blue tint. They seemed woven of blue air. To compare
them with cobweb or with gauze would be to liken them to something
infinitely grosser than themselves.

In all cases a distant candle-flame, when looked at through the cloud, was
sensibly undimmed.

.

.

.

.

§ 2. ON THE BLUE COLOUR OF THE SKY, AND THE
POLARISATION OF SKYLIGHT.

[Footnote: In my 'Lectures on Light' (Longman), the polarisation of light
will be found briefly, but, I trust, clearly explained.]

1869.

After the communication to the Royal Society of the foregoing brief account
of a new Series of Chemical Reactions produced by Light, the experiments
upon this subject were continued, the number of substances thus acted on
being considerably increased.

I now, however, beg to direct attention to two questions glanced at
incidentally in the preceding pages — the blue colour of the sky, and the
polarisation of skylight. Reserving the historic treatment of the subject for a
more fitting occasion, I would merely mention now that these questions
constitute, in the opinion of our most eminent authorities, the two great
standing enigmas of meteorology. Indeed it was the interest manifested in
them by Sir John Herschel, in a letter of singular speculative power,
addressed to myself, that caused me to enter upon the consideration of these
questions so soon.

The apparatus with which I work consists, as already stated, of a glass tube
about a yard in length, and from 2.5 to 3 inches internal diameter. The
vapour to be examined is introduced into this tube in the manner already
described, and upon it the condensed beam of the electric lamp is permitted
to act, until the neutrality or the activity of the substance has been declared.

It has hitherto been my aim to render the chemical action of light upon
vapours visible. For this purpose substances have been chosen, one at least
of whose products of decomposition under light shall have a boiling-point
so high, that as soon as the substance is formed it shall be precipitated. By
graduating the quantity of the vapour, this precipitation may be rendered of
any degree of fineness, forming particles distinguishable by the naked eye,
or far beyond the reach of our highest microscopic powers. I have no reason

to doubt that particles may be thus obtained, whose diameters constitute but
a small fraction of the length of a wave of violet light.

In all cases when the vapours of the liquids employed are sufficiently
attenuated, no matter what the liquid may be, the visible action commences
with the formation of a blue cloud. But here I must guard myself against all
misconception as to the use of this term. The 'cloud' here referred to is
totally invisible in ordinary daylight. To be seen, it requires to be
surrounded by darkness, it only being illuminated by a powerful beam of
light. This blue cloud differs in many important particulars from the finest
ordinary clouds, and might justly have assigned to it an intermediate
position between such clouds and true vapour. With this explanation, the
term 'cloud,' or 'incipient cloud,' or 'actinic cloud,' as I propose to employ it,
cannot, I think, be misunderstood.

I had been endeavouring to decompose carbonic acid gas by light. A faint
bluish cloud, due it may be, or it may not be, to the residue of some vapour
previously employed, was formed in the experimental tube. On looking
across this cloud through a Nicol's prism, the line of vision being horizontal,
it was found that when the short diagonal of the prism was vertical, the
quantity of light reaching the eye was greater than when the long diagonal
was vertical. When a plate of tourmaline was held between the eye and the
bluish cloud, the quantity of light reaching the eye when the axis of the
prism was perpendicular to the axis of the illuminating beam, was greater
than when the axes of the crystal and of the beam were parallel to each
other.

This was the result all round the experimental tube. Causing the crystal of
tourmaline to revolve round the tube, with its axis perpendicular to the
illuminating beam, the quantity of light that reached the eye was in all its
positions a maximum. When the crystallographic axis was parallel to the
axis of the beam, the quantity of light transmitted by the crystal was a
minimum.

From the illuminated bluish cloud, therefore, polarised light was discharged,
the direction of maximum polarisation being at right angles to the
illuminating beam; the plane of vibration of the polarised light was
perpendicular to the beam. [Footnote: This is still an undecided point; but

the probabilities are so much in its favour, and it is in my opinion so much
preferable to have a physical image on which the mind can rest, that I do
not hesitate to employ the phraseology in the text.]

Thin plates of selenite or of quartz, placed between the Nicol and the actinic
cloud, displayed the colours of polarised light, these colours being most
vivid when the line of vision was at right angles to the experimental tube.
The plate of selenite usually employed was a circle, thinnest at the centre,
and augmenting uniformly in thickness from the centre outwards. When
placed in its proper position between the Nicol and the cloud, it exhibited a
system of splendidly-coloured rings.

The cloud here referred to was the first operated upon in the manner
described. It may, however, be greatly improved upon by the choice of
proper substances, and by the application, in proper quantities, of the
substances chosen. Benzol, bisulphide of carbon, nitrite of amyl, nitrite of
butyl, iodide of allyl, iodide of isopropyl, and many other substances may
be employed. I will take the nitrite of butyl as illustrative of the means
adopted to secure the best result, with reference to the present question.

And here it may be mentioned that a vapour, which when alone, or mixed
with air in the experimental tube, resists the action of light, or shows but a
feeble result of this action, may, when placed in proximity with another gas
or vapour, exhibit vigorous, if not violent action. The case is similar to that
of carbonic acid gas, which, diffused in the atmosphere, resists the
decomposing action of solar light, but when placed in contiguity with
chlorophyl in the leaves of plants, has its molecules shaken asunder.

Dry air was permitted to bubble through the liquid nitrite of butyl, until the
experimental tube, which had been previously exhausted, was filled with the
mixed air and vapour. The visible action of light upon the mixture after
fifteen minutes' exposure was slight. The tube was afterwards filled with
half an atmosphere of the mixed air and vapour, and a second half-
atmosphere of air which had been permitted to bubble through fresh
commercial hydrochloric acid. On sending the beam through this mixture,
the tube, for a moment, was optically empty. But the pause amounted only
to a small fraction of a second, a dense cloud being immediately
precipitated upon the beam.

This cloud began blue, but the advance to whiteness was so rapid as almost
to justify the application of the term instantaneous. The dense cloud, looked
at perpendicularly to its axis, showed scarcely any signs of polarisation.
Looked at obliquely the polarisation was strong.

The experimental tube being again cleansed and exhausted, the mixed air
and nitrite-of-butyl vapour was permitted to enter it until the associated
mercury column was depressed 1/10 of an inch. In other words, the air and
vapour, united, exercised a pressure not exceeding 1/300th of an atmosphere.
Air, passed through a solution of hydrochloric acid, was then added, till the
mercury column was depressed three inches. The condensed beam of the
electric light was passed for some time through this mixture without
revealing anything within the tube competent to scatter the light. Soon,
however, a superbly blue cloud was formed along, the track of the beam,
and it continued blue sufficiently long to permit of its thorough
examination. The light discharged from the cloud, at right angles to its own
length, was at first perfectly polarised. It could be totally quenched by the
Nicol. By degrees the cloud became of whitish blue, and for a time the
selenite colours, obtained by looking at it normally, were exceedingly
brilliant. The direction of maximum polarisation was distinctly at right
angles to the illuminating beam. This continued to be the case as long as the
cloud maintained a decided blue colour, and even for some time after the
blue had changed to whitish blue. But, as the light continued to act, the
cloud became coarser and whiter, particularly at its centre, where it at length
ceased to discharge polarised light in the direction of the perpendicular,
while it continued to do so at both ends.

But the cloud which had thus ceased to polarise the light emitted normally,
showed vivid selenite colours when looked at obliquely, proving that the
direction of maximum polarisation changed with the texture of the cloud.
This point shall receive further illustration subsequently.

A blue, equally rich and more durable, was obtained by employing the
nitrite-of-butyl vapour in a still more attenuated condition. The instance
here cited is representative. In all cases, and with all substances, the cloud
formed at the commencement, when the precipitated particles are
sufficiently fine, is blue, and it can be made to display a colour rivalling that

of the purest Italian sky. In all cases, moreover, this fine blue cloud polarises
perfectly the beam which illuminates it, the direction of polarisation
enclosing an angle of 90° with the axis of the illuminating beam.

It is exceedingly interesting to observe both the perfection and the decay of
this polarisation. For ten or fifteen minutes after its first appearance the light
from a vividly illuminated actinic cloud, looked at perpendicularly, is
absolutely quenched by a Nicol's prism with its longer diagonal vertical. But
as the sky-blue is gradually rendered impure by the growth of the particles
— in other words, as real clouds begin to be formed — the polarisation
begins to decay, a portion of the light passing through the prism in all its
positions. It is worthy of note, that for some time after the cessation of
perfect polarisation, the residual light which passes, when the Nicol is in its
position of minimum transmission, is of a gorgeous blue, the whiter light of
the cloud being extinguished. [Footnote: This shows that particles too large
to polarise the blue, polarise perfectly light of lower refrangibility.] When
the cloud texture has become sufficiently coarse to approximate to that of
ordinary clouds, the rotation of the Nicol ceases to have any sensible effect
on the quantity of light discharged normally.

The perfection of the polarisation, in a direction perpendicular to the
illuminating beam, is also illustrated by the following experiments: A
Nicol's prism, large enough to embrace the entire beam of the electric lamp,
was placed between the lamp and the experimental tube. A few bubbles of
air, carried through the liquid nitrite of butyl, were introduced into the tube,
and they were followed by about three inches (measured by the mercurial
gauge) of air which had passed through aqueous hydrochloric acid. Sending
the polarised beam through the tube, I placed myself in front of it, my eye
being on a level with its axis, my assistant occupying a similar position
behind the tube. The short diagonal of the large Nicol was in the first
instance vertical, the plane of vibration of the emergent beam being
therefore also vertical. As the light continued to act, a superb blue cloud,
visible to both my assistant and myself, was slowly formed. But this cloud,
so deep and rich when looked at from the positions mentioned, utterly
disappeared when looked at vertically downwards, or vertically upwards.
Reflection from the cloud was not possible in these directions. When the
large Nicol was slowly turned round its axis, the eye of the observer being

on the level of the beam, and the line of vision perpendicular to it, entire
extinction of the light emitted horizontally occurred when the longer
diagonal of the large Nicol was vertical. But now a vivid blue cloud was
seen when looked at downwards or upwards. This truly fine experiments,
which I contemplated making on my own account, was first definitely
suggested by a remark in a letter addressed to me by Professor Stokes.

As regards the polarisation of skylight, the greatest stumbling-block has
hitherto been, that, in accordance with the law of Brewster, which makes the
index of refraction the tangent of the polarising angle, the reflection which
produces perfect polarisation would require to be made in air upon air; and
indeed this led many of our most eminent men, Brewster himself among the
number, to entertain the idea of aerial molecular reflection. [Footnote: 'The
cause of the polarisation is evidently a reflection of the sun's light upon
something. The question is on what? Were the angle of maximum
polarisation 76°, we should look to water or ice as the reflecting body,
however inconceivable the existence in a cloudless atmosphere and a hot
summer's day of unevaporated molecules (particles?) of water. But though
we were once of this opinion, careful observation has satisfied us that 90°,
or thereabouts, is the correct angle, and that therefore whatever be the body
on which the light has been reflected, if polarised by a single reflection, the
polarising angle must be 45°, and the index of refraction, which is the
tangent of that angle, unity; in other words, the reflection would require to
be made in air upon air!' (Sir John Herschel, 'Meteorology,' par. 233.)

Any particles, if small enough, will produce both the colour and the
polarisation of the sky. But is the existence of small water-particles on a hot
summer's day in the higher regions of our atmosphere inconceivable? It is
to be remembered that the oxygen and nitrogen of the air behave as a
vacuum to radiant heat, the exceedingly attenuated vapour of the higher
atmosphere being therefore in practical contact with the cold of space.]

I have, however, operated upon substances of widely different refractive
indices, and therefore of very different polarising angles as ordinarily
defined, but the polarisation of the beam, by the incipient cloud, has thus far
proved itself to be absolutely independent of the polarising angle. The law
of Brewster does not apply to matter in this condition, and it rests with the
undulatory theory to explain why. Whenever the precipitated particles are

sufficiently fine, no matter what the substance forming the particles may be,
the direction of maximum polarisation is at right angles to the illuminating
beam, the polarising angle for matter in this condition being invariably 45°.

Suppose our atmosphere surrounded by an envelope impervious to light, but
with an aperture on the sunward side through which a parallel beam of solar
light could enter and traverse the atmosphere. Surrounded by air not directly
illuminated, the track of such a beam would resemble that of the parallel
beam of the electric lamp through an incipient cloud. The sunbeam would
be blue, and it would discharge laterally light in precisely the same
condition as that discharged by the incipient cloud. In fact, the azure
revealed by such a beam would be to all intents and purposes that which I
have called a 'blue cloud.' Conversely our 'blue cloud' is, to all intents and
purposes, an artificial sky.' [Footnote: The opinion of Sir John Herschel,
connecting the polarisation and the blue colour of the sky, is verified by the
foregoing results. 'The more the subject [the polarisation of skylight] is
considered,' writes this eminent philosopher, 'the more it will be found beset
with difficulties, and its explanation when arrived at will probably be found
to carry with it that of the blue colour of the sky itself, and of the great
quantity of light it actually does send down to us.' 'We may observe, too,' he
adds, 'that it is only where the purity of the sky is most absolute that the
polarisation is developed in its highest degree, and that where there is the
slightest perceptible tendency to cirrus it is materially impaired.' This
applies word for word to our 'incipient clouds.' ]

But, as regards the polarisation of the sky, we know that not only is the
direction of maximum polarisation at right angles to the track of the solar
beams, but that at certain angular distances, probably variable ones, from
the sun, 'neutral points,' or points of no polarisation, exist, on both sides of
which the planes of atmospheric polarisation are at right angles to each
other. I have made various observations upon this subject which are
reserved for the present; but, pending the more complete examination of the
question, the following facts bearing upon it may be submitted.

The parallel beam employed in these experiments tracked its way through
the laboratory air, exactly as sunbeams are seen to do in the dusty air of
London. I have reason to believe that a great portion of the matter thus
floating in the laboratory air consists of organic particles, which are capable

of imparting a perceptibly bluish tint to the air. These also showed, though
far less vividly, all the effects of polarisation obtained with the incipient
clouds. The light discharged laterally from the track of the illuminating
beam was polarised, though not perfectly, the direction of maximum
polarisation being at right angles to the beam. At all points of the beam,
moreover, throughout its entire length, the light emitted normally was in the
same state of polarisation. Keeping the positions of the Nicol and the
selenite constant, the same colours were observed throughout the entire
beam, when the line of vision was perpendicular to its length.

The horizontal column of air, thus illuminated, was 18 feet long, and could
therefore be looked at very obliquely. I placed myself near the end of the
beam, as it issued from the electric lamp, and, looking through the Nicol
and selenite more and more obliquely at the beam, observed the colours
fading until they disappeared. Augmenting the obliquity the colours
appeared once more, but they were now complementary to the former ones.

Hence this beam, like the sky, exhibited a neutral point, on opposite sides of
which the light was polarised in planes at right angles to each other.

Thinking that the action observed in the laboratory might be caused, in
some way, by the vaporous fumes diffused in its air, I had the light removed
to a room at the top of the Royal Institution. The track of the beam was seen
very finely in the air of this room, a length of 14 or 15 feet being attainable.
This beam exhibited all the effects observed with the beam in the laboratory.
Even the uncondensed electric light falling on the floating matter showed,
though faintly, the effects of polarisation.

When the air was so sifted as to entirely remove the visible floating matter,
it no longer exerted any sensible action upon the light, but behaved like a
vacuum. The light is scattered and polarised by particles, not by molecules
or atoms.

By operating upon the fumes of chloride of ammonium, the smoke of brown
paper, and tobacco-smoke, I had varied and confirmed in many ways those
experiments on neutral points, when my attention was drawn by Sir Charles
Wheatstone to an important observation communicated to the Paris
Academy in 1860 by Professor Govi, of Turin.[Footnote: Comptes Rendus,'

tome li, pp. 360 and 669.] M. Govi had been led to examine a beam of light
sent through a room in which were successively diffused the smoke of
incense, and tobacco-smoke. His first brief communication stated the fact of
polarisation by such smoke; but in his second communication he announced
the discovery of a neutral point in the beam, at the opposite sides of which
the light was polarised in planes at right angles to each other.

But unlike my observations on the laboratory air, and unlike the action of
the sky, the direction of maximum polarisation in M. Govi's experiments
enclosed a very small angle with the axis of the illuminating beam. The
question was left in this condition, and I am not aware that M. Govi or any
other investigator has pursued it further.

I had noticed, as before stated, that as the clouds formed in the experimental
tube became denser, the polarisation of the light discharged at right angles
to the beam became weaker, the direction of maximum polarisation
becoming oblique to the beam. Experiments on the fumes of chloride of
ammonium gave me also reason to suspect that the position of the neutral
point was not constant, but that it varied with the density of the illuminated
fumes.

The examination of these questions led to the following new and remarkable
results: The laboratory being well filled with the fumes of incense, and
sufficient time being allowed for their uniform diffusion, the electric beam
was sent through the smoke. From the track of the beam polarised light was
discharged; but the direction of maximum polarisation, instead of being
perpendicular, now enclosed an angle of only 12° or 13° with the axis of the
beam.

A neutral point, with complementary effects at opposite sides of it, was also
exhibited by the beam. The angle enclosed by the axis of the beam, and a
line drawn from the neutral point to the observer's eye, measured in the first
instance 66°.

The windows of the laboratory were now opened for some minutes, a
portion of the incense-smoke being permitted to escape. On again darkening
the room and turning on the light, the line of vision to the neutral point was
found to enclose, with the axis of the beam, an angle of 63°.

The windows were again opened for a few minutes, more of the smoke
being permitted to escape. Measured as before, the angle referred to was
found to be 54°.

This process was repeated three additional times the neutral point was found
to recede lower and lower down the beam, the angle between a line drawn
from the eye to the neutral point and the axis of the beam falling
successively from 54° to 49°, 43° and 33°.

The distances, roughly measured, of the neutral point from the lamp,
corresponding to the foregoing series of observations, were these :—

1st observation 2 feet 2 inches.

2nd observation 2 feet 6 inches.

3rd observation 2 feet 10 inches.

4th observation 3 feet 2 inches.

5th observation 3 feet 7 inches.

6th observation 4 feet 6 inches.

At the end of this series of experiments the direction of maximum
polarisation had again become normal to the beam.

The laboratory was next filled with the fumes of gunpowder. In five
successive experiments, corresponding to five different densities of the
gunpowder-smoke, the angles enclosed between the line of vision to the

neutral point and the axis of the beam, were 63 degrees, 50°, 47°, 42°, and
38° respectively.

After the clouds of gunpowder had cleared away, the laboratory was filled
with the fumes of common resin, rendered so dense as to be very irritating
to the lungs. The direction of maximum polarisation enclosed, in this case,
an angle of 12°, or thereabouts, with the axis of the beam. Looked at, as in
the former instances, from a position near the electric lamp, no neutral point
was observed throughout the entire extent of the beam.

When this beam was looked at normally through the selenite and Nicol, the
ring-system, though not brilliant, was distinct. Keeping the eye upon the
plate of selenite, and the line of vision perpendicular, the windows were
opened, the blinds remaining undrawn. The resinous fumes slowly
diminished, and as they did so the ring-system became paler. It finally
disappeared. Continuing to look in the same direction, the rings revived, but
now the colours were complementary to the former ones. The neutral point
had passed me in its motion down the beam, consequent upon the
attenuation of the fumes of resin.

With the fumes of chloride of ammonium substantially the same results
were obtained. Sufficient, however, has been here stated to illustrate the
variability of the position of the neutral point.[Footnote: Brewster has
proved the variability of the position of the neutral point for skylight with
the sun's altitude, a result obviously connected with the foregoing
experiments.]

By a puff of tobacco-smoke, or of condensed steam, blown into the
illuminated beam, the brilliancy of the selenite colours may be greatly
enhanced. But with different clouds two different effects are produced. Let
the ring-system observed in the common air be brought to its maximum
strength, and then let an attenuated cloud of chloride of ammonium be
thrown into the beam at the point looked at; the ring system flashes out with
augmented brilliancy, but the character of the polarisation remains
unchanged. This is also the case when phosphorus, or sulphur, is burned
underneath the beam, so as to cause the fine particles of phosphorus or of
sulphur to rise into the light. With the sulphur-fumes the brilliancy of the

colours is exceedingly intensified; but in none of these cases is there any
change in the character of the polarisation.

But when a puff of the fumes of hydrochloric acid, hydriodic acid, or nitric
acid is thrown into the beam, there is a complete reversal of the selenite
tints. Each of these clouds twists the plane of polarisation 90°, causing the
centre of the ring-system to change from black to white, and the rings
themselves to emit their complementary colours. [Footnote: Sir John
Herschel suggested to me that this change of the polarisation from positive
to negative may indicate a change from polarisation by reflection to
polarisation by refraction. This thought repeatedly occurred to me while
looking at the effects; but it will require much following up before it
emerges into clearness.]

Almost all liquids have motes in them sufficiently numerous to polarise
sensibly the light, and very beautiful effects may be obtained by simple
artificial devices. When, for example, a cell of distilled water is placed in
front of the electric lamp, and a thin slice of the beam is permitted to pass
through it, scarcely any polarised light is discharged, and scarcely any
colour produced with a plate of selenite. But if a bit of soap be agitated in
the water above the beam, the moment the infinitesimal particles reach the
light the liquid sends forth laterally almost perfectly polarised light; and if
the selenite be employed, vivid colours flash into existence. A still more
brilliant result is obtained with mastic dissolved in a great excess of alcohol.

The selenite rings, in fact, constitute an extremely delicate test as to the
collective quantity of individually invisible particles in a liquid.
Commencing with distilled water, for example, a thick slice of light is
necessary to make the polarisation of its suspended particles sensible. A
much thinner slice suffices for common water; while, with Bruecke's
precipitated mastic, a slice too thin to produce any sensible effect with most
other liquids, suffices to bring out vividly the selenite colours.

.

.

.

.

§ 3. THE SKY OF THE ALPS.

The vision of an object always implies a differential action on the retina of
the observer. The object is distinguished from surrounding space by its
excess or defect of light in relation to that space. By altering the
illumination, either of the object itself or of its environment, we alter the
appearance of the object. Take the case of clouds floating in the atmosphere
with patches of blue between them. Anything that changes the illumination
of either alters the appearance of both, that appearance depending, as stated,
upon differential action. Now the light of the sky, being polarised, may, as
the reader of the foregoing pages knows, be in great part quenched by a
Nicol's prism, while the light of a common cloud, being unpolarised, cannot
be thus extinguished. Hence the possibility of very remarkable variations,
not only in the aspect of the firmament, which is really changed, but also in
the aspect of the clouds, which have that firmament as a background. It is
possible, for example, to choose clouds of such a depth of shade that when
the Nicol quenches the light behind them, they shall vanish, being
undistinguishable

from the residual dull tint which outlives the extinction of the brilliancy of
the sky. A cloud less deeply shaded, but still deep enough, when viewed
with the naked eye, to appear dark on a bright ground, is suddenly changed
to a white cloud on a dark ground by the quenching of the light behind it.
When a reddish cloud at sunset chances to float in the region of maximum
polarisation, the quenching of the surrounding light causes it to flash with a
brighter crimson. Last Easter eve the Dartmoor sky, which had just been
cleansed by a snow-storm, wore a very wild appearance. Round the horizon
it was of steely brilliancy, while reddish cumuli and cirri floated
southwards. When the sky was quenched behind them these floating masses
seemed like dull embers suddenly blown upon; they brightened like a fire.

In the Alps we have the most magnificent examples of crimson clouds and
snows, so that the effects just referred to may be here studied under the best
possible conditions. On August 23, 1869, the evening Alpenglow was very
fine, though it did not reach its maximum depth and splendour. The side of
the Weisshorn seen from the Bel Alp, being turned from the sun, was tinted

mauve; but I wished to observe one of the rose-coloured buttresses of the
mountain. Such a one was visible from a point a few hundred feet above the
hotel. The Matterhorn also, though for the most part in shade, had a crimson
projection, while a deep ruddy red lingered along its western shoulder. Four
distinct peaks and buttresses of the Dom, in addition to its dominant head —
all covered with pure snow — were reddened by the light of sunset. The
shoulder of the Alphubel was similarly coloured, while the great mass of the
Fletschorn was all a-glow, and so was the snowy spine of the Monte Leone.

Looking at the Weisshorn through the Nicol, the glow of its protuberance
was strong or weak according to the position of the prism. The summit also
underwent striking changes. In one position of the prism it exhibited a pale
white against a dark background; in the rectangular position it was a dark
mauve against a light background. The red of the Matterhorn changed in a
similar manner; but the whole mountain also passed through wonderful
changes of definition. The air at the time was filled with a silvery haze, in
which the Matterhorn almost disappeared. This could be wholly quenched
by the Nicol, and then the mountain sprang forth with astonishing solidity
and detachment from the surrounding air. The changes of the Dom were still
more wonderful. A vast amounts of light could be removed from the sky
behind it, for it occupied the position of maximum polarisation. By a little
practice with the Nicol it was easy to render the extinction of the light, or its
restoration, almost instantaneous. When the sky was quenched, the four
minor peaks and buttresses, and the summit of the Dom, together with the
shoulder of the Alphubel, glowed as if set suddenly on fire. This was
immediately dimmed by turning the Nicol through an angle of 90°. It was
not the stoppage of the light of the sky behind the mountains alone which
produced this startling effect; the air between them and me was highly
opalescent, and the quenching of this intermediate glare augmented
remarkably the distinctness of the mountains.

On the morning of August 24 similar effects were finely shown. At 10 A.M.
all three mountains, the Dom, the Matterhorn, and the Weisshorn, were
powerfully affected by the Nicol. But in this instance also, the line drawn to
the Dom being very nearly perpendicular to the solar beams, the effects on
this mountain were

most striking. The grey summit of the Matterhorn, at the same time, could
scarcely be distinguished from the opalescent haze around it; but when the
Nicol quenched the haze, the summit became instantly isolated, and stood
out in bold definition. It is to be remembered that in the production of these
effects the only things changed are the sky behind, and the luminous haze in
front of the mountains; that these are changed because the light emitted
from the sky and from the haze is plane polarised light, and that the light
from the snows and from the mountains, being sensibly unpolarised, is not
directly affected by the Nicol. It will also be understood that it is not the
interposition of the haze as an opaque body that renders the mountains
indistinct, but that it is the light of the haze which dims and bewilders the
eye, and thus weakens the definition of objects seen through it.

These results have a direct bearing upon what artists call 'aerial perspective.'
As we look from the summit of Mont Blanc, or from a lower elevation, at
the serried crowd of peaks, especially if the mountains be darkly coloured
— covered with pines, for example — every peak and ridge is separated
from the mountains behind it by a thin blue haze which renders the relations
of the mountains as to distance unmistakable. When this haze is regarded
through the Nicol perpendicular to the sun's rays, it is in many cases wholly
quenched, because the light which it emits in this direction is wholly
polarised. When this happens, aerial perspective is abolished, and
mountains very differently distant appear to rise in the same vertical plane.
Close to the Bel Alp for instance, is the gorge of the Massa, and beyond the
gorge is a high ridge darkened by pines. This ridge may be projected upon
the dark slopes at the opposite side of the Rhone valley, and between both
we have the blue haze referred to, throwing the distant mountains far away.
But at certain hours of the day the haze may be quenched, and then the
Massa ridge and the mountains beyond the Rhone seem almost equally
distant from the eye. The one appears, as it were, a vertical continuation of
the other. The haze varies with the temperature and humidity of the
atmosphere. At certain times and places it is almost as blue as the sky itself;
but to see its colour, the attention must be withdrawn from the mountains
and from the trees which cover them. In point of fact, the haze is a piece of
more or less perfect sky; it is produced in the same manner, and is subject to
the same laws, as the firmament itself. We live in the sky, not under it.

These points were further elucidated by the deportment of the selenite,
plate, with which the readers of the foregoing pages are so well acquainted.
On some of the sunny days of August the haze in the valley of the Rhone, as
looked at from the Bel Alp, was very remarkable. Towards evening the sky
above the mountains opposite to my place of observation yielded a series of
the most splendidly-coloured iris-rings; but on lowering the selenite until it
had the darkness of the pines at the opposite side of the Rhone 'valley,
instead of the darkness of space, as a background, the colours were not
much diminished in brilliancy. I should estimate the distance across the
valley, as the crow flies, to the opposite mountain, at nine miles; so that a
body of air of this thickness can, under favourable circumstances, produce
chromatic effects of polarisation almost as vivid as those produced by the
sky itself.

Again: the light of a landscape, as of most other things, consists of two
parts; the one, coming purely from superficial reflection, is always of the
same colour

as the light which falls upon the landscape; the other Part reaches us from a
certain depth within the objects which compose the landscape, and it is this
portion of the total light which gives these objects their distinctive colours.
The white light of the sun enters all substances to a certain depth, and is
partly ejected by internal reflection; each distinct substance absorbing and
reflecting the light, in accordance with the laws of its own molecular
constitution. Thus the solar light is sifted by the landscape, which appears in
such colours and variations of colour as, after the sifting process, reach the
observer's eye. Thus the bright green of grass, or the darker colour of the
pine, never comes to us alone, but is always mingled with an amounts of
light derived from superficial reflection. A certain hard brilliancy is
conferred upon the woods and meadows by this superficially-reflected light.
Under certain circumstances, it may be quenched by a Nicol's prism, and we
then obtain the true colour of the grass and foliage. Trees and meadows,
thus regarded, exhibit a richness and softness of tint which they never show
as long as the superficial light is permitted to mingle with the true interior
emission. The needles of the pines show this effect very well, large-leaved
trees still better; while a glimmering field of maize exhibits the most
extraordinary variations when looked at through the rotating Nicol.

Thoughts and questions like those here referred to took me, in August 1869,
to the top of the Aletschhorn. The effects described in the foregoing
paragraphs were for the most part reproduced on the summit of the
mountain. I scanned the whole of the sky with my Nicol. Both alone, and in
conjunction with the selenite, it pronounced the perpendicular to the solar
beams to be the direction of maximum polarisation.

But at no portion of the firmament was the polarisation complete. The
artificial sky produced in the experiments recorded in the preceding pages
could, in this respect, be rendered far more perfect than the natural one;
while the gorgeous 'residual blue' which makes its appearance when the
polarisation of the artificial sky ceases to be perfect, was strongly contrasted
with the lack-lustre hue which, in the case of the firmament, outlived the
extinction of the brilliancy. With certain substances, however, artificially
treated, this dull residue may also be obtained.

All along the arc from the Matterhorn to Mont Blanc the light of the sky
immediately above the mountains was powerfully acted upon by the Nicol.
In some cases the variations of intensity were astonishing. I have already
said that a little practice enables the observer to shift the Nicol from one
position to another so rapidly as to render the alternative extinction and
restoration of the light immediate. When this was done along the arc to
which I have referred, the alternations of light and darkness resembled the
play of sheet lightning behind the mountains. There was an element of awe
connected with the suddenness with which the mighty masses, ranged along
the line referred to, changed their aspect and definition under the operation
of the prism.

-----

The physical reason of the blueness of both natural and artificial skies is, I
trust, correctly given in the essay on the Scientific use of the Imagination
published in the second volume of these Fragments.

.

.

.

.

--------------------

.

.

V. ON DUST AND DISEASE.

[Footnote: A discourse delivered before the Royal Institution of Great
Britain, January 21, 1870.]

Experiments on Dusty Air.

SOLAR light, in passing through a dark room, reveals its track by
illuminating the dust floating in the air. 'The sun,' says Daniel Culverwell,
'discovers atomes, though they be invisible by candle-light, and makes them
dance naked in his beams.'

In my researches on the decomposition of vapours by light, I was compelled
to remove these 'atoms' and this dust. It was essential that the space
containing the vapours should embrace no visible thing — that no substance
capable of scattering light in the slightest sensible degree should, at the
outset of an experiments, be found in the wide 'experimental tube' in which
the vapour was enclosed.

For a long time I was troubled by the appearance there of floating matter,
which, though invisible in diffuse daylight, was at once revealed by a
powerfully condensed beam. Two U-tubes were placed in succession in the
path of the air, before it entered the liquid whose vapour was to be carried
into the experimental tube. One of the U-tubes contained fragments of
marble wetted with a strong solution of caustic potash; the other, fragments
of glass wetted with concentrated sulphuric acid which, while yielding no
vapour of its own, powerfully absorbs the aqueous vapour of the air.
[Footnote: The apparatus is figured at p. 98.] To my astonishment, the air
of the Royal Institution, sent through these tubes at a rate sufficiently slow

to dry it, and to remove its carbonic acid, carried into the experimental tube
a considerable amounts of mechanically suspended matter, which was
illuminated when the beam passed through the tube. The effect was
substantially the same when the air was permitted to bubble through the
liquid acid, and through the solution of potash.

I tried to intercept this floating matter in various ways; and on October 5,
1868, prior to sending the air through the drying apparatus, it was carefully
permitted to pass over the tip of a spirit-lamp flame. The floating matter no
longer appeared, having been burnt up by the flame. It was therefore
organic matter. I was by no means prepared for this result; having
previously thought that the dust of our air was, in great part, inorganic and
non-combustible. [Footnote: According to an analysis kindly furnished to
me by Dr. Percy, the dust collected from the walls of the British Museum
contains fully 50 per cent. of inorganic matter. I have every confidence in
the results of this distinguished chemist; they show that the floating dust of
our rooms is, as it were, winnowed from the heavier matter. As bearing
directly upon this point I may quote the following passage from Pasteur:
'Mais ici se présente une remarque: la poussière que Pon trouve à la surface
de tous les corps est soumise constamment à des courants d'air, qui doivent
soulever des particules les plus légères, au nombre desquelles se trouvent,
sans doute, de préférence les corpuscules organisés, oeufs ou spores, moins
lourds généralement que les particules minérales.']

I had constructed a small gas-furnace, now much employed by chemists,
containing a platinum tube, which could be heated to vivid redness.
[Footnote: Pasteur was, I believe, the first to employ such a tube.] The tube
contained a roll of platinum gauze, which, while it permitted the air to pass
through it, ensured the practical contact of the dust with the incandescent
metal. The air of the laboratory was permitted to enter the experimental
tube, sometimes through the cold, and sometimes through the heated, tube
of platinum. In the first column of the following fragment of a long table the
quantity of air operated on is expressed by the depression of the mercury
gauge of the air-pump. In the second column the condition of the platinum
tube is mentioned, and in the third the state of the air in the experimental
tube.

Quantity of air State of platinum tube State of experimental tube

15 inches Cold Full of particles.

30 inches Red-hot Optically empty.

The phrase 'optically empty' shows that when the conditions of perfect
combustion were present, the floating matter totally disappeared.

-----

In a cylindrical beam, which strongly illuminated the dust of the laboratory,
I placed an ignited spirit-lamp. Mingling with the flame, and round its rim,
were seen curious wreaths of darkness resembling an intensely black smoke.
On placing the flame at some distance below the beam, the same dark
masses stormed upwards. They were blacker than the blackest smoke ever
seen issuing from the funnel of a steamer; and their resemblance to smoke
was so perfect as to lead the most practised observer to conclude that the
apparently pure flame of the alcohol lamp required but a beam of sufficient
intensity to reveal its clouds of liberated carbon.

But is the blackness smoke? This question presented itself in a moment and
was thus answered: A red-hot poker was placed underneath the beam: from
it the black wreaths also ascended. A large hydrogen flame was next
employed, and it produced those whirling masses of darkness, far more
copiously than either the spirit-flame or poker. Smoke was therefore out of
the question. [Footnote: In none of the public rooms of the United States
where I had the honour to lecture was this experiment made. The organic
dust was too scanty. Certain rooms in England — the Brighton Pavilion, for
example — also lack the necessary conditions.]

What, then, was the blackness? It was simply that of stellar space; that is to
say, blackness resulting from the absence from the track of the beam of all

matter competent to scatter its light. When the flame was placed below the
beam the floating matter was destroyed in situ; and the air, freed from this
matter, rose into the beam, jostled aside the illuminated particles, and
substituted for their light the darkness due to its own perfect transparency.
Nothing could more forcibly illustrate the invisibility of the agent which
renders all things visible. The beam crossed, unseen, the black chasm
formed by the transparent air, while, at both sides of the gap, the thick-
strewn particles shone out like a luminous solid under the powerful
illumination.

It is not, however, necessary to burn the particles to produce a stream of
darkness. Without actual combustion, currents may be generated which shall
displace the floating matter, and appear dark amid the surrounding
brightness. I noticed this effect first on placing a red-hot copper ball below
the beam, and permitting it to remain there until its temperature had fallen
below that of boiling water. The dark currents, though much enfeebled,
were still produced. They may also be produced by a flask filled with hot
water.

To study this effect a platinum wire was stretched across the beam, the two
ends of the wire being connected with the two poles of a voltaic battery. To
regulate the strength of the current a rheostat was placed in the circuit.
Beginning with a feeble current the temperature of the wire was gradually
augmented; but long before it reached the heat of ignition, a flat stream of
air rose from it, which when looked at edgeways appeared darker and
sharper than one of the blackest lines of Fraunhofer in the purified
spectrum. Right and left of this dark vertical band the floating matter rose
upwards, bounding definitely the non-luminous stream of air. What is the
explanation? Simply this: The hot wire rarefied the air in contact with it, but
it did not equally lighten the floating matter. The convection current of pure
air therefore passed upwards among the inert particles, dragging them after
it right and left, but forming between them an impassable black partition.
This elementary experiments enables us to render an account of the dark
currents produced by bodies at a temperature below that of combustion.

But when the platinum wire is intensely heated, the floating matter is not
only displaced, but destroyed. I stretched a wire about 4 inches long through
the air of an ordinary glass shade resting on cotton-wool, which also

surrounded the rim. The wire being raised to a white heat by an electric
current, the air expanded, and some of it was forced through the cotton-
wool. When the current was interrupted, and the air within the shade cooled,
the returning air did not carry motes along with it, being filtered by the
wool. At the beginning of this experiments the shade was charged with
floating matter; at the end of half an hour it was optically empty.

On the wooden base of a cubical glass shade, a cubic foot in volume,
upright supports were fixed, and from one support to the other 38 inches of
platinum wire were stretched in four parallel lines. The ends of the platinum
wire were soldered to two stout copper wires which passed through the base
of the shade and could be connected with a battery. As in the last
experiments the shade rested upon cotton-wool. A beam sent through the
shade revealed the suspended matter. The platinum wire was then raised to
whiteness. In five minutes there was a sensible diminution of the matter, and
in ten minutes it was totally consumed.

Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to exclude all
floating particles, produce, when poured or blown into the beam, the
darkness of stellar space. Coal-gas does the same. An ordinary glass shade,
placed in the air with its mouth downwards, permits the track of the beam to
be seen crossing it. When coal-gas or hydrogen is allowed to enter the shade
by a tube reaching to its top, the gas gradually fills the shade from above
downwards. As soon as it occupies the space crossed by the beam, the
luminous track is abolished. Lifting the shade so as to bring the common
boundary of gas and air above the beam, the track flashes forth. After the
shade is full, if it be inverted, the pure gas passes upwards like a black
smoke among the illuminated particles.

.

The Germ Theory of Contagious Disease.

There is no respite to our contact with the floating matter of the air; and the
wonder is, not that we should suffer occasionally from its presence, but that
so small a portion of it, and even that but rarely diffused over large areas,
should appear to be deadly to man. And what is this portion? It was some
time ago the current belief that epidemic diseases generally were propagated

by a kind of malaria, which consisted of organic matter in a state of motor-
decay; that when such matter was taken into the body through the lungs,
skin, or stomach, it had the power of spreading there the destroying process
by which itself had been assailed. Such a power was visibly exerted in the
case of yeast. A little leaven was seen to leaven the whole lump — a mere
speck of matter, in this supposed state of decomposition, being apparently
competent to propagate indefinitely its own decay. Why should not a bit of
rotten malaria act in a similar manner within the human frame? In 1836 a
very wonderful reply was given to this question. In that year Cagniard de la
Tour discovered the yeast-plant — a living organism, which when placed in
a proper medium feeds, grows, and reproduces itself, and in this way carries
on the process which we name fermentation. By this striking discovery
fermentation was connected with organic growth.

Schwann, of Berlin, discovered the yeast-plant independently about the
same time; and in February, 1837, he also announced the important result,
that when a decoction of meat is effectually screened from ordinary air, and
supplied solely with calcined air, putrefaction never sets in. Putrefaction,
therefore, he affirmed to be caused, not by the air, but by something which
could be destroyed by a sufficiently high temperature. The results of
Schwann were confirmed by the independent experiments of Helmholtz,
Ure, and Pasteur, while other methods, pursued by Schultze, and by
Schroeder and Dusch, led to the same result.

But as regards fermentation, the minds of chemists, influenced probably by
the great authority of Gay-Lussac, fell back upon the old notion of matter in
a state of decay. It was not the living yeast-plant, but the dead or dying parts
of it, which, assailed by oxygen, produced the fermentation. Pasteur,
however, proved the real 'ferments,' mediate or immediate, to be organised
beings which find in the reputed ferments their necessary food.

Side by side with these researches and discoveries, and fortified by them
and others, has run the germ theory of epidemic disease. The notion was
expressed by Kircher, and favoured by Linnaeus, that epidemic diseases
may be due to germs which float in the atmosphere, enter the body, and
produce disturbance by the development within the body of parasitic life.
The strength of this theory consists in the perfect parallelism of the
phenomena of contagious disease with those of life. As a planted acorn

gives birth to an oak, competent to produce a whole crop of acorns, each
gifted with the power of reproducing its parent tree; and as thus from a
single seedling a whole forest may spring; so, it is contended, these
epidemic diseases literally plant their seeds, grow, and shake abroad new
germs, which, meeting in the human body their proper food and
temperature, finally take possession of whole populations. There is nothing
to my knowledge in pure chemistry which resembles the power of
propagation and self-multiplication possessed by the matter which produces
epidemic disease. If you sow wheat you do not get barley; if you sow small-
pox you do not get scarlet-fever, but small-pox indefinitely multiplied, and
nothing else. The matter of each contagious disease reproduces itself as
rigidly as if it were (as Miss Nightingale puts it) dog or cat.

.

Parasitic Diseases of Silkworms. Pasteur's Researches.

It is admitted on all hands that some diseases are the product of parasitic
growth. Both in man and in lower creatures, the existence of such diseases
has been demonstrated. I am enabled to lay before you an account of an
epidemic of this kind, thoroughly investigated and successfully combated
by M. Pasteur. For fifteen years a plague had raged among the silkworms of
France. They had sickened and died in multitudes, while those that
succeeded in spinning their cocoons furnished only a fraction of the normal
quantity of silk. In 1853 the silk culture of France produced a revenue of
one hundred and thirty millions of francs. During the twenty previous years
the revenue had doubled itself, and no doubt was entertained as to its further
augmentation. The weight of the cocoons produced in 1853 was 26,000,000
kilogrammes; in 1865 it had fallen to 4,000,000, the fall entailing, in a
single year, a loss of 100,000,000 francs.

The country chiefly smitten by this calamity happened to be that of the
celebrated chemist Dumas, now perpetual secretary of the French Academy
of Sciences. He turned to his friend, colleague, and pupil, Pasteur, and
besought him, with an earnestness which the circumstances rendered almost
personal, to undertake the investigation of the malady. Pasteur at this time
had never seen a silkworm, and he urged his inexperience in reply to his
friend. But Dumas knew too well the qualities needed for such an enquiry to

accept Pasteur's reason for declining it. 'Je mets,' said he, 'un prix extréme à
voir votre attention fixée sur la question qui intéresse mon pauvre pays; la
misére surpasse tout ce que vous pouvez imaginer.' Pamphlets about the
plague had been showered upon the public, the monotony of waste paper
being broken, at rare intervals, by a more or less useful publication. 'The
Pharmacopoeia of the Silkworm,' wrote M. Cornalia in 1860, 'is now as
complicated as that of man. Gases, liquids, and solids have been laid under
contribution. From chlorine to sulphurous acid, from nitric acid to rum,
from sugar to sulphate of quinine, — all has been invoked in behalf of this
unhappy insect.' The helpless cultivators, moreover, welcomed with ready
trustfulness every new remedy, if only pressed upon them with sufficient
hardihood. It seemed impossible to diminish their blind confidence in their
blind guides. In 1863 the French Minister of Agriculture signed an
agreement to pay 500,000 francs for the use of a remedy, which its promoter
declared to be infallible. It was tried in twelve different departments of
France, and found perfectly useless. In no single instance was it successful.
It was under these circumstances that M. Pasteur, yielding to the entreaties
of his friend, betook himself to Alais in the beginning of June, 1865. As
regards silk husbandry, this was the most important department in France,
and it was the most sorely smitten by the plague.

The silkworm had been previously attacked by muscardine, a disease
proved by Bassi to be caused by a vegetable parasite. This malady was
propagated annually by the parasitic spores. Wafted by winds they often
sowed the disease in places far removed from the centre of infection.
Muscardine is now said to be very rare, a deadlier malady having taken its
place. This new disease is characterised by the black spots which cover the
silkworms; hence the name pébrine, first applied to the plague by M. de
Quatrefages, and adopted by Pasteur. Pébrine declares itself in the stunted
and unequal growth of the worms, in the languor of their movements, in
their fastidiousness as regards food, and in their premature death. The
course of discovery as regards the epidemic is this: In 1849 Guérin
Méneville noticed in the blood of silkworms vibratory corpuscles, which he
supposed from their motions to be endowed with independent life. Filippi,
however, showed that the motion of the corpuscles was the well-known
Brownian motion; but he committed the error of supposing the corpuscles to
be normal to the life of the insect. Possessing the power of indefinite self-

multiplication, they are really the cause of its mortality — the form and
substance of its disease. This was well described by Cornalia; while Lebert
and Frey subsequently found the corpuscles not only in the blood, but in all
the tissues of the insect. Osimo, in 1857, discovered them in the eggs; and
on this observation Vittadiani founded, in 1859, a practical method of
distinguishing healthy from diseased eggs. The test often proved fallacious,
and it was never extensively applied.

These living corpuscles take possession of the intestinal canal, and spread
thence throughout the body of the worm. They fill the silk cavities, the
stricken insect often going automatically through the motions of spinning,
without any material to work upon. Its organs, instead of being filled with
the clear viscous liquid of the silk, are packed to distension by the
corpuscles. On this feature of the plague Pasteur fixed his entire attention.
The cycle of the silkworm's life is briefly this: From the fertile egg comes
the little worm, which grows, and casts its skin. This process of moulting is
repeated two or three times at intervals during the life of the insect. After the
last moulting the worm climbs the brambles placed to receive it, and spins
among them its cocoon. It passes thus into a chrysalis; the chrysalis
becomes a moth, and the moth, when liberated, lays the eggs which form
the starting-point of a new cycle. Now Pasteur proved that the plague-
corpuscles might be incipient in the egg, and escape detection; they might
also be germinal in the worm, and still baffle the microscope. But as the
worm grows, the corpuscles grow also, becoming larger and more defined.
In the aged chrysalis they are more pronounced than in the worm; while in
the moth, if either the egg or the worm from which it comes should have
been at all stricken, the corpuscles infallibly appear, offering no difficulty of
detection. This was the first great point made out in 1865 by Pasteur. The
Italian naturalists, as aforesaid, recommended the examination of the eggs
before risking their incubation. Pasteur showed that both eggs and worms
might be smitten, and still pass muster, the culture of such eggs or such
worms being sure to entail disaster. He made the moth his starting-point in
seeking to regenerate the race.

Pasteur made his first communication on this subject to the Academy of
Sciences in September, 1865. It raised a cloud of criticism. Here, forsooth,
was a chemist rashly quitting his proper métier and presuming to lay down

the law for the physician and biologist on a subject which was eminently
theirs. 'On trouva étrange que je fusse si peu au courant de la question; on
m'opposa des travaux qui avaient paru depuis longtemps en Italie, dont les
résultats montraient l'inutilité de mes efforts, et l'impossibilité d'arriver à un
résultat pratique dans la direction que je m'étais engagé. Que mon ignorance
fut grande au sujet des recherches sans nombre qui avaient paru depuis
quinze années.' Pasteur heard the buzz, but he continued his work. In
choosing the eggs intended for incubation, the cultivators selected those
produced in the successful 'educations' of the year. But they could not
understand the frequent and often disastrous failures of their selected eggs;
for they did not know, and nobody prior to Pasteur was competent to tell
them, that the finest cocoons may envelope doomed corpusculous moths. It
was not, however, easy to make the cultivators accept new guidance. To
strike their imagination, and if possible determine their practice, Pasteur hit
upon the expedient of prophecy. In 1866 he inspected, at St. Hippolyte-du-
Fort, fourteen different parcels of eggs intended for incubation. Having
examined a sufficient number of the moths which produced these eggs, he
wrote out the prediction of what would occur in 1867, and placed the
prophecy as a sealed letter in the hands of the Mayor of St. Hippolyte.

In 1867 the cultivators communicated to the mayor their results. The letter
of Pasteur was then opened and read, and it was found that in twelve out of
fourteen cases there was absolute conformity between his prediction and the
observed facts. Many of the groups had perished totally; the others had
perished almost totally; and this was the prediction of Pasteur. In two out of
the fourteen cases, instead of the prophesied destruction, half an average
crop was obtained. Now, the parcels of eggs here referred to were
considered healthy by their owners. They had been hatched and tended in
the firm hope that the labour expended on them would prove remunerative.
The application of the moth-test for a few minutes in 1866, would have
saved the labour and averted the disappointment. Two additional parcels of
eggs were at the same time submitted to Pasteur. He pronounced them
healthy; and his words were verified by the production of an excellent crop.
Other cases of prophecy still more remarkable, because more circumstantial,
are recorded in Pasteur's work.

Pasteur subjected the development of the corpuscles to a searching
investigation, and followed out with admirable skill and completeness the
various modes by which the plague was propagated. From moths perfectly
free from corpuscles he obtained healthy worms, and selecting 10, 20, 30,
50, as the case might be, he introduced into the worms the corpusculous
matter. It was first permitted to accompany the food. Let its take a single
example out of many. Rubbing up a small corpusculous worm in water, he
smeared the mixture over the mulberry-leaves. Assuring himself that the
leaves had been eaten, he watched the consequences from day to day. Side
by side with the infected worms he reared their fellows, keeping them as
much as possible out of the way of infection. These constituted his 'lot
témoin,' — his standard of comparison. On April 16, 1868, he thus infected
thirty worms. Up to the 23rd they remained quite well. On the 25th they
seemed well, but on that day corpuscles were found in the intestines of two
of them. On the 27th, or eleven days after the infected repast, two fresh
worms were examined, and not only was the intestinal canal found in each
case invaded, but the silk organ itself was charged with corpuscles. On the
28th the twenty-six remaining worms were covered by the black spots of
pébrine. On the 30th the difference of size between the infected and non-
infected worms was very striking, the sick worms being not more than two-
thirds of the bulk of the healthy ones. On May 2 a worm which had just
finished its fourth moulting was examined. Its whole body was so filled
with the parasite as to excite astonishment that it could live.

The disease advanced, the worms died and were examined, and on May 11
only six out of the thirty remained. They were the strongest of the lot, but on
being searched they also were found charged with corpuscles. Not one of
the thirty worms had escaped; a single meal had poisoned them all. The
standard lot, on the contrary, spun their fine cocoons, two only of their
moths being proved to contain any trace of the parasite, which had doubtless
been introduced during the rearing of the worms.

As his acquaintance with the subject increased, Pasteur's desire for precision
augmented, and he finally counted the growing number of corpuscles seen
in the field of his microscope from day to day. After a contagious repast the
number of worms containing the parasite gradually augmented until finally
it became cent. per cent. The number of corpuscles would at the same time

rise from 0 to 1, to 10, to 100, and sometimes even to 1,000 or 1,500 in the
field of his microscope. He then varied the mode of infection. He inoculated
healthy worms with the corpusculous matter, and watched the consequent
growth of the disease. He proved that the worms inoculate each other by the
infliction of visible wounds with their claws. In various cases he washed the
claws, and found corpuscles in the water. He demonstrated the spread of
infection by the simple association of healthy and diseased worms. By their
claws and their dejections, the diseased worms spread infection. It was no
hypothetical infected medium — no problematical pythogenic gas — that
killed the worms, but a definite organism. The question of infection at a
distance was also examined, and its existence demonstrated. As might be
expected from Pasteur's antecedents, the investigation was exhaustive, the
skill and beauty of his manipulation finding fitting correlatives in the
strength and clearness of his thought.

The following quotation from Pasteur's work clearly shows the relation in
which his researches stand to the important question on which he was
engaged:

-----

Place (he says) the most skilful educator, even the most expert microscopist,
in presence of large educations which present the symptoms described in
our experiments; his judgment will necessarily be erroneous if he confines
himself to the knowledge which preceded my researches. The worms will
not present to him the slightest spot of pébrine; the microscope will not
reveal the existence of corpuscles; the mortality of the worms will be null or
insignificant; and the cocoons leave nothing to be desired. Our observer
would, therefore, conclude without hesitation that the eggs produced will be
good for incubation. The truth is, on the contrary, that all the worms of these
fine crops have been poisoned; that from the beginning they carried in them
the germ of the malady; ready to multiply itself beyond measure in the
chrysalides and the moths, thence to pass into the eggs and smite with
sterility the next generation. And what is the first cause of the evil concealed
under so deceitful an exterior? In our experiments we can, so to speak,
touch it with our fingers. It is entirely the effect of a single corpusculous
repast; an effect more or less prompt according to the epoch of life of the
worm that has eaten the poisoned food.

-----

Pasteur describes in detail his method of securing healthy eggs. It is nothing
less than a mode of restoring to France her ancient silk husbandry. The
justification of his work is to be found in the reports which reached him of
the application and the unparalleled success of his method, while editing his
researches for final publication. In both France and Italy his method has
been pursued with the most surprising results. But it was an up-hill fight
which led to this triumph.

'Ever,' he says, 'since the commencement of these researches, I have been
exposed to the most obstinate and unjust contradictions; but I have made it a
duty to leave no trace of these conflicts in this book.' And in reference to
parasitic diseases, generally, he uses the following weighty words: 'Il est au
pouvoir de l'homme de faire disparaitre de la surface du globe les maladies
parasitaires, si, comme c'est ma conviction, la doctrine des générations
spontanées est une chimère.'

Pasteur dwells upon the ease with which an island like Corsica might be
absolutely isolated from the silkworm epidemic. And with regard to other
epidemics, Mr. Simon describes an extraordinary case of insular exemption,
for the ten years extending from 1851 to 1860. Of the 627 registration
districts of England, one only had an entire escape from diseases which, in
whole or in part, were prevalent in all the others: 'In all the ten years it had
not a single death by measles, nor a single death by small-pox, nor a single
death by scarlet-fever. And why? Not because of its general sanitary merits,
for it had an average amounts of other evidence of unhealthiness. Doubtless,
the reason of its escape was that it was insular. It was the district of the
Scilly Isles; to which it was most improbable that any febrile contagion
should come from without. And its escape is an approximative proof that, at
least for those ten years, no contagium of measles, nor any contagium of
scarlet-fever, nor any contagium of smallpox had arisen spontaneously
within its limits.' It may be added that there were only seven districts in
England in which no death from diphtheria occurred, and that, of those
seven districts, the district of the Scilly Isles was one.

A second parasitic disease of silkworms, called in France la flacherie, co-
existent with pébrine, but quite distinct from it, has also been investigated

by Pasteur. Enough, however, has been said to send the reader interested in
these questions to the original volumes for further information. To one
important practical point M. Pasteur, in a letter to myself, directs attention:

-----

Permettez-moi de terminer ces quelques lignes que je dois dicter, vaincu que
je suis par la maladie, en vous faisant observer que vous rendriez service
aux Colonies de la Grande-Bretagne en répandant la connaissance de ce
livre, et des principes que j'établis touchant la maladie des vers à soie.
Beaucoup de ces colonies pourraient cultiver le mûrier avec succés, et, en
jetant les yeux sur mon ouvrage, vous vous convaincrez aisement qu'il est
facile aujourd'hui, nonseulement d'éloigner la maladie régnante, mais en
outre de donner aux récoltes de la soie une prospérité qu'elles n'ont jamais
eue.

Origin and Propagation of Contagious Matter.

Prior to Pasteur, the most diverse and contradictory opinions were
entertained as to the contagious character of pébrine; some stoutly affirmed
it, others as stoutly denied it. But on one point all were agreed. I They
believed in the existence of a deleterious medium, rendered epidemic by
some occult and mysterious influence, to which was attributed the cause of
the disease.' Those acquainted with our medical literature will not fail to
observe an instructive analogy here. We have on the one side accomplished
writers ascribing epidemic diseases to 'deleterious media' which arise
spontaneously in crowded hospitals and ill-smelling drains. According to
them, the contagia of epidemic disease are formed de novo in a putrescent
atmosphere. On the other side we have writers, clear, vigorous, with well-
defined ideas and methods of research, contending that the matter which
produces epidemic disease comes always from a parent stock. It behaves as
germinal matter, and they do not hesitate to regard it as such. They no more
believe in the spontaneous generation of such diseases, than they do in the
spontaneous generation of mice. Pasteur, for example, found that pébrine
had been known for an indefinite time as a disease among silkworms. The
development of it which he combated was merely the expansion of an

already existing power — the bursting into open conflagration of a
previously smouldering fire. There is nothing surprising in this. For though
epidemic disease requires a special contagium to produce it, surrounding
conditions must have a potent influence on its development. Common seeds
may be duly sown, but the conditions of temperature and moisture may be
such as to restrict, or altogether prevent, the subsequent growth. Looked at,
therefore, from the point of view of the germ theory, the exceptional energy
which epidemic disease from time to time exhibits, is in harmony with the
method of Nature. We sometimes hear diphtheria spoken of as if it were a
new disease of the last twenty years; but Mr. Simon tells me that about three
centuries ago tremendous epidemics of it began to rage in Spain (where it
was named Garrotillo), and soon afterwards in Italy; and that since that time
the disease has been well known to all successive generations of doctors. In
or about 1758, for instance, Dr. Starr, of Liskeard, in a communication to
the Royal Society, particularly described the disease, with all the characters
which have recently again become familiar, but under the name of morbus
strangulatorius, as then severely epidemic in Cornwall. This fact is the
more interesting, as diphtheria, in its more modern reappearance, again
showed predilection for that remote county. Many also believe that the
Black Death, of five centuries ago, has disappeared as mysteriously as it
came; but Mr. Simon finds that it is believed to be prevalent at this hour in
some of the north-western parts of India.

Let me here state an item of my own experience. When I was at the Bel Alp
in 1869, the English chaplain received letters informing him of the breaking
out of scarlet-fever among his children. He lived, if I remember rightly, on
the healthful eminence of Dartmoor, and it was difficult to imagine how
scarlet-fever could have been wafted to the place. A drain ran close to his
house, and on it his suspicions were manifestly fixed. Some of our medical
writers would fortify him in this notion, and thus deflect him from the truth,
while those of another, and, in my opinion, a wiser school, would deny to a
drain, however foul, the power of generating de novo a specific disease.
After close enquiry he recollected that a hobby-horse had been used both by
his boy and another, who, a short time previously, had passed through
scarlet-fever.

Drains and cesspools, indeed, are by no means in such evil odour as they
used to be. A fetid Thames and a low death-rate occur from time to time
together in London. For, if the special matter or germs of epidemic disorder
be not present, a corrupt atmosphere, however obnoxious otherwise, will not
produce the disorder. But, if the germs be present, defective drains and
cesspools become the potent distributors of disease and death. Corrupted air
may promote an epidemic, but cannot produce it. On the other hand,
through the transport of the special germ or virus, disease may develop itself
in regions where the drainage is good and the atmosphere pure.

If you see a new thistle growing in your field, you feel sure that its seed has
been wafted thither. Just as sure does it seem that the contagious matter of
epidemic disease has been transplanted to the place where it newly appears.
With a clearness and conclusiveness s not to be surpassed, Dr. William
Budd has traced such diseases from place to place; showing how they plant
themselves, at distinct foci, among populations subjected to the same
atmospheric influences, just as grains of corn might be carried in the pocket
and sown. Hildebrand, to whose remarkable work, 'Du Typhus contagieux,'
Dr. de Mussy has directed my attention, gives the following striking case,
both of the durability and the transport of the virus of scarlatina: 'Un habit
noir que j'avais en visitant une malade attaquée de scarlatina, et que je portai
de Vienne en Podolie, sans l'avoir mis depuis plus d'un an et demi, me
communiqua, dès que je fus arrivé, cette maladie contagieuse, que je
répandis ensuite dans cette province, où elle était jusqu'alors presque
inconnue.' Some years ago Dr. de Mussy himself was summoned to a
country house in Surrey, to see a young lady who was suffering from a
dropsy, evidently the consequence of scarlatina. The original disease, being
of a very mild character, had been quite overlooked; but circumstances were
recorded which could leave no doubt upon the mind as to the nature and
cause of the complaint. But then the question arose, How did the young lady
catch the scarlatina? She had come there on a visit two months previously,
and it was only after she had been a month in the house that she was taken
ill. The housekeeper at length cleared up the mystery. The young lady, on
her arrival, had expressed a wish to occupy a room in an isolated tower. Her
desire was granted; and in that room, six months previously, a visitor had
been confined with an attack of scarlatina. The room had been swept and
whitewashed, but the carpets had been permitted to remain.

Thousands of cases could probably be cited in which the disease has shown
itself in this mysterious way, but where a strict examination has revealed its
true parentage and extraction. Is it, then, philosophical to take refuge in the
fortuitous concourse of atoms as a cause of specific disease, merely because
in special cases the parentage may be indistinct? Those best acquainted with
atomic nature, and who are most ready to admit, as regards even higher
things than this, the potentialities of matter, will be the last to accept these
rash hypotheses.

.

The Germ Theory applied to Surgery.

Not only medical but still more especially surgical science is now seeking
light and guidance from this germ theory. Upon it the antiseptic system of
Professor Lister of Edinburgh is founded. As already stated, the germ theory
of putrefaction was started by Schwann; but the illustrations of this theory
adduced by Professor Lister are of such public moment as not only to
justify, but to render imperative, their introduction here.

Schwann's observations (says Professor Lister) did not receive the attention
which they appeared to me to have deserved. The fermentation of sugar was
generally allowed to be occasioned by the Torula cerevisiae; but it was not
admitted that putrefaction was due to an analogous agency. And yet the two
cases present a very striking parallel. In each a stable chemical compound,
sugar in the one case, albumen in the other, undergoes extraordinary
chemical changes under the influence of an excessively minute quantity of a
substance which, regarded chemically, we should suppose inert. As an
example of this in the case of putrefaction, let us take a circumstance often
witnessed in the treatment of large chronic abscesses. In order to guard
against the access of atmospheric air, we used to draw off the matter by
means of a canula and trocar, such as you see here, consisting of a silver
tube with a sharp-pointed steel rod fitted into it, and projecting beyond it.
The instrument, dipped in oil, was thrust into the cavity of the abscess, the
trocar was withdrawn, and the pus flowed out through the canula, care being
taken by gentle pressure over the part to prevent the possibility of
regurgitation. The canula was then drawn out with due precaution against
the reflux of air. This method was frequently successful as to its immediate

object, the patient being relieved from the mass of the accumulated fluid,
and experiencing no inconvenience from the operation. But the pus was
pretty certain to reaccumulate in course of time, and it became necessary
again and again to repeat the process. And unhappily there was no absolute
security of immunity from bad consequences. However carefully the
procedure was conducted, it sometimes happened, even though the puncture
seemed healing by first intention, that feverish symptoms declared
themselves in the course of the first or second day, and, on inspecting the
seat of the abscess, the skin was perhaps seen to be red, implying the
presence of some cause of irritation, while a rapid reaccumulation of the
fluid was found to have occurred. Under these circumstances, it became
necessary to open the abscess by free incision, when a quantity, large in
proportion to the size of the abscess, say, for example, a quart, of pus
escaped, fetid from putrefaction. Now, how had this change been brought
about? Without the germ theory, I venture to say, no rational explanation of
it could have been given. It must have been caused by the introduction of
something from without. Inflammation of the punctured wound, even
supposing it to have occurred, would not explain the phenomenon. For mere
inflammation, whether acute or chronic, though it occasions the formation
of pus, does not induce Putrefaction. The pus originally evacuated was
perfectly sweet, and we know of nothing to account for the alteration in its
quality but the influence of something derived from the external world. And
what could that something be? The dipping of the instrument in oil, and the
subsequent precautions, prevented the entrance of oxygen. Or even if you
allowed that a few atoms of the gas did enter, it would be an extraordinary
assumption to make that these could in so short a time effect such changes
in so large a mass of albuminous material. Besides, the pyogenic membrane
is abundantly supplied with capillary vessels, through which arterial blood,
rich in oxygen, is perpetually flowing; and there can be little doubt that the
pus, before it was evacuated at all, was liable to any action which the
element might be disposed to exert upon it.

On the oxygen theory, then, the occurrence of putrefaction under these
circumstances is quite inexplicable. But if you admit the germ theory, the
difficulty vanishes at once. The canula and trocar having been lying exposed
to the air, dust will have been deposited upon them, and will be present in
the angle between the trocar and the silver tube, and in that protected

situation will fail to be wiped off when the instrument is thrust through the
tissues. Then when the trocar is withdrawn, some portions of this dust will
naturally remain upon the margin of the canula, which is left projecting into
the abscess, and nothing is more likely than that some particles may fail to
be washed off by the stream of out-flowing pus, but may be dislodged when
the tube is taken out, and left behind in the cavity. The germ theory tells us
that these particles of dust will be pretty sure to contain the germs of
putrefactive organisms, and if one such is left in the albuminous liquid, it
will rapidly develop at the high temperature of the body, and account for all
the phenomena.

But striking as is the parallel between putrefaction in this instance and the
vinous fermentation, as regards the greatness of the effect produced,
compared with the minuteness and the inertness, chemically speaking, of
the cause, you will naturally desire further evidence of the similarity of the
two processes. You can see with the microscope the Torula of fermenting
must or beer. Is there, you may ask, any organism to be detected in the
putrefying pus? Yes, gentlemen, there is. If any drop of the putrid matter is
examined with a good glass, it is found to be teeming with myriads of
minute jointed bodies, called vibrios, which indubitably proclaim their
vitality by the energy of their movements. It is not an affair of probability,
but a fact, that the entire mass of that quart of pus has become peopled with
living organisms as the result of the introduction of the canula and trocar;
for the matter first let out was as free from vibrios as it was from
putrefaction. If this be so, the greatness of the chemical changes that have
taken place in the pus ceases to be surprising. We know that it is one of the
chief peculiarities of living structures that they possess extraordinary
powers of effecting chemical changes in materials in their vicinity, out of all
proportion to their energy as mere chemical compounds. And we can hardly
doubt that the animalcules which have been developed in the albuminous
liquid, and have grown at its expense, must have altered its constitution, just
as we ourselves alter that of the materials on which we feed. [Footnote:
'Introductory Lecture before the University of Edinburgh.']

In the operations of Professor Lister care is taken that every portion of tissue
laid bare by the knife shall be defended from germs; that if they fall upon
the wound they should be killed as they fall. With this in view he showers

upon his exposed surfaces the spray of dilute carbolic acid, which is
particularly deadly to the germs, and he surrounds the wound in the most
careful manner with antiseptic bandages. To those accustomed to strict
experiment it is manifest that we have a strict experimenter here — a man
with a perfectly distinct object in view, which he pursues with never-tiring
patience and unwavering faith. And the result, in his hospital practice, as
described by himself, has been, that even in the midst of abominations too
shocking to be mentioned here, and in the neighbourhood of wards where
death was rampant from pyaemia, erysipelas, and hospital gangrene, he was
able to keep his patients absolutely free from these terrible scourges. Let me
here recommend to your attention Professor Lister's 'Introductory Lecture
before the University of Edinburgh,' which I have already quoted; his paper
on The Effect of the Antiseptic System of Treatment on the Salubrity of a
Surgical Hospital;' and the article in the 'British Medical Journal' of January
14, 1871.

If, instead of using carbolic acid spray, he could surround his wounds with
properly filtered air, the result would, he contends, be the same. In a room
where the germs not only float but cling to clothes and walls, this would be
difficult, if not impossible. But surgery is acquainted with a class of wounds
in which the blood is freely mixed with air that has passed through the
lungs, and it is a most remarkable fact that such air does not produce
putrefaction. Professor Lister, as far as I know, was the first to give a
philosophical interpretation of this fact, which he describes and comments
upon thus:

I have explained to my own mind the remarkable fact that in simple fracture
of the ribs, if the lung be punctured by a fragment, the blood effused into the
pleural cavity, though freely mixed with air, undergoes no decomposition.
The air is sometimes pumped into the pleural cavity in such abundance that,
making its way through the wound in the pleura costalis, it inflates the
cellular tissue of the whole body. Yet this occasions no alarm to the surgeon
(although if the blood in the pleura were to putrefy, it would infallibly
occasion dangerous suppurative pleurisy). Why air introduced into the
pleural cavity through a wounded lung, should have such wholly different
effects from that entering directly through a wound in the chest, was to me a
complete mystery until I heard of the germ theory of putrefaction, when it at

once occurred to me that it was only natural that air should be filtered of
germs by the air-passages, one of whose offices is to arrest inhaled particles
of dust, and prevent them from entering the air-cells.

-----

I shall have occasion to refer to this remarkable hypothesis farther on.

The advocates of the germ theory, both of putrefaction and epidemic
disease, hold that both arise, not from the air, but from something contained
in the air. They hold, moreover, that this 'something' is not a vapour nor a
gas, nor indeed a molecule of any kind, but a particle. [Footnote: As
regards size, there is probably no sharp line of division between molecules
and particles; the one gradually shades into the other. But the distinction
that I would draw is this: the atom or the molecule, if free, is always part of
a gas, the particle is never so. A particle is a bit of liquid or solid matter,
formed by the Aggregation of atoms or molecules.] The term 'particulate
'has been used in the Reports of the Medical Department of the Privy
Council to describe this supposed constitution of contagious matter; and Dr.
Sanderson's experiments render it in the highest degree probable, if they do
not actually demonstrate, that the virus of small-pox is 'particulate.' Definite
knowledge upon this point is of exceeding importance, because in the
treatment of particles methods are available which it would be futile to
apply to molecules.

The Luminous beam as a means of Research.

My own interference with this great question, while sanctioned by eminent
names, has been also an object of varied and ingenious attack. On this point
I will only say that when angry feeling escapes from behind the intellect,
where it may be useful as an urging force, and places itself athwart the
intellect, it is liable to produce all manner of delusions. Thus my censors,
for the most part, have levelled their remarks against positions which were
never assumed, and against claims which were never made. The simple
history of the matter is this: During the autumn of 1868 I was much
occupied with the observations referred to at the beginning of this discourse,

and in part described in the preceding article. For fifteen years it had been
my habit to make use of floating dust to reveal the paths of luminous beams
through the air; but until 1868 I did not intentionally reverse the process,
and employ a luminous beam to reveal and examine the dust. In a paper
presented to the Royal Society in December, 1869, the observations which
induced me to give more special attention to the question of spontaneous
generation, and the germ theory of epidemic disease, are thus described:

The Floating Matter of the Air.

Prior to the discovery of the foregoing action (the chemical action of light
upon vapours, Fragment IV.), and also during the experiments just referred
to, the nature of my work compelled me to aim at obtaining experimental
tubes absolutely clean upon the surface, and absolutely free within from
suspended matter. Neither condition is, however, easily attained.

For however well the tubes might be washed and polished, and however
bright and pure they might appear in ordinary daylight, the electric beam
infallibly revealed signs and tokens of dirt. The air was always present, and
it was sure to deposit some impurity. All chemical processes, not conducted
in a vacuum, are open to this disturbance. When the experimental tube was
exhausted, it exhibited no trace of floating matter, but on admitting the air
through the U-tubes (containing caustic potash and sulphuric acid), a dust-
cone more or less distinct was always revealed by the powerfully condensed
electric beam.

The floating motes resembled minute particles of liquid which had been
carried mechanically from the U-tubes into the experimental tube.
Precautions were therefore taken to prevent any such transfer. They
produced little or no mitigation. I did not imagine, at the time, that the dust
of the external air could find such free passage through the caustic potash
and sulphuric acid. This, however, was the case; the motes really came from
without. They also passed with freedom through a variety of aethers and
alcohols. In fact, it requires long-continued action on the part of an acid first
to wet the motes and afterwards to destroy them. By carefully passing the
air through the flame of a spirit lamp, or through a platinum tube heated to

bright redness, the floating matter was sensibly destroyed. It was therefore
combustible, in other words, organic, matter. I tried to intercept it by a large
respirator of cotton-wool. Close pressure was necessary to render the wool
effective. A plug of the wool, rammed pretty tightly into the tube through
which the air passed, was finally found competent to hold back the motes.
They appeared from time to time afterwards, and gave me much trouble; but
they were invariably traced in the end to some defect in the purifying
apparatus — to some crack or flaw in the sealing-wax employed to render
the tubes air-tight. Thus through proper care, but not without a great deal of
searching out of disturbances, the experimental tube, even when filled with
air or vapour, contains nothing competent to scatter the light. The space
within it has the aspect of an absolute vacuum.

An experimental tube in this condition I call optically empty.

The simple apparatus employed in these experiments will be at once
understood by reference to a figure printed in the last article (Fig. 3.) s s' is
the glass experimental tube, which has varied in length from 1 to 5 feet, and
which may be from 2 to 3 inches in diameter. From the end s, the pipe p p'
passes to an air-pump. Connected with the other end s' we have the flask F,
containing the liquid whose vapour is to be examined; then follows a U-
tube, T, filled with fragments of clean glass, wetted with sulphuric acid;
then a second U-tube, T, containing fragments of marble, wetted with
caustic potash; and finally a narrow straight tube t t', containing a tolerably
tightly fitting plug of cotton-wool. To save the air-pump gauge from the
attack of such vapours as act on mercury, as also to facilitate observation, a
separate barometer tube was employed.

Through the cork which stops the flask F two glass tubes, a and b, pass air-
tight. The tube a ends immediately under the cork; the tube b, on the
contrary, descends to the bottom of the flask and dips into the liquid. The
end of the tube b is drawn out so as to render very small the orifice through
which the air escapes into the liquid.

The experimental tube s s' being exhausted, a cock at the end s' is turned
carefully on. The air passes slowly through the cotton-wool, the caustic
potash, and the sulphuric acid in succession. Thus purified, it enters the
flask F and bubbles through the liquid. Charged with vapour, it finally

passes into the experimental tube, where it is submitted to examination. The
electric lamp L placed at the end of the experimental tube furnishes the
necessary beam.

-----

The facts here forced upon my attention had a bearing too evident to be
overlooked. The inability of air which had been filtered through cotton-wool
to generate animalcular life, had been demonstrated by Schroeder and
Pasteur: here the cause of its impotence was rendered evident to the eye.
The experiment proved that no sensible amount of light was scattered by the
molecules of the air; that the scattered light always arose from suspended
particles; and the fact that the removal of these abolished simultaneously
the power of scattering light and of originating life, obviously detached the
life-originating power from the air, and fixed it on something suspended in
the air. Gases of all kinds passed with freedom through the plug of cotton-
wool; hence the thing whose removal by the cotton-wool rendered the gas
impotent, could not itself have been matter in the gaseous condition. It at
once occurred to me that the retina, protected as it was, in these
experiments, from all extraneous light, might be converted into a new and
powerful instrument of demonstration in relation to the germ theory.

But the observations also revealed the danger incurred in experiments of
this nature; showing that without an amount of care far beyond that hitherto
bestowed upon them, such experiments left the door open to errors of the
gravest description. It was especially manifest that the chemical method
employed by Schultze in his experiments, and so often resorted to since,
might lead to the most erroneous consequences; that neither acids nor
alkalies had the power of rapid destruction hitherto ascribed to them. In
short, the employment of the luminous beam rendered evident the cause of
success in experiments rigidly conducted like those of Pasteur; while it
made equally evident the certainty of failure in experiments less severely
carried out.

Dr. Bennett's Experiments.

But I do not wish to leave an assertion of this kind without illustration.
Take, then, the well-conceived experiments of Dr. Hughes Bennett,
described before the Royal Society of Surgeons in Edinburgh on January 17,
1868. [Footnote: 'British Medical Journal,' 13, pt. ii. 1868.] Into flasks
containing decoctions of liquorice-root, hay, or tea, Dr. Bennett, by an
ingenious method, forced air. The air was driven through two U-tubes, the
one containing a solution of caustic potash, the other sulphuric acid. 'All the
bent tubes were filled with fragments of pumice-stone to break up the air, so
as to prevent the possibility of any germs passing through in the centre of
bubbles.' The air also passed through a Liebig's bulb containing sulphuric
acid, and also through a bulb containing gun-cotton.

It was only natural for Dr. Bennett to believe that his 'bent tubes' entirely cut
off the germs. Previous to the observations just referred to, I also believed in
their efficacy. But these observations destroy any such notion. The gun-
cotton, moreover, will fail to arrest the whole of the floating matter, unless it
is tightly packed, and there is no indication in Dr. Bennett's memoir that it
was so packed. On the whole, I should infer, from the mere inspection of Dr.
Bennett's apparatus, the very results which he has described — a retardation
of the development of life, a total absence of it in some cases, and its
presence in others.

In his first series of experiments, eight flasks were fed with sifted air, and
five with common air. In ten or twelve days all the five had fungi in them;
whilst it required from four to nine months to develop fungi in the others. In
one of the eight, moreover, even after this interval no fungi appeared. In a
second series of experiments there was a similar exception. In a third series
the cork stoppers used in the first and second series were abandoned, and
glass stoppers employed. Flasks containing decoctions of tea, beef, and hay
were filled with common air, and other flasks with sifted air. In every one of
the former fungi appeared and in not one of the latter. These experiments
simply ruin the doctrine that Dr. Bennett finally espouses.

In all these negative cases, the prepared air was forced into the infusion
when it was boiling hot. Dr. Bennett made a fourth series of experiments, in
which, previous to forcing in the air, he permitted the flasks to cool. Into
four bottles thus treated he forced prepared air, and after a time found fungi
in all of them. What is his conclusion? Not that the boiling hot liquid,

employed in his first experiments, had destroyed such germs as had run the
gauntlet of his apparatus; but that air which, previous to being sealed up,
had been exposed to a temperature of 212°, is too rare to support life. This
conclusion is so remarkable that it ought to be stated in Dr. Bennett's own
words. 'It may be easily conceived that air subjected to a boiling
temperature is so expanded as scarcely to merit the name of air, and that it is
more or less unfit for the purpose of sustaining animal or vegetable life.'

Now numerical data are attainable here, and as a matter of fact I live and
flourish for a considerable portion of each year in a medium of less density
than that which Dr. Bennett describes as scarcely meriting the name of air.
The inhabitants of the higher Alpine chalets, with their flocks and herds, and
the grasses which support these, do the same; while the chamois rears its
kids in air rarer still. Insect life, moreover, is sometimes exhibited with
monstrous prodigality at Alpine heights.

In a fifth series of experiments sixteen bottles were filled with infusions.
Into four of them, while cold, ordinary unheated and unsifted air was
pumped. In these four bottles fungi were developed. Into four other bottles,
containing a boiling infusion, ordinary air was also pumped — no fungi
were here developed. Into four other bottles containing an infusion which
had been boiled and permitted to cool, sifted air was pumped — no fungi
were developed. Finally, into four bottles containing a boiling infusion
sifted air was pumped no fungi were developed. Only, therefore, in the four
cases where the infusions were cold infusions, and the air ordinary air, did
fungi appear.

Dr. Bennett does not draw from his experiments the conclusion to which
they so obviously point. On them, on the contrary, he founds a defence of
the doctrine of spontaneous generation, and a general theory of spontaneous
development. So strongly was he impressed with the idea that the germs
could not possibly pass through his potash and sulphuric acid tubes, that the
appearance of fungi, even in a small minority of cases, where the air had
been sent through these tubes, was to him conclusive evidence of the
spontaneous origin of such fungi. And he accounts for the absence of life in
many of his experiments by an hypothesis which will not bear a moment's
examination. But, knowing that organic particles may pass unscathed
through alkalies and acids, the results of Dr. Bennett are precisely what

ought wider the circumstances to be expected. Indeed, their harmony with
the conditions now revealed is a proof of the honesty and accuracy with
which they were executed.

The caution exercised by Pasteur both in the execution of his experiments,
and in the reasoning based upon them, is perfectly evident to those who,
through the practice of severe experimental enquiry, have rendered
themselves competent to judge of good experimental work. He found germs
in the mercury used to isolate his air. He was never sure that they did not
cling to the instruments he employed, or to his own person. Thus when he
opened his hermetically sealed flasks upon the Mer de Glace, he had his eye
upon the file used to detach the drawn-out necks of his bottles; and he was
careful to stand to leeward when each flask was opened. Using these
precautions, he found the glacier air incompetent, in nineteen cases out of
twenty, to generate life; while similar flasks, opened amid the vegetation of
the lowlands, were soon crowded with living things. M. Pouchet repeated
Pasteur's experiments in the Pyrenees, adopting the precaution of holding
his flasks above his head, and obtaining a different result. Now great care
would be needed to render this procedure a real precaution. The luminous
beam at once shows us its possible effect. Let smoking brown paper be
placed at the open mouth of a glass shade, so that the smoke shall ascend
and fill the shade. A beam sent through the shade forms a bright track
through the smoke. When the closed fist is placed underneath the shade, a
vertical wind of surprising violence, considering the small elevation of
temperature, rises from the band, displacing by comparatively dark air the
illuminated smoke. Unless special care were taken such a wind would rise
from M. Pouchet's body as he held his flasks above his head, and thus the
precaution of Pasteur, of not coming between the wind and the flask, would
be annulled.

Let me now direct attention to another result of Pasteur, the cause and
significance of which are at once revealed by the luminous beam. He
prepared twenty one flasks, each containing a decoction of yeast, filtered
and clear. He boiled the decoction so as to destroy whatever germs it might
contain, and, while the space above the liquid was filled with pure steam, he
sealed his flasks with a blow-pipe. He opened ten of them in the deep, damp
caves of the Paris Observatory, and eleven of them in the courtyard of the

establishment. Of the former, one only showed signs of life subsequently. In
nine out of the ten flasks no organisms of any kind were developed. In all
the others organisms speedily appeared.

Now here is an experiment conducted in Paris, on which we can throw
obvious light in London. Causing our luminous beam to pass through a
large flask filled with the air of this room, and charged with its germs and its
dust, the beam is seen crossing the flask from side to side. But here is
another similar flask, which cuts a clear gap out of the beam. It is filled with
unfiltered air, and still no trace of the beam is visible. Why? By pure
accident I stumbled on this flask in our apparatus room, where it had
remained quiet for some time. Acting upon this obvious suggestion I set
aside three other flasks, filled, in the first instance, with mote-laden air.
They are now optically empty. Our former experiments proved that the life-
producing particles attach themselves to the fibres of cotton-wool. In the
present experiment the motes have been brought by gentle air-currents,
established by slight differences of temperature within our closed vessels,
into contact with the interior surface, to which they adhere. The air of these
flasks has deposited its dust, germs and all, and is practically free from
suspended matter.

I had a chamber erected, the lower half of which is of wood, its upper half
being enclosed by four glazed window-frames. It tapers to a truncated cone
at the top. It measures in plan 3 ft. by 2 ft. 6 in., and its height is 5 ft. 10 in.
On February 6 it was closed, every crevice that could admit dust, or cause
displacement of the air, being carefully pasted over with paper. The electric
beam at first revealed the dust within the chamber as it did in the air of the
laboratory. The chamber was examined almost daily; a perceptible
diminution of the floating matter being noticed as time advanced. At the end
of a week the chamber was optically empty, exhibiting no trace of matter
competent to scatter the light. Such must have been the case in the stagnant
caves of the Paris Observatory. Were our electric beam sent through the air
of these caves its track would be invisible; thus showing the indissoluble
association of the scattering of light by air and its power to generate life.

I will now turn to what seems to me a more interesting application of the
luminous beam than any hitherto described. My reference to Professor
Lister's interpretation of the fact, that air which has passed through the lungs

cannot produce putrefaction, is fresh in your memories. 'Why air,' said he,
'introduced into the pleural cavity, through a wounded lung, should have
such wholly different effects from that entering through a permanently open
wound, penetrating from without, was to me a complete mystery, till I heard
of the germ -theory of putrefaction, when it at once occurred to me that it
was only natural that the air should be filtered of germs by the air passages,
one of whose offices is to arrest inhaled particles of. dust, and prevent them
from entering the air-cells.'

Here is a surmise which bears the stamp of genius, but which needs
verification. If, for the words 'it is only natural' we were authorised to write
'it is perfectly certain,' the demonstration would be complete. Such
demonstration is furnished by experiments with a beam of light. One
evening, towards the close of 1869, while pouring various pure gases across
the dusty track of a luminous beam, the thought occurred to me of using my
breath instead of the gases. I then noticed, for the first time, the
extraordinary darkness produced by the expired air, towards the end of the
expiration. Permit me to repeat the experiment in your presence. I fill my
lungs with ordinary air and breathe through a glass tube across the beam.
The condensation of the aqueous vapour of the breath is shown by the
formation of a luminous white cloud of delicate texture. We abolish this
cloud by drying the breath previous to its entering the beam; or, still more
simply, by warming the glass tube. The luminous track of the beam is for a
time uninterrupted by the breath, because the dust returning from the lungs
makes good, in great part, the particles displaced. After a time, however, an
obscure disk appears in the beam, the darkness of which increases, until
finally, towards the end of the expiration, the beam is, as it were, pierced by
an intensely black hole, in which no particles whatever can be discerned.
The deeper air of the lungs is thus proved to be absolutely free from
suspended matter. It is therefore in the precise condition required by
Professor Lister's explanation. This experiment may be repeated any
number of times with the same result. I think it must be regarded as a
crowning piece of evidence both of the correctness of Professor Lister's
views and of the impotence, as regards vital development, of optically pure
air. [Footnote: Dr. Burden Sanderson draws attention to the important
observation of Brauell, which shows that the contagium of a pregnant
animal, suffering from splenic fever, is not found in the blood of the foetus;

the placental apparatus acting as a filter, and holding back the infective
particles. ]

.

Application of Luminous beams to Water.

The method of examination here pursued is also applicable to water. It is in
some sense complementary to that of the microscope, and may, I think,
materially aid enquiries conducted with that instrument. In microscopic
examination attention is directed to a small portion of the liquid, and the
aim is to detect the individual particles. By the present method a large
portion of the liquid is illuminated, the collective action of the particles
being revealed, by the scattered light. Care is taken to defend the eye from
the access of all other light, and, thus defended, it becomes an organ of
inconceivable delicacy. Indeed, an amount of impurity so infinitesimal as to
be scarcely expressible in numbers, and the individual particles of which are
so small as wholly to elude the microscope, may, when examined by the
method alluded to, produce not only sensible, but striking, effects upon the
eye.

We will apply the method, in the first place, to an experiment of M. Pouchet
intended to prove conclusively that animalcular life is developed in cases
where no antecedent germs could possibly exist. He produced water from
the combustion of hydrogen in air, justly arguing that no germ could survive
the heat of a hydrogen flame. But he overlooked the fact that his aqueous
vapour was condensed in the air, and was allowed as water to trickle
through the air. Indeed the experiment is one of a number by which workers
like M. Pouchet are differentiated from workers like Pasteur. I will show
you some water, produced by allowing a hydrogen flame to play upon a
polished silver condenser, formed by the bottom of a silver basin,
containing ice. The collected liquid is pellucid in the common light; but in
the condensed electric beam it is seen to be laden with particles, so thick-
strewn and minute as to produce a continuous luminous cone. In passing
through the air the water loaded itself with this matter; and the deportment
of such water could obviously have no influence in deciding this great
question.

We are invaded with dirt not only in the air we breathe, but in the water we
drink. To prove this I take the bottle of water intended to quench your
lecturer's thirst; which, in the track of the beam, simply reveals itself as
dirty water. And this water is no worse than the other London waters.
Thanks to the kindness of Professor Frankland, I have been furnished with
specimens of the water of eight London companies. They are all laden with
impurities mechanically suspended. But you will ask whether filtering will
not remove the suspended matter? The grosser matter, undoubtedly, but not
the more finely divided matter. Water may be passed any number of times
through bibulous paper, it will continue laden with fine matter. Water passed
through Lipscomb's charcoal filter, or through the filters of the Silicated
Carbon Company, has its grosser matter removed, but it is thick with fine
matter. Nine-tenths of the light scattered by these suspended particles is
perfectly polarised in a direction at right angles to the beam, and this release
of the particles from the ordinary law of polarisation is a demonstration of
their smallness. I should say by far the greater number of the particles
concerned in this scattering are wholly beyond the range of the microscope,
and no ordinary filter can intercept such particles. It is next to impossible,
by artificial means, to produce a pure water. Mr. Hartley, for example, some
time ago distilled water while surrounded by hydrogen, but the water was
not free from floating matter. It is so hard to be clean in the midst of dirt. In
water from the Lake of Geneva, which has remained long without being
stirred, we have an approach to the pure liquid. I have a bottle of it here,
which was carefully filled for me by my distinguished friend Soret. The
track of the beam through it is of a delicate sky-blue; there is scarcely a
trace of grosser matter.

The purest water that I have seen — probably the purest which has been
seen hitherto — has been obtained from the fusion of selected specimens of
ice. But extraordinary. precautions are required to obtain this degree of
purity. The following apparatus has been constructed for this purpose:
Through the plate of an air-pump passes the shank of a large funnel,
attached to which below the plate is a clean glass bulb. In the funnel is
placed a block of the most transparent ice, and over the funnel a glass
receiver. This is first exhausted and refilled several times with air, filtered
by its passage through cotton-wool, the ice being thus surrounded by pure
moteless air. But the ice has previously been in contact with mote-filled air;

it is therefore necessary to let it wash its own surface, and also to wash the
bulb which is to receive the water of liquefaction. The ice is permitted to
melt, the bulb is filled and emptied several times, until finally the large
block dwindles to a small one. We may be sure that all impurity has been
thus removed from the surface of the ice. The water obtained in this way is
the purest hitherto obtained. Still I should hesitate to call it absolutely pure.
When condensed light is sent through it, the track of the beam is not
invisible, but of the most exquisitely delicate blue. This blue is purer than
that of the sky, so that the matter which produces it must be finer than that
of the sky. It may be and indeed has been, contended that this blue is
scattered by the very molecules of the water, and not by matter suspended in
the water. But when we remember that this perfection of blue is approached
gradually through stages of less perfect blue; and when we consider that a
blue in all respects similar is demonstrably obtainable from particles
mechanically suspended, we should hesitate, I think, to conclude that we
have arrived here at the last stage of purification. The evidence, I think,
points distinctly to the conclusion that, could we push the process of
purification still farther, even this last delicate trace of blue would
disappear.

Chalk-water. Clark's Softening Process.

But is it not possible to match the water of the Lake of Geneva here in
England? Undoubtedly it is. We have in England a kind of rock which
constitutes at once an exceedingly clean recipient and a natural filter, and
from which we can obtain water extremely free from mechanical impurities.
I refer to the chalk formation, in which large quantities of water are held in
store. Our chalk hills are in most cases covered with thin layers of soil, and
with very scanty vegetation. Neither opposes much obstacle to the entry of
the rain into the chalk, where any organic impurity which the water may
carry in is soon oxidised and rendered harmless. Those who have scampered
like myself over the downs of Hants and Wilts will remember the scarcity of
water in these regions. In fact, the rainfall, instead of washing the surface
and collecting in streams, sinks into the fissured chalk and percolates
through it. When this formation is suitably tapped, we obtain water of
exceeding briskness and purity. A large glass globe, filled with the water of

a well near Tring, shows itself to be wonderfully free from mechanical
impurity. Indeed, it stands to reason that water wholly withdrawn from
surface contamination, and percolating through so clean a substance, should
be pure. It has been a subject much debated, whether the supply of excellent
water which the chalk holds in store could not be rendered available for
London. Many of the most eminent engineers and chemists have ardently
recommended this source, and have sought to show, not only that its purity
is unrivalled, but that its quantity is practically inexhaustible. Data sufficient
to test this are now, I believe, in existence; the number of wells sunk in the
chalk being so considerable, and the quantity of water which they yield so
well known.

But this water, so admirable as regards freedom from mechanical impurity,
labours under the disadvantage of being rendered very hard by the carbonate
of lime which it holds in solution. The chalk-water in the neighbourhood of
Watford contains about seventeen grains of carbonate of lime per gallon.
This, in the old terminology, used to be called seventeen degrees of
hardness. This hard water is bad for tea, bad for washing, and it furs our
boilers, because the lime held in solution is precipitated by boiling. If the
water be used cold, its hardness must be neutralised at the expense of soap,
before it will give a lather. These are serious objections to the use of chalk-
water in London. But they are successfully met by the fact that such water
can be softened inexpensively, and on a grand scale. I had long known the
method of softening water called Clark's process, but not until recently,
under the guidance of Mr. Homersham, did I see proof of its larger
applications. The chalk-water is softened for the supply of the city of
Canterbury; and at the Chiltern Hills it is softened for the supply of Tring
and Aylesbury. Caterham also enjoys the luxury.

I have visited all these places, and made myself acquainted with the works.
At Canterbury there are three reservoirs covered in and protected, by a
concrete roof and layers of pebbles, both from the summer's heat and the
winter's cold. Each reservoir holds 120,000 gallons of water. Adjacent to
these reservoirs are others containing pure slaked lime — the so-called
'cream of lime.' These being filled with water, the lime and water are
thoroughly mixed by air forced by an engine through apertures in the
bottom of the reservoir. The water soon dissolves all the lime it is capable of

dissolving. The mechanically suspended lime is then allowed to subside to
the bottom, leaving a perfectly transparent lime-water behind.

The softening process is this: Into one of the empty reservoirs is introduced
a certain quantity of the clear lime-water, and after this about nine times the
quantity of the chalk-water. The transparency immediately disappears — the
mixture of the two clear liquids becoming thickly turbid, through the
precipitation of carbonate of lime. The precipitate is crystalline and heavy,
and in about twelve hours a layer of pure white carbonate of lime is formed
at the bottom of the reservoir, with a water of extraordinary beauty and
purity overhead. A few days ago I pitched some halfpence into a reservoir
sixteen feet deep at the Chiltern Hills. This depth hardly dimmed the coin.
Had I cast in a pin, it could have been seen at the bottom. By this process of
softening, the water is reduced from about seventeen degrees of hardness, to
three degrees of hardness. It yields a lather immediately. Its temperature is
constant throughout the year. In the hottest summer it is cool, its
temperature being twenty degrees above the freezing point; and it does not
freeze in winter if conveyed in proper pipes. The reservoirs are covered; a
leaf cannot blow into them, and no surface contamination can reach the
water. It passes direct from the main into the house tap; no cisterns are
employed, and the supply is always fresh and pure. This is the kind of water
which is supplied to the fortunate people of Tring, Caterham, and
Canterbury.

-----

The foregoing article, as far as it relates to the theory which ascribes
epidemic disease to the development of low parasitic life within the human
life, was embodied in a discourse delivered before the Royal Institution in
January 1870. In June 1871, after a brief reference to the polarisation of
light by cloudy matter, I ventured to recur to the subject in these terms:
What is the practical use of these curiosities? If we exclude the interest
attached to the observation of new facts, and the enhancement of that
interest through the knowledge that facts often become the exponents of
laws, these curiosities are in themselves worth little. They will not enable us
to add to our stock of food, or drink, or clothes, or jewellery. But though
thus shorn of all usefulness in themselves, they may, by carrying thought
into places which it would not otherwise have entered, become the

antecedents of practical consequences. In looking, for example, at our
illuminated dust, we may ask ourselves what it is. How does it act, not upon
a beam of light, but upon our own bodies? The question then assumes a
practical character. We find on examination that this dust is mainly organic
matter — in part living, in part dead. There are among it particles of ground
straw, torn rags, smoke, the pollen of flowers, the spores of fungi, and the
germs of other things. But what have they to do with the animal economy?
Let me give you an illustration to which my attention has been lately drawn
by Mr. George Henry Lewes, who writes to me thus:

'I wish to direct your attention to the experiments of von Recklingshausen
should you happen not to know them. They are striking confirmations of
what you say of dust and disease. Last spring, when I was at his laboratory
in Wuerzburg, I examined with him blood that had been three weeks, a
month, and five weeks, out of the body, preserved in little porcelain cups
under glass shades. This blood was living and growing. Not only were the
Amoeba-like movements of the white corpuscles present, but there were
abundant evidences of the growth and development of the corpuscles. (I
also saw a frog's heart still pulsating which had been removed from the
body I forget how many days, but certainly more than a week). There were
other examples of the same persistent vitality, or absence of putrefaction.
Von Recklingshausen did not attribute this to the absence of germs — germs
were not mentioned by him; but when I asked him how he represented the
thing to himself, he said the whole mystery of his operation consisted in
keeping the blood free from dirt. The instruments employed were raised to a
red heat just before use; the thread was silver thread and was similarly
treated; and the porcelain cups, though not kept free from air, were kept free
from currents. He said he often had failures, and these he attributed to
particles of dust having escaped his precautions.'

Professor Lister, who has founded upon the removal or destruction of this
'dirt' momentous improvements in surgery, tells us the effect of its
introduction into the blood of wounds. The blood would putrefy and
become fetid; and when you examine more closely what putrefaction
means, you find the putrefying substance swarming with infusorial life, the
germs of which have been derived from the atmospheric dust.

We are now assuredly in the midst of practical matters; and with your
permission I will refer once more to a question which has recently occupied
a good deal of public attention. As regards the lowest forms of life, the
world is divided, and has for a long time been divided, into two parties, the
one affirming that we have only to submit absolutely dead matter to certain
physical conditions, to evolve from it living things; the other (without
wishing to set bounds to the power of matter) affirming that, in our day, life
has never been found to arise independently of pre-existing life. I belong to
the party which claims life as a derivative of life. The question has two
factors — the evidence, and the mind that judges of the evidence; and it
may be purely a mental set or bias on my part that causes me throughout
this long discussion, to see, on the one side, dubious facts and defective
logic, and on the other side firm reasoning and a knowledge of what rigid
experimental enquiry demands. But, judged of practically, what, again, has
the question of Spontaneous Generation to do with us? Let us see. There are
numerous diseases of men and animals that are demonstrably the products
of parasitic life, and such diseases may take the most terrible epidemic
forms, as in the case of the silkworms of France, referred to at an earlier part
of this article. Now it is in the highest degree important to know whether the
parasites in question are spontaneously developed, or whether they have
been wafted from without to those afflicted with the disease. The means of
prevention, if not of cure, would be widely different in the two cases.

But this is not all. Besides these universally admitted cases, there is the
broad theory, now broached and daily growing in strength and clearness —
daily, indeed, gaining more and more of assent from the most successful
workers and profound thinkers of the medical profession itself — the
theory, namely, that contagious disease, generally, is of this parasitic
character. Had I any cause to regret having introduced this theory to your
notice more than a year ago, that regret should now be expressed. I would
certainly renounce in your presence whatever leaning towards the germ
theory my words might then have betrayed. But since the time referred to
nothing has occurred to shake my conviction of the truth of the theory. Let
me briefly state the grounds on which its supporters rely. From their
respective viruses you may plant typhoid fever, scarlatina, or small-pox.
What is the crop that arises from this husbandry? As surely as a thistle rises
from a thistle seed, as surely as the fig comes from the fig, the grape from

the grape, the thorn from the thorn, so surely does the typhoid virus increase
and multiply into typhoid fever, the scarlatina virus into scarlatina, the
small-pox virus into small-pox. What is the conclusion that suggests itself
here? It is this: That the thing which we vaguely call a virus is to all intents
and purposes a seed. Excluding the notion of vitality, in the whole range of
chemical science you cannot point to an action which illustrates this perfect
parallelism with the phenomena of life — this demonstrated power of self-
multiplication and reproduction. The germ theory alone accounts for the
phenomena.

In cases of epidemic disease, it is not on bad air or foul drains that the
attention of the physician of the future will primarily be fixed, but upon
disease germs, which no bad air or foul drains can create, but which may be
pushed by foul air into virulent energy of reproduction. You may think I am
treading on dangerous ground, that I am putting forth views that may
interfere with salutary practice. No such thing. If you wish to learn the
impotence of medical practice in dealing with contagious diseases, you have
only to refer to the Harveian oration for 1871, by Sir William Gull. Such
diseases defy the physician. They must run their course, and the utmost that
can be done for them is careful nursing. And this, though I do not specially
insist upon it, would favour the idea of their vital origin. For if the seeds of
contagious disease be themselves living things, it may be difficult to destroy
either them or their progeny, without involving their living habitat in the
same destruction.

It has been said, and it is sure to be repeated, that I am quitting my own
métier, in speaking of these things. Not so. I am dealing with a question on
which minds accustomed to weigh the value of experimental evidence are
alone competent to decide, and regarding which, in its present condition,
minds so trained are as capable of forming an opinion as regarding the
phenomena of magnetism or radiant heat. 'The germ theory of disease,' it
has been said, 'appertains to the biologist and the physician.' Where, I would
ask in reply, is the biologist or physician, whose researches, in connection
with this subject, could for one instant be compared to those of the chemist
Pasteur? It is not the philosophic members of the medical profession who
are dull to the reception of truth not originated within the pale of the
profession itself. I cannot better conclude this portion of my story than by

reading to you an extract from a letter addressed to me some time ago by
Dr. William Budd, of Clifton, to whose insight and energy the town of
Bristol owes so much in the way of sanitary improvement.

'As to the germ theory itself,' writes Dr. Budd, that is a matter on which I
have long since made up my mind. From the day when I first began to think
of these subjects I have never had a doubt that the specific cause of
contagious fevers must be living organisms.

'It is impossible, in fact, to make any statement bearing upon the essence or
distinctive characters of these fevers, without using terms which are of all
others the most distinctive of life. Take up the writings of the most violent
opponent of the germ theory, and, ten to one, you will find them full of such
terms as "propagation," "self-propagation," "reproduction," 61 self-
multiplication," and so on. Try as he may — if he has anything to say of
those diseases which is characteristic of them — he cannot evade the use of
these terms, or the exact equivalents to them. While perfectly applicable to
living things, these terms express qualities which are not only inapplicable
to common chemical agents, but, as far as I can see, actually inconceivable
of them.'

.

.

Cotton-wool Respirator.

Once, then, established within the body, this evil form of life, if you will
allow me to call it so, must run its course. Medicine as yet is powerless to
arrest its progress, and the great point to be aimed at is to prevent its access
to the body. It was with this thought in my mind that I ventured to
recommend, more than a year ago, the use of cotton-wool respirators in
infectious places. I would here repeat my belief in their efficacy if properly
constructed. But I do not wish to prejudice the use of these respirators, by
connecting them indissolubly with the germ theory. There are too many
trades in England where life is shortened and rendered miserable by the
introduction of matters into the lungs which might be kept out of them. Dr.
Greenhow has shown the stony grit deposited in the lungs of stonecutters.

The black lungs of colliers is another case in point. In fact, a hundred
obvious cases might be cited, and others that are not obvious might be
added to them. We should not, for example, think that printing implied
labour where the use of cotton-wool respirators might come into play; but
the fact is that the dust arising from the sorting of the type is very
destructive of health. I went some time ago into a manufactory in one of our
large towns, where iron vessels are enamelled by coating them with a
mineral powder, and subjecting them to a heat sufficient to fuse the powder.
The organisation of the establishment was excellent, and one thing only was
needed to make it faultless. In a large room a number of women were
engaged covering the vessels. The air was laden with the fine dust, and their
faces appeared as white and bloodless as the powder with which they
worked. By the use of cotton-wool respirators these women might be caused
to breathe air as free from suspended matter as that of the open street. Over
a year ago a Lancashire seedsman wrote to me, stating that during the seed
season his men suffered horribly from irritation and fever, so that many of
them left his service. He asked for help, and I gave him my advice. At the
conclusion of the season, this year, he wrote to inform me that he had folded
a little cotton-wool. in muslin, and tied it in front of the mouth; and that
with this simple defence he had passed through the season in comfort, and
without a single complaint from his men.

Against the use of such a respirator the obvious objection arises, that it
becomes wet and heated by the breath. While casting about for a remedy for
this, a friend forwarded to me from Newcastle a form of respirator invented
by Mr. Carrick, a hotel-keeper at Glasgow, which, by a slight modification,
may be caused to meet the case perfectly. The respirator, with its back in
part removed, is shown in fig. 4. Under the partition of wire-gauze q r, is a
space intended by Mr. Carrick for 'medicated substances,' and which may be
filled with cotton-wool. The mouth is placed against the aperture o, which
fits closely round the lips, and the filtered air enters the mouth through a
light valve v, which is lifted by the act of inhalation.

During exhalation this valve closes; the breath escapes by a second valve,
v', into the open. air. The wool is thus kept dry and cool; the air in passing
through it being filtered of everything it holds in suspension. The respirator
has since taken other forms.

FIG. 4.

-----

Fireman's Respirator.

We have thus been led by our first unpractical experiments into a thicket of
practical considerations. But another step is possible. Admiring, as I do, the
bravery of our firemen, and hearing that smoke was a more serious enemy
than flame itself, I thought of devising a fireman's respirator.

Our fire-escapes are each in charge of a single man, and it would be of
obvious importance to place it in the power of each of those men to penetrate
through the densest smoke, into the recesses of a house, and there to rescue
those who would otherwise be suffocated or burnt. Cotton-wool, which so
effectually arrested dust, was first tried; but, though found soothing in
certain gentle kinds of smoke, it was no match for the pungent fumes of a
resinous fire. For the purpose of catching the atmospheric germs, M. Pouchet
spread a film of glycerine on a plate of glass, urged air against the film, and
examined the dust which stuck to it. The moistening of the cotton-wool with

glycerine was a decided improvement; still the respirator only enabled us to
remain in dense smoke for three or four minutes, after which the irritation
became unendurable. Reflection suggested that, besides the smoke, there
must be numerous hydrocarbons produced, which, being in a state of vapour,
would be very imperfectly arrested by the cotton-wool. These, in all
probability, were the cause of the residual irritation; and if these could be
removed, a practically perfect respirator might possibly be obtained.

I state the reasoning exactly as it occurred to my mind. Its result will be
anticipated by many present. All bodies possess the power of condensing, in
a greater or less degree, gases and vapours upon their surfaces, and when the
condensing body is very porous, or in a fine state of division, the force of
condensation may produce very remarkable effects. Thus, a clean piece of
platinum-foil placed in a mixture of oxygen and hydrogen so squeezes the
gases together as to cause them to combine; and if the experiment be made
with care, the heat of combination may raise the platinum to bright redness.

The promptness of this action is greatly augmented by reducing the platinum
to a state of fine division. A pellet of 'spongy platinum,' for instance, plunged
into a mixture of oxygen and hydrogen, causes the gases to explode
instantly. In virtue of its extreme porosity, a similar power is possessed by
charcoal. It is not strong enough to cause the oxygen and hydrogen to
combine like the spongy platinum, but it so squeezes the more condensable
vapours, and acts with such condensing power upon the oxygen of the air, as
to bring both within the combining distance, thus enabling the oxygen to
attack and destroy the vapours in the pores of the charcoal. In this way,
effluvia of all kinds may be virtually burnt up; and this is the principle of the
excellent charcoal respirators invented by Dr. Stenhouse. Armed with one of
these, you may go into the foulest-smelling places without having your nose
offended.

But, while powerful to arrest vapours, the charcoal respirator is ineffectual
as regards smoke. The smoke-particles get freely through the respirator. With
a number of such respirators, tested in a proper room, from half a minute to a
minute was the limit of endurance. This might be exceeded by Faraday's
simple method of emptying the lungs completely, and then filling them
before going into a smoky atmosphere. In fact, each solid smoke particle is
itself a bit of charcoal, and carries on it, and in it, its little load of irritating
vapour. It is this, far more than the particles of carbon themselves, that
produces the irritation. Hence two causes of offence are to be removed: the
carbon particles which convey the irritant by adhesion and condensation, and
the free vapour which accompanies the particles. The cotton-wool moistened
with glycerine I knew would arrest the first; fragments of charcoal I hoped
would stop the second. In the first fireman's respirator, Mr. Carrick's
arrangement of two valves, the one for inhalation, the other for exhalation,
was preserved. But the portion of the respirator which holds the filtering and
absorbent substances, was prolonged to a depth of four or five inches (see
fig. 5.) Under the partition of wire-gauze q r at the bottom of the space
which fronts the mouth was placed a layer of cotton-wool, c, moistened with
glycerine; then a thin layer of dry wool, c'; then a layer of charcoal
fragments; and finally a second thin layer of dry cotton-wool. The
succession of the layers may be changed without prejudice to the action. A
wire-gauze cover, shown in plan under fig. 5, keeps the substances from
falling out of the respirator. A layer of caustic lime may be added for the
absorption of carbonic acid; but in the densest smoke that we have hitherto

employed, it has not been found necessary, nor is it shown in the figure. In a
flaming building, indeed, the mixture of air with the smoke never permits the
carbonic acid to become so dense as to be irrespirable; but in a place where
the gas is present in undue quantity, the fragments of lime would materially
mitigate its action.

In a small cellar-like chamber with a stone flooring and stone walls, the first
experiments were made. We Placed there furnaces containing resinous pine-
wood, lighted the wood, and, placing over it a lid which prevented too brisk
a circulation of the air, generated dense volumes of smoke. With our eyes
protected by suitable glasses, my assistant and I have remained for half an
hour and more in smoke so dense and pungent that a single inhalation,
through the undefended mouth, would be perfectly unendurable. We might
have prolonged our stay for hours.

FIG. 5.

Having thus far perfected the instrument, I wrote to the chief officer of the
Metropolitan Fire Brigade, asking him whether such a respirator would be of
use to him. His reply was prompt; it would be most valuable. He had,
however, made himself acquainted with every contrivance of the kind in this
and other countries, and had found none of them of any practical use. He
offered to come and test it here, or to place a room at my disposal in the City.
At my request he came here, accompanied by three of his men. Our small
room was filled with smoke to their entire satisfaction. The three men went
successively into it, and remained there as long as Captain Shaw wished
them. On coming out they said that they had not suffered the slightest
inconvenience; that they could have remained all day in the smoke. Captain
Shaw then tested the respirator with the same result, and he afterwards took
great interest in the perfecting of the instrument.

-----

Various ameliorations and improvements have recently been introduced into
the smoke respirator. The hood of Captain Shaw has been improved upon by
the simple and less expensive mouthpiece of Mr. Sinclair; and this, in its
turn, has been simplified and improved by my assistant Mr. John Cottrell.
The respirator is now in considerable demand, and it has already done good
practical service. Care is, however, necessary, in moistening the wool with
glycerine. It must be carefully teazed, so that the individual fibres may be
moistened, and clots must be avoided. I cannot recommend the layers of
moistened flannel which, in some cases, have been used instead of cotton-
wool: nothing equals the wool, when carefully treated.

An experiment made last year brought out very conspicuously the necessity
of careful packing, and the enormous comparative power of resisting smoke
irritation possessed by our firemen, and the able officer who commands
them. Having heard from Captain Shaw that, in some recent very trying
experiments, he had obtained the best effects from dry cotton-wool, and
thinking that I could not have been mistaken in my first results, which
proved the dry so much inferior to the moistened wool and its associated
charcoal, I proposed to Captain Shaw to bring the matter to a test at his
workshops in the City. He was good enough to accept my proposal, and

thither I went on May 7, 1874. The smoke was generated in a confined space
from wet straw, and it was certainly very diabolical.

At this season of the year I am usually somewhat shorn of vigour, and
therefore not in the best condition for severe experiments; still I wished to
test the matter in my own person. With a respirator which had been in use
some days previously, and which was not carefully packed, I followed a
fireman into the smoke, he being provided with a dry-wool respirator. I was
compelled to quit the place in about three minutes, while the fireman
remained there for six or seven minutes.

I then tried his respirator upon myself, and found that with it I could not
remain more than a minute in the smoke; in fact the first inhalation provoked
coughing.

Thinking that Captain Shaw himself might have lungs more like mine than
those of his fireman, I proposed that we should try the respirators together;
but he informed me that his lungs were very strong. He was, however, good
enough to accede to my request. Before entering the den a second time I
repacked my respirator, with due care, and entered the smoke in company
with Captain Shaw. I could hear him breathe long slow inhalations; his
labour was certainly greater than mine, and after the lapse of seven minutes I
heard him cough. In seven and a half minutes he had to quit the place, thus
proving that his lungs were able to endure the irritation seven times as long
as mine could bear it. I continued in the smoke, with hardly any discomfort,
for sixteen minutes, and certainly could have remained in it much longer.
The advantage arising from the glycerine was thus placed beyond question.

During this time I was in a condition to render very material assistance to a
person in danger of suffocation.

Helmholtz on Hay Fever.

In my lecture on Dust and Disease in 1870, I referred to an experiment made
by Helmholtz upon himself which strikingly connected hay fever with
animalcular life. About a year ago I received from Professor Binz of Bonn a

short, but important paper, embracing Helmholtz's account of his
observation, to which Professor Binz has added some remarks of his own.
The paper, being mainly intended for English medical men, was published in
English, and though here and there its style might be amended, I think it
better to publish it unaltered.

From what I have observed (says Professor Binz) of recent English
publications on the subject of hay fever, I am led to suppose that English
authorities are inaccurately acquainted with the discovery of Professor
Helmholtz, as far back as 1868, of the existence of uncommon low
organisms in the nasal secretions in this complaint, and of the possibility of
arresting their action by the local employment of quinine. I therefore purpose
to republish the letter in which he originally announced these facts to myself,
and to add some further observations on this topic. The letter is as follows :
— [Footnote: Cf. Virchow's 'Archiv.' vol. xlvi. p. 100]

'I have suffered, as well as I can remember, since the year 1847, from the
peculiar catarrh called by the English "hay fever," the speciality of which
consists in its attacking its victims regularly in the hay season (myself-
between May 20 and the end of June), that it ceases in the cooler weather,
but on the other hand quickly reaches a great intensity if the patients expose
themselves to heat and sunshine. An extraordinary violent sneezing then sets
in, and a strongly corrosive thin discharge, with which much epithelium is
thrown off. This increases, after a few hours, to a painful inflammation of
the mucous membrane and of the outside of the nose, and excites fever with
severe headache and great depression, if the patient cannot withdraw himself
from the heat and the sunshine. In a cool room, however, these symptoms
vanish as quickly as they come on, and there then only remains for a few
days a lessened discharge and soreness, as if caused by the loss of
epithelium. I remark, by the way, that in all my other years I had very little
tendency to catarrh or catching cold, while the hay fever has never failed
during the twenty-one years of which I have spoken, and has never attacked
me earlier or later in the year than the times named. The condition is
extremely troublesome, and increases, if one is obliged to be much exposed
to the sun, to an excessively severe malady.

'The curious dependence of the disease on the season of the year suggested
to me the thought that organisms might be the origin of the mischief. In

examining the secretion I regularly found, in the last five years, certain
vibrio-like bodies in it, which at other times I could not observe in my nasal
secretion. . . . They are very small, and can only be recognised with the
immersion-lens of a very good Hartnack's microscope. It is characteristic of
the common isolated single joints that they contain four nuclei in a row, of
which two pairs are more closely united. The length of the joints is 0.004
millimetre. Upon the warm objective-stage they move with moderate
activity, partly in, mere vibration, partly shooting backwards and forwards in
the direction of their long axis; in lower temperatures they are very inactive.
Occasionally one finds them arranged in rows upon each other, or in
branching series. Observed some days in the moist chamber, they vegetated
again, and appeared somewhat larger and more conspicuous than
immediately after their excretion. It is to be noticed that only that kind of
secretion contains them which is expelled by violent sneezings; that which
drops slowly does not contain any. They stick tenaciously enough in the
lower cavities and recesses of the nose.

'When I saw your first notice respecting the poisonous action of quinine
upon infusoria, I determined at once to make an experiment with that
substance, thinking that these vibrionic bodies, even if they did not cause the
whole illness, still could render it much more unpleasant through their
movements and the decompositions caused by them. For that reason I made
a neutral solution of sulphate of quinine, which did not contain much of the
salt (1·800), but still was effective enough, and caused moderate irritation
on the mucus membrane of the nose. I then lay flat on my back, keeping my
head very low, and poured with a pipette about four cubic centimetres into
both nostrils. Then I turned my head about in order to let the liquid flow in
all directions.

'The desired effect was obtained immediately, and remained for some hours;
I could expose myself to the sun without fits of sneezing and the other
disagreeable symptoms coming on. It was sufficient to repeat the treatment
three times a day, even under the most unfavourable circumstances, in order
to keep myself quite free. [Footnote: There is no foundation for the
objection that syringing the nose could not cure the asthma which
accompanies hay fever; for this asthma is only the reflex effect arising from
the irritation of the nose. — B.] There were then no such vibrios in the
secretion. If I only go out in the evening, it suffices to inject the quinine once

a day, just before going. After continuing this treatment for some days the
symptoms disappear completely, but if I leave off they return till towards the
end of June.

'My first experiments with quinine date from the summer of 1867; this year
(1868) I began at once as soon as the first traces of the illness appeared, and
I have thus been able to stop its development completely.

'I have hesitated as yet in publishing the matter, because I have found no
other patient [Footnote: Helmholtz, now Professor of Physics at the
University of Berlin, is, although M.D., no medical practitioner. — B.] on
whom I could try the experiment. There is, it seems to me, no doubt,
considering the extraordinary regularity in the recurrence and course of the
illness, that quinine had here a most quick and decided effect. And this again
makes my hypothesis very probable, that the vibrios, although of no specific
form but a very frequent one, are at least the cause of the rapid increase of
the symptoms in warm air, as heat excites them to lively action.

I should be very glad if the above lines would induce medical men in
England — the haunt of hay fever — to test the observation of Helmholtz.
To most patients the application with the pipette may be too difficult or
impossible; I have therefore already suggested the use of Weber's very
simple but effective nose-douche. Also it will be advisable to apply the
solution of quinine tepid. It can, further, not be repeated often enough that
quinine is frequently adulterated, especially with cinchona, the action of
which is much less to be depended upon.

Dr. Frickhoefer, of Schwalbach, has communicated to me a second case in
which hay fever was cured by local application of quinine. [Footnote: Cf.
Virchow's 'Archiv.' (1870), vol. li. p. 176.] Professor Busch, of Bonn,
authorises me to say that he succeeded in two cases of 'catarrhus aestivus' by
the same method: a third patient was obliged to abstain from the use of
quinine, as it produced an unbearable irritation of the sensible nerves of the
nose. In the autumn of 1872 Helmholtz told me that his fever was quite
cured, and that in the meantime two other patients had, by his advice, tried
this method, and with the same success. [Footnote: Prof. Helmholtz, whom I
had the pleasure of meeting in Switzerland last year, then told me that he

was quite convinced that hay fever was produced by the pollen afloat in
early summer in the atmosphere.]

.

.

.

.

--------------------

.

.

VI. VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE.

1870.

THE opening of the Eclipse Expedition was not propitious. Portsmouth, on
Monday, December 5, 1870, was swathed by fog, which was intensified by
smoke, and traversed by a drizzle of fine rain. At six P.M. I was on board the
'Urgent.' On Tuesday morning the weather was too thick to permit of the
ship's being swung and her compasses calibrated. The Admiral of the port, a
man of very noble presence, came on board. Under his stimulus the energy
which the weather had damped appeared to become more active, and soon
after his departure we steamed down to Spithead. Here the fog had so far
lightened as to enable the officers to swing the ship.

At three P.M. on Tuesday, December 6, we got away, gliding successively
past Whitecliff Bay, Bembridge, Sandown, Shanklin, Ventnor, and St.
Catherine's Lighthouse. On Wednesday morning we sighted the Isle of
Ushant, on the French side of the Channel. The northern end of the island
has been fretted by the waves into detached tower-like masses of rock of
very remarkable appearance. In the Channel the sea was green, and opposite
Ushant it was a brighter green. On Wednesday evening we committed
ourselves to the Bay of Biscay. The roll of the Atlantic was full, but not

violent. There had been scarcely a gleam of sunshine throughout the day, but
the cloud-forms were fine, and their apparent solidity impressive. On
Thursday morning the green of the sea was displaced by a deep indigo blue.
The whole of Thursday we steamed across the bay. We had little blue sky,
but the clouds were again grand and varied — cirrus, stratus, cumulus, and
nimbus, we had them all. Dusky hair-like trails were sometimes dropped
from the distant clouds to the sea.

These were falling showers, and they. sometimes occupied the whole
horizon, while we steamed across the rainless circle which was thus
surrounded. Sometimes we plunged into the rain, and once or twice, by
slightly changing our course, avoided a heavy shower. From time to time
perfect rainbows spanned the heavens from side to side. At times a bow
would appear in fragments, showing the keystone of the arch midway in air,
and its two buttresses on the horizon. In all cases the light of the bow could
be quenched by a Nicol's prism, with its long diagonal tangent to the arc.
Sometimes gleaming patches of the firmament were seen amid the clouds.
When viewed in the proper direction, the gleam could be quenched by a
Nicol's prism, a dark aperture being thus opened into stellar space.

At sunset on Thursday the denser clouds were fiercely fringed, while
through the lighter ones seemed to issue the glow of a conflagration. On
Friday morning we sighted Cape Finisterre — the extreme end of the arc
which sweeps from Ushant round the Bay of Biscay. Calm spaces of blue, in
which floated quietly scraps of cumuli, were behind us, but in front of us
was a horizon of portentous darkness. It continued thus threatening
throughout the day. Towards evening the wind strengthened to a gale, and at
dinner it was difficult to preserve the plates and dishes from destruction. Our
thinned company hinted that the rolling had other consequences. It was very
wild when we went to bed. I slumbered and slept, but after some time was
rendered anxiously conscious that my body had become a kind of projectile,
with the ship's side for a target. I gripped the edge of my berth to save
myself from being thrown out. Outside, I could hear somebody say that he
had been thrown from his berth, and sent spinning to the other side of the
saloon. The screw laboured violently amid the lurching; it incessantly
quitted the water, and, twirling in the air, rattled against its bearings, causing
the ship to shudder from stem to stern. At times the waves struck us, not with
the soft impact which might be expected from a liquid, but with the sudden

solid shock of battering-rams. 'No man knows the force of water,' said one of
the officers,' until he has experienced a storm at sea.' These blows followed
each other at quicker intervals, the screw rattling after each of them, until,
finally, the delivery of a heavier stroke than ordinary seemed to reduce the
saloon to chaos. Furniture crashed, glasses rang, and alarmed enquiries
immediately followed. Amid the noises I heard one note of forced laughter;
it sounded very ghastly. Men tramped through the saloon, and busy voices
were heard aft, as if something there had gone wrong.

I rose, and not without difficulty got into my clothes. In the after-cabin,
under the superintendence of the able and energetic navigating lieutenant,
Mr. Brown, a group of blue-jackets were working at the tiller-ropes. These
had become loose, and the helm refused to answer the wheel. High moral
lessons might be gained on shipboard, by observing what steadfast
adherence to an object can accomplish, and what large effects are heaped up
by the addition of infinitesimals. The tiller-rope, as the blue-jackets strained
in concert, seemed hardly to move; still it did move a little, until finally, by
timing the pull to the lurching of the ship, the mastery of the rudder was
obtained. I had previously gone on deck. Round the saloon-door were a few
members of the eclipse party, who seemed in no mood for scientific
observation. Nor did I; but I wished to see the storm. I climbed the steps to
the poop, exchanged a word with Captain Toynbee, the only member of the
party to be seen on the poop, and by his direction made towards a cleat not
far from the wheel. [Footnote: The cleat is a T-shaped mass of metal
employed for the fastening of ropes.] Round it I coiled my arms. With the
exception of the men at the wheel, who stood as silent as corpses, I was
alone.

I had seen grandeur elsewhere, but this was a new form of grandeur to me.
The 'Urgent' is long and narrow, and during our expedition she lacked the
steadying influence of sufficient ballast. She was for a time practically
rudderless, and lay in the trough of the sea. I could see the long ridges, with
some hundreds of feet between their crests, rolling upon the ship perfectly
parallel to her sides. As they approached, they so grew upon the eye as to
render the expression 'mountains high' intelligible. At all events, there was
no mistaking their mechanical might, as they took the ship upon their
shoulders, and swung her like a pendulum. The deck sloped sometimes at an
angle which I estimated at over forty-five degrees; wanting my previous

Alpine practice, I should have felt less confidence in my grip of the cleat.
Here and there the long rollers were tossed by interference into heaps of
greater height. The wind caught their crests, and scattered them over the sea,
the whole surface of which was seething white. The aspect of the clouds was
a fit accompaniment to the fury of the ocean. The moon was almost full — at
times concealed, at times revealed, as the scud flew wildly over it. These
things appealed to the eye, while the ear was filled by the groaning of the
screw and the whistle and boom of the storm.

Nor was the outward agitation the only object of interest to me. I was at once
subject and object to myself, and watched with intense interest the workings
of my own mind. The 'Urgent' is an elderly ship. She had been built, I was
told, by a contracting firm for some foreign Government, and had been
diverted from her first purpose when converted into a troop-ship. She had
been for some time out of work, and I had heard that one of her boilers, at
least, needed repair. Our scanty but excellent crew, moreover, did not belong
to the 'Urgent,' but had been gathered from other ships. Our three lieutenants
were also volunteers. All this passed swiftly through my mind as the steamer
shook under the blows of the waves, and I thought that probably no one on
board could say how much of this thumping and straining the 'Urgent' would
be able to bear. This uncertainty caused me to look steadily at the worst, and
I tried to strengthen myself in the face of it.

But at length the helm laid hold of the water, and the ship was got gradually
round to face the waves. The rolling diminished, a certain amount of
pitching taking its place. Our speed had fallen from eleven knots to two. I
went again to bed. After a space of calm, when we seemed crossing the
vortex of a storm, heavy tossing recommenced. I was afraid to allow myself
to fall asleep, as my berth was high, and to be pitched out of it might be
attended with bruises, if not with fractures. From Friday at noon to Saturday
at noon we accomplished sixty-six miles, or an average of less than three
miles an hour. I overheard the sailors talking about this storm. The 'Urgent,'
according to those that knew her, had never previously experienced anything
like it. [Footnote: 'There is, it will be seen, a fair agreement between these
impressions and those so vigorously described by a scientific correspondent
of the 'Times.']

All through Saturday the wind, though somewhat sobered, blew dead against
us. The atmospheric effects were exceedingly fine. The cumuli resembled
mountains in shape, and their peaked summits shone as white as Alpine
snows. At one place this resemblance was greatly strengthened by a vast area
of cloud, uniformly illuminated, and lying like a névé below the peaks. From
it fell a kind of cloud-river strikingly like a glacier. The horizon at sunset
was remarkable — spaces of brilliant green between clouds of fiery red,
Rainbows had been frequent throughout the day, and at night a perfectly
continuous lunar bow spanned the heavens from side to side. Its colours
were feeble; but, contrasted with the black ground against which it rested, its
luminousness was extraordinary.

Sunday morning found us opposite to Lisbon, and at midnight we rounded
Cape St. Vincent, where the lurching seemed disposed to recommence.
Through the kindness of Lieutenant Walton, a cot had been slung for me. It
hung between a tiller-wheel and a flue, and at one A.M. I was roused by the
banging of the cot against its boundaries. But the wind was now behind us,
and we went along at a speed of eleven knots. We felt certain of reaching
Cadiz by three. But a new lighthouse came in sight, which some affirmed to
be Cadiz Lighthouse, while the surrounding houses were declared to be
those of Cadiz itself. these statements, the navigating lieutenant changed his
course, and steered for the place. A pilot came on board, and he informed us
that we were before the mouth of the Guadalquivir, and that the lighthouse
was that of Cipiòna. Cadiz was still some eighteen miles distant.

We steered towards the city, hoping to get into the harbour before dark. But
the pilot who would have guided us had been snapped up by another vessel,
and we did not get in. We beat about during the night, and in the morning
found ourselves about fifteen miles from Cadiz. The sun rose behind the city,
and we steered straight into the light. The three-towered cathedral stood in
the midst, round which swarmed apparently a multitude of chimney-stacks.
A nearer approach showed the chimneys to be small turrets. A pilot was
taken on board; for there is a dangerous shoal in the harbour. The appearance
of the town as the sun shone upon its white and lofty walls was singularly
beautiful. We cast anchor; some officials arrived and demanded a clean bill
of health. We had none. They would have nothing to do with us; so the
yellow quarantine flag was hoisted, and we waited for permission to land the
Cadiz party. After some hours' delay the English consul and vice-consul

came on board, and with them a Spanish officer ablaze with gold lace and
decorations. Under slight pressure the requisite permission had been granted.
We landed our party, and in the afternoon weighed anchor. Thanks to the
kindness of our excellent paymaster, I was here transferred to a more roomy
berth.

Cadiz soon sank beneath the sea, and we sighted in succession Cape
Trafalgar, Tarifa, and the revolving light of Ceuta. The water was very calm,
and the moon rose in a quiet heaven. She swung with her convex surface
downwards, the common boundary between light and shadow being almost
horizontal. A pillar of reflected light shimmered up to us from the slightly
rippled sea. I had previously noticed the phosphorescence of the water, but
tonight it was stronger than usual, especially among the foam at the bows. A
bucket let down into the sea brought up a number of the little sparkling
organisms which caused the phosphorescence. I caught some of them in my
hand. And here an appearance was observed which was new to most of us,
and strikingly beautiful to all. Standing at the bow and looking forwards, at a
distance of forty or fifty yards from the ship, a number of luminous
streamers were seen rushing towards us. On nearing the vessel they rapidly
turned, like a comet round its perihelion, placed themselves side by side,
and, in parallel trails of light, kept up with the ship. One of them placed itself
right in front of the bow as a pioneer. These comets of the sea were joined at
intervals by others. Sometimes as many as six at a time would rush at us,
bend with extraordinary rapidity round a sharp curve, and afterwards keep us
company. I leaned over the bow, and scanned the streamers closely. The
frontal portion of each of them revealed the outline of a porpoise. The rush
of the creatures through the water had started the phosphorescence, every
spark of which was converted by the motion of the retina into a line of light.
Each porpoise was thus wrapped in a luminous sheath. The phosphorescence
did not cease at the creature's tail, but was carried many porpoise-lengths
behind it.

To our right we had the African hills, illuminated by the moon. Gibraltar
Rock at length became visible, but the town remained long hidden by a belt
of haze, through which at length the brighter lamps struggled. It was like the
gradual resolution of a nebula into stars. As the intervening depth became
gradually less, the mist vanished more and more, and finally all the lamps
shone through it They formed a bright foil to the sombre mass of rock above

them. The sea was so calm and the scene so lovely that Mr. Huggins and
myself stayed on deck till near midnight, when the ship was moored. During
our walking to and fro a striking enlargement of the disk of Jupiter was
observed, whenever the heated air of the funnel came between us and the
planet. On passing away from the heated air, the flat dim disk would
immediately shrink to a luminous point. The effect was one of visual
persistence. The retinal image of the planet was set quivering in all azimuths
by the streams of heated air, describing in quick succession minute lines of
light, which summed themselves to a disk of sensible area.

At six o'clock next morning, the gun at the Signal Station on the summit of
the rock, boomed. At eight the band on board the 'Trafalgar' training-ship,
which was in the harbour, struck up the national anthem; and immediately
afterwards a crowd of mite-like cadets swarmed up the rigging. After the
removal of the apparatus belonging to the Gibraltar party we went on shore.
Winter was in England when we left, but here we had the warmth of
summer. The vegetation was luxuriant — palm-trees, cactuses, and aloes, all
ablaze with scarlet flowers. A visit to the Governor was proposed, as an act
of necessary courtesy, and I accompanied Admiral Ommaney and Mr.
Huggins to 'the Convent,' or Government House. We sent in our cards,
waited for a time, and were then conducted by an orderly to his Excellency.
He is a fine old man, over six feet high, and of frank military bearing. He
received us and conversed with us in a very genial manner. He took us to see
his garden, his palms, his shaded promenades, and his orange-trees loaded
with fruit, in all of which he took manifest delight. Evidently 'the hero of
Kars' had fallen upon quarters after his own heart. He appeared full of good
nature, and engaged us on the spot to dine with him that day.

We sought the town-major for a pass to visit the lines. While awaiting his
arrival I purchased a stock of white glass bottles, with a view to experiments
on the colour of the sea. Mr. Huggins and myself, who wished to see the
rock, were taken by Captain Salmond to the library, where a model of
Gibraltar is kept, and where we had a useful preliminary lesson. At the
library we met Colonel Maberly, a courteous and kindly man, who gave us
good advice regarding our excursion. He sent an orderly with us to the
entrance of the lines. The orderly handed us over to an intelligent Irishman,
who was directed to show us everything that we desired to see, and to hide
nothing from us. We took the 'upper line,' traversed the galleries hewn

through the limestone; looked through the embrasures, which opened like
doors in the precipice, towards the hills of Spain; reached St. George's hall,
and went still higher, emerging on the summit of one of the noblest cliffs I
have ever seen.

Beyond were the Spanish lines, marked by a line of white sentry-boxes;
nearer were the English lines, less conspicuously indicated; and between
both was the neutral ground. Behind the Spanish lines rose the conical hill
called the Queen of Spain's Chair. The general aspect of the mainland from
the rock is bold and rugged. Doubling back from the galleries, we struck
upwards towards the crest, reached the Signal Station, where we indulged in
'shandy-gaff' and bread and cheese. Thence to O'Hara's Tower, the highest
point of the rock. It was built by a former Governor, who, forgetful of the
laws of terrestrial curvature, thought he might look from the tower into-the
port of Cadiz. The tower is riven, and it may be climbed along the edges of
the crack. We got to the top of it; thence descended the curious
Mediterranean Stair — a zigzag, mostly of steps down a steeply falling
slope, amid palmetto brush, aloes, and prickly pear.

Passing over the Windmill Hill, we were joined at the 'Governor's Cottage'
by a car, and drove afterwards to the lighthouse at Europa Point. The tower
was built, I believe, by Queen Adelaide, and it contains a fine dioptric
apparatus of the first order, constructed by Messrs. Chance, of Birmingham.
At the appointed hour we were at the Convent. During dinner the same
genial traits which appeared in the morning were still more conspicuous. The
freshness of the Governor's nature showed itself best when he spoke of his
old antagonist in arms, Mouravieff. Chivalry in war is consistent with its
stern prosecution. These two men were chivalrous, and after striking the last
blow became friends for ever. Our kind and courteous reception at Gibraltar
is a thing to be remembered with pleasure.

On December 15 we committed ourselves to the Mediterranean. The views
of Gibraltar with which we are most acquainted represent it as a huge ridge;
but its aspect, end on, both from the Spanish lines and from the other side, is
truly noble. There is a sloping bank of sand at the back of the rock, which I
was disposed to regard simply as the débris of the limestone. I wished to let
myself down upon it, but had not the time. My friend Mr. Busk, however,
assures me that it is silica, and that the same sand constitutes the adjacent

neutral ground. There are theories afloat as to its having been blown from
Sahara. The Mediterranean throughout this first day, and indeed throughout
the entire voyage to Oran, was of a less deep blue than the

Atlantic. Possibly the quantity of organisms may have modified the colour.
At night the phosphorescence was startling, breaking suddenly out along the
crests of the waves formed by the port and starboard bows. Its strength was
not uniform. Having flashed brilliantly for a time, it would in part subside,
and afterwards regain its vigour. Several large phosphorescent masses of
weird appearance also floated past.

On the morning of the 16th we sighted the fort and lighthouse of Marsa el
Kibir, and beyond them the white walls of Oran lying in the bight of a bay,
sheltered by dominant hills. The sun was shining brightly; during our whole
voyage we had not had so fine a day. The wisdom which had led us to
choose Oran as our place of observation seemed demonstrated. A rather
excitable pilot came on board, and he guided us in behind the Mole, which
had suffered much damage the previous year from an unexplained outburst
of waves from the Mediterranean. Both port and bow anchors were cast in
deep water. With three huge hawsers the ship's stem was made fast to three
gun-pillars fixed in the Mole; and here for a time the 'Urgent' rested from her
labours.

M. Janssen, who had rendered his name celebrated by his observations of the
eclipse in India in 1868, when he showed the solar flames to be eruptions of
incandescent hydrogen, was already encamped in the open country about
eight miles from Oran. On December 2 he had quitted Paris in a balloon,
with a strong young sailor as his assistant, had descended near the mouth of
the Loire, seen M. Gambetta, and received from him encouragement and aid.
On the day of our arrival his encampment was visited by Mr. Huggins, and
the kind and courteous Engineer of the Port drove me subsequently, in his
own phaeton, to the place. It bore the best repute as regards freedom from
haze and fog, and commanded an open outlook; but it was inconvenient for
us on account of its distance from the ship. The place next in repute was the
railway station, between two and three miles distant from the Mole. It was
inspected, but, being enclosed, was abandoned for an eminence in an
adjacent garden, the property of Mr. Hinshelwood, a Scotchman who had
settled some years previously as an Esparto merchant in Oran. [Footnote:

Esparto is a kind of grass now much used in the manufacture of paper.] He,
in the most liberal manner, placed his ground at the disposition of the party.
Here the tents were pitched, on the Saturday, by Captain Salmond and his
intelligent corps of sappers, the instruments being erected on the Monday
under cover of the tents.

Close to the railway station runs a new loopholed wall of defence, through
which the highway passes into the open country. Standing on the highway,
and looking southwards, about twenty yards to the right is a small bastionet,
intended to carry a gun or two. Its roof I thought would form an admirable
basis for my telescope, while the view of the surrounding country was
unimpeded in all directions. The authorities kindly allowed me the use of
this bastionet. Two men, one a blue-jacket named Elliot, and the other a
marine named Hill, were placed at my disposal by Lieutenant Walton; and,
thus aided, on Monday morning I mounted my telescope. The instrument
was new to me, and some hours of discipline were spent in mastering all the
details of its manipulation.

Mr. Huggins joined me, and we visited together the Arab quarter of Oran.
The flat-roofed houses appeared very clean and white. The street was filled
with loiterers, and the thresholds were occupied by picturesque groups.
Some of the men were very fine. We saw many straight, manly fellows who
must have been six feet four in height. They passed us with perfect
indifference, evincing no anger, suspicion, or curiosity, hardly caring in fact
to glance at us as we passed. In one instance only during my stay at Oran
was I spoken to by an Arab. He was a tall, good-humoured fellow, who came
smiling up to me, and muttered something about 'les Anglais.' The mixed
population of Oran is picturesque in the highest degree: the Jews, rich and
poor, varying in their costumes as their wealth varies; the Arabs more
picturesque still, and of all shades of complexion — the negroes, the
Spaniards, the French, all grouped together, each race preserving its own
individuality, formed a picture intensely interesting to me.

On Tuesday, the 20th, I was early at the bastionet. The night had been very
squally. The sergeant of the sappers had taken charge of our key, and on
Tuesday morning Elliot went for it. He brought back the intelligence that the
tents had been blown down, and the instruments overturned. Among these
was a large and valuable equatorial from the Royal Observatory, Greenwich.

It seemed hardly possible that this instrument, with its wheels and verniers
and delicate adjustments, could have escaped uninjured from such a fall.
This, however, was the case; and during the day all the overturned
instruments were restored to their places, and found to be in practical
working order. This and the following day were devoted to incessant
schooling. I had come out as a general stargazer, and not with the intention
of devoting myself to the observation of any particular phenomenon. I
wished to see the whole — the first contact, the advance of the moon, the
successive swallowing up of the solar spots, the breaking of the last line of
crescent by the lunar mountains into Bailey's beads, the advance of the
shadow through the air, the appearance of the corona and prominences at the
moment of totality, the radiant streamer; of the corona, the internal structure
of the flames, a glance through a polariscope, a sweep round the landscape
with the naked eye, the reappearance of the soar limb through Bailey's
beads, and, finally, the retreat of the lunar shadow through the air.

I was provided with a telescope of admirable definition, mounted, adjusted,
packed, and most liberally placed at my disposal by Mr. Warren De La Rue.
The telescope grasped the whole of the sun, and a considerable portion of the
space surrounding it. But it would not take in the extreme limits of the
corona. For this I had lashed on to the large telescope a light but powerful
instrument, constructed by Ross, and lent to me by Mr. Huggins. I was also
furnished with an excellent binocular by Mr. Dallmeyer. In fact, no man
could have been more efficiently supported.

It required a strict parcelling out of the interval of totality to embrace in it
the entire series of observations. These, while the sun remained visible, were
to be made with an unsilvered diagonal eye-piece, which reflected but a
small fraction of the sun's light, this fraction, being still further toned down
by a dark glass. At the moment of totality the dark glass was to be removed,
and a silver reflector pushed in, so as to get the maximum of light from the
corona and prominences The time of totality was distributed as follows:

1. Observe approach of shadow through the air:
totality.

2. Telescope 30 seconds.

3. Finder 30 seconds.

4. Double image prism 15 seconds.

5. Naked eye. 10 seconds.

6. Finder or binocular 20 seconds.

7. Telescope. 20 seconds.

8. Observe retreat of shadow.

In our rehearsals Elliot stood beside me, watch in hand, and furnished with a
lantern. He called out at the end of each interval, while I moved from
telescope to finder, from finder to polariscope, from polariscope to naked
eye, from naked eye back to finder, from finder to telescope, abandoning the
instrument finally to observe the retreating shadow. All this we went over
twenty times, while looking at the actual sun, and keeping him in the middle
of the field. It was my object to render the repetition of the lesson so
mechanical as to leave no room for flurry, forgetfulness, or excitement.
Volition was not to be called upon, nor judgment exercised, but a well-
beaten path of routine was to be followed. Had the opportunity occurred, I
think the programme would have been strictly carried out.

But the opportunity did not occur. For several days the weather had been ill-
natured. We had wind so strong As to render the hawsers at the stern of the
'Urgent' as rigid as iron, and to destroy the navigating lieutenant's sleep. We
had clouds, a thunder-storm, and some rain. Still the hope was held out that

the atmosphere would cleanse itself, and if it did we were promised air of
extraordinary limpidity. Early on the 22nd we were all at our posts. Spaces
of blue in the early morning gave us some encouragement, but all depended
on the relation of these spaces to the surrounding clouds. Which of them
were to grow as the day advanced? The wind was high, and to secure the
steadiness of my instrument I was forced to retreat behind a projection of the
bastionet, place stones upon its stand, and, further, to avail myself of the
shelter of a sail. My practised men fastened the sail at the top, and loaded it
with boulders at the bottom. It was tried severely, but it stood firm.

The clouds and blue spaces fought for a time with varying success. The sun
was bidden and revealed at intervals, hope oscillating in synchronism with
the changes of the sky. At the moment of first contact a dense cloud
intervened; but a minute or two afterwards the cloud had passed, and the
encroachment of the black body of the moon was evident upon the solar
disk. The moon marched onward, and I saw it at frequent intervals; a large
group of spots were approached and swallowed up. Subsequently I caught
sight of the lunar limb as it cut through the middle of a large spot. The spot
was not to be distinguished from the moon, but rose like a mountain above
it. The clouds, when thin, could be seen as grey scud drifting across the
black surface of the moon; but they thickened more and more, and made the
intervals of clearness scantier. During these moments I watched with an
interest bordering upon fascination the march of the silver sickle of the sun
across the field of the telescope. It was so sharp and so beautiful. No trace of
the lunar limb could be observed beyond the sun's boundary. Here, indeed, it
could only be relieved by the corona, which was utterly cut off by the dark
glass. The blackness of the moon beyond the sun was, in fact, confounded
with the blackness of space.

Beside me was Elliot with the watch and lantern, while Lieutenant Archer, of
the Royal Engineers, had the kindness to take charge of my note-book. I
mentioned, and he wrote rapidly down, such things as seemed worthy of
remembrance. Thus my hands and mind were entirely free; but it was all to
no purpose. A patch of sunlight fell and rested upon the landscape some
miles away. It was the only illuminated spot within view. But to the north-
west there was still a space of blue which might. reach us in time. Within
seven minutes of totality another space towards the zenith became very dark.
The atmosphere was, as it were, on the brink of a precipice, being charged

with humidity, which required but a slight chill to bring it down in clouds.
This was furnished by the withdrawal of the solar beams: the clouds did
come down, covering up the space of blue on which our hopes had so long
rested. I abandoned the telescope and walked to and fro in despair. As the
moment of totality approached, the descent towards darkness was as obvious
as a falling stone. I looked towards a distant ridge, where the darkness would
first appear. At the moment a fan of beams, issuing from the hidden sun, was
spread out over the southern heavens. These beams are bars of alternate light
and shade, produced in illuminated haze by the shadows of floating cloudlets
of varying density. The beams are practically parallel, but by an effect of
perspective they appear divergent, having the sun, in fact, for their point of
convergence. The darkness took possession of the ridge referred to, lowered
upon M. Janssen's observatory, passed over the southern heavens, blotting
out the beams as if a sponge had been drawn across them. It then took
successive possession of three spaces of blue sky in the south-eastern
atmosphere. I again looked towards the ridge. A glimmer as of day-dawn
was behind it, and immediately afterwards the fan of beams, which had been
for more than two minutes absent, revived. The eclipse of 1870 had ended,
and, as far as the corona and flames were concerned, we had been defeated.

Even in the heart of the eclipse the darkness was by no means perfect. Small
print could be read. In fact, the clouds which rendered the day a dark one, by
scattering light into the shadow, rendered the darkness less intense than it
would have been had the atmosphere been without cloud. In the more open
spaces I sought for stars, but could find none. There was a lull in the wind
before and after totality, but during the totality the wind was strong. I waited
for some time on the bastionet, hoping to get a glimpse of the moon on the
opposite border of the sun, but in vain. The clouds continued, and some rain
fell. The day brightened somewhat afterwards, and, having packed all up, in
the sober twilight Mr. Crookes and myself climbed the heights above the fort
of Vera Cruz. From this eminence we had a very noble view over the
Mediterranean and the flanking African hills. The sunset was remarkable,
and the whole outlook exceedingly fine.

The able and well-instructed medical officer of the 'Urgent,' Mr. Goodman,
observed the following temperatures during the progress of the eclipse:

Hour Deg.

11.45 56

11.55 55

12.10 54

12.37 53

12.39 52

12.43 51

1.5 52

1.27 53

1.44 56

2.10 57

The minimum temperature occurred some minutes after totality, when a
slight rain fell.

The wind was so strong on the 23rd that Captain Henderson would not
venture out. Guided by Mr. Goodman, I visited a cave in a remarkable
stratum of shell-breccia, and, thanks to my guide, secured specimens. Mr.
Busk informs me that a precisely similar breccia, is found at Gibraltar, at
approximately the same level. During the afternoon, Admiral Ommaney and
myself drove to the fort of Marsa el Kibir. The fortification is of ancient
origin, the Moorish arches being still there in decay, but the fort is now very
strong. About four or five hundred fine-looking dragoons were looking after
their horses, waiting for a lull to enable them to embark for France. One of
their officers was wandering in a very solitary fashion over the fort. We had
some conversation with him. He had been at Sedan, had been taken prisoner,
but had effected his escape. He shook his head when we spoke of the
termination of the war, and predicted its long continuance. There was
bitterness in his tone as he spoke of the charges of treason so lightly levelled
against French commanders.

The green waves raved round the promontory on which the fort stands,
smiting the rocks, breaking into foam, and jumping, after impact, to a height
of a hundred feet and more into the air. As we returned our vehicle broke
down through the loss of a wheel. The Admiral went on board, while I
remained long watching the agitated sea. The little horses of Oran well merit
a passing word. Their speed and endurance, both of which are heavily drawn
upon by their drivers, are extraordinary.

The wind sinking, we lifted anchor on the 24th. For some hours we went
pleasantly along; but during the afternoon the storm revived, and it blew
heavily against us all the night. When we came opposite the Bay of Almeria,
on the 25th, the captain turned the ship, and steered into the bay, where,
under the shadow of the Sierra Nevada, we passed Christmas night in peace.
Next morning 'a rose of dawn' rested on the snows of the adjacent
mountains, while a purple haze was spread over the lower hills. I had no
notion that Spain possessed so fine a range of mountains as the Sierra
Nevada. The height is considerable, but the form also is such as to get the
maximum of grandeur out of the height. We weighed anchor at eight A.M.,
passing for a time through shoal water, the bottom having been evidently
stirred up. The adjacent land seemed eroded in a remarkable manner. It has
its floods, which excavate these valleys and ravines, and leave those singular
ridges behind. Towards evening I climbed the mainmast, and, standing on

the cross-trees, saw the sun set amid a blaze of fiery clouds. The wind was
strong and bitterly cold, and I was glad to slide back to the deck along a
rope, which stretched from the mast-head to the ship's side. That night we
cast anchor beside the Mole of Gibraltar.

On the morning of the 27th, in company with two friends, I drove to the
Spanish lines, with the view of seeing the rock from that side. It is an
exceedingly noble mass. The Peninsular and Oriental mail-boat had been
signalled and had come. Heavy duties called me homeward, and by
transferring myself from the 'Urgent' to the mail-steamer I should gain three
days. I hired a boat, rowed to the steamer, learned that she was to start at
one, and returned with all speed to the 'Urgent.' Making known to Captain
Henderson my wish to get away, he expressed doubts as to the possibility of
reaching the mail-steamer in time. With his accustomed kindness, he
however placed a boat at my disposal. Four hardy fellows and one of the
ship's officers jumped into it; my luggage, hastily thrown together, was
tumbled in, and we were immediately on our way. We had nearly four miles
to row in about twenty minutes; but we hoped the mail-boat might not be
punctual. For a time we watched her anxiously; there was no motion; we
came nearer, but the flags were not yet hauled in. The men put forth all their
strength, animated by the exhortations of the officer at the helm. The
roughness of the sea rendered their efforts to some extent nugatory: still we
were rapidly approaching the steamer. At length she moved, punctual almost
to the minute, at first slowly, but soon with quickened pace.

We turned to the left, so as to cut across her bows. Five minutes' pull would
have brought us up to her. The officer waved his cap and I my hat. 'If they
could only see us, they might back to us in a moment.' But they did not see
us, or if they did, they paid us no attention. I returned to the 'Urgent,'
discomfited, but grateful to the fine fellows who had wrought so hard to
carry out my wishes.

Glad of the quiet, in the sober afternoon I took a walk towards Europa Point.
The sky darkened and heavy squalls passed at intervals. Private theatricals
were at the Convent, and the kind and courteous Governor had sent cards to
the eclipse party. I failed in my duty in not going. St. Michael's Cave is said
to rival, if it does not outrival, the Mammoth Cave of Kentucky. On the 28th
Mr. Crookes, Mr. Carpenter, and myself, guided by a military policeman

who understood his work, explored the cavern. The mouth is about 1,100
feet above the sea. We zigzagged up to it, and first were led into an aperture
in the rock, at some height above the true entrance of the cave. In this upper
cavern we saw some tall and beautiful stalactite pillars.

The water drips from the roof charged with bicarbonate of lime. Exposed to
the air, the carbonic acid partially escapes, and the simple carbonate of lime,
which is hardly at all soluble in water, deposits itself as a solid, forming
stalactites and stalagmites. Even the exposure of chalk or limestone water to
the open air partially softens it. A specimen of the Redbourne water exposed
by Professors Graham, Miller, and Hofmann, in a shallow basin, fell from
eighteen degrees to nine degrees of hardness. The softening process of Clark
is virtually a hastening of the natural process. Here, however, instead of
being permitted to evaporate, half the carbonic acid is appropriated by lime,
the half thus taken up, as well as the remaining half, being precipitated. The
solid precipitate is permitted to sink, and the clear supernatant liquid is
limpid soft water.

We returned to the real mouth of St. Michael's Cave, which is entered by a
wicket. The floor was somewhat muddy, and the roof and walls were wet.
We soon found ourselves in the midst of a natural temple, where tall
columns sprang complete from floor to roof, while incipient columns were
growing to meet each other, upwards and downwards. The water which
trickles from the stalactite, after having in part yielded up its carbonate of
lime, falls upon the floor vertically underneath, and there builds the
stalagmite. Consequently, the pillars grow from above and below
simultaneously, along the same vertical. It is easy to distinguish the
stalagmitic from the stalactitic portion of the pillars. The former is always
divided into short segments by protuberant rings, as if deposited periodically,
while the latter presents a uniform surface. In some cases the points of
inverted cones of stalactite rested on the centres of pillars of stalagmite. The
process of solidification and the consequent architecture were alike
beautiful.

We followed our guide through various branches and arms of the cave,
climbed and descended steps, halted at the edges of dark shafts and
apertures, and squeezed ourselves through narrow passages. From time to
time we halted, while Mr. Crookes illuminated with ignited magnesium wire,

the roof, columns, dependent spears, and graceful drapery of the stalactites.
Once, coming to a magnificent cluster of icicle-like spears, we helped
ourselves to specimens. There was some difficulty in detaching the more
delicate ones, their fragility was so great. A consciousness of vandalism,
which smote me at the time, haunts me still; for, though our requisitions
were moderate, this beauty ought not to be at all invaded. Pendent from the
roof, in their natural habitat, nothing can exceed their delicate beauty; they
live, as it were, surrounded by organic connections. In London they are
curious, but not beautiful. Of gathered shells Emerson writes:

I wiped away the weeds and foam,
And brought my sea-born treasures home
But the poor, unsightly, noisome things
Had left their beauty on the shore,
With the sun, and the sand, and the wild uproar.

The promontory of Gibraltar is so burrowed with caverns that it has been
called the Hill of Caves. They are apparently related to the geologic
disturbances which the rock has undergone. The earliest of these is the tilting
of the once horizontal strata. Suppose a force of torsion to act upon the
promontory at its southern extremity near Europa Point, and suppose the
rock to be of a partially yielding character; such a force would twist the
strata into screw-surfaces, the greatest amount of twisting being endured
near the point of application of the force. Such a twisting the rock appears to
have suffered; but instead of the twist fading gradually and uniformly off, in
passing from south to north, the want of uniformity in the material has
produced lines of dislocation where there are abrupt changes in the amount
of twist. Thus, at the northern end of the rock the dip to the west is nineteen
degrees; in the Middle Hill, it is thirty-eight degrees; in the centre of the
South hill, or Sugar Loaf, it is fifty-seven degrees. At the southern extremity
of the Sugar Loaf the strata are vertical, while farther to the south they
actually turn over and dip to the east.

The rock is thus divided into three sections, separated from each other by
places of dislocation, where the strata are much wrenched and broken. These
are called the Northern and Southern Quebrada, from the Spanish 'Tierra
Quebrada,' or broken ground. It is at these places that the inland caves of
Gibraltar are almost exclusively found. Based on the observations of Dr.

Falconer and himself, an excellent and most interesting account of these
'caves, and of the human remains and works of art which they contain, was
communicated by Mr. Busk to the meeting of the Congress of Prehistoric
Archaeology at Norwich, and afterwards printed in the 'Transactions' of the
Congress. [Footnote: In this essay Mr. Busk refers to the previous labours of
Mr. Smith, of Jordan Hill, to whom we owe most of our knowledge of the
geology of the rock.] Long subsequent to the operation of the twisting force
just referred to, the promontory underwent various changes of level. There
are sea-terraces and layers of shell-breccia along its flanks, and numerous
caves which, unlike the inland ones, are the product of marine erosion. The
Ape's Hill, on the African side of the strait, Mr. Busk informs me has
undergone similar disturbances. [Footnote: No one can rise from the perusal
of Mr. Busk's paper without a feeling of admiration for the principal
discoverer and indefatigable explorer of the Gibraltar caves, the late
Captain Frederick Brome.]

-----

In the harbour of Gibraltar, on the morning of our departure, I resumed a
series of observations on the colour of the sea. On the way out a number of
specimens had been collected, with a view to subsequent examination. But
the bottles were claret bottles, of doubtful purity. At Gibraltar, therefore, I
purchased fifteen white glass bottles, with ground glass stoppers, and at
Cadiz, thanks to the friendly guidance of Mr. Cameron, I secured a dozen
more. These seven-and-twenty bottles were filled with water, taken at
different places between Oran and Spithead.

And here let me express my warmest acknowledgments to Captain
Henderson, the commander of H.M.S. 'Urgent,' who aided me in my
observations in every possible way. Indeed, my thanks are due to all the
officers for their unfailing courtesy and help. The captain placed at my
disposal his own coxswain, an intelligent fellow named Thorogood, who
skilfully attached a cord to each bottle, weighted it with lead, cast it into the
sea, and, after three successive rinsings, filled it under my own eyes. The
contact of jugs, buckets, or other vessels was thus avoided; and even the
necessity of pouring out the water, afterwards, through the dirty London air.

The mode of examination applied to these bottles has been already
described. [Footnote: See "On Dust and Disease".] The liquid is illuminated
by a powerfully condensed beam, its condition being revealed through the
light scattered by its suspended particles. 'Care is taken to defend the eye
from the access of all other light, and, thus defended, it becomes an organ of
inconceivable delicacy.' Were water of uniform density perfectly free from
suspended matter, it would, in my opinion, scatter no light at all. The track of
a luminous beam could not be seen in such water. But 'an amount of
impurity so infinitesimal as to be scarcely expressible in numbers, and the
individual particles of which are so small as wholly to elude the microscope,
may, when examined by the method alluded to, produce not only sensible,
but striking, effects upon the eye.'

The results of the examination of nineteen bottles filled at various places
between Gibraltar and Spithead, are here tabulated:

No. Locality Colour of Sea Appearance in
Luminous beam

1 Gibraltar Harbour. Green Thick with fine
particles

2 Two miles from Gibraltar Clearer green Thick with very fine
particles

3 Off Cabreta Point Bright green Still thick, but less so

4 Off Cabreta Point Black-indigo Much less thick, very
pure

5 Off Tarifa Undecided Thicker than No. 4

6 Beyond Tarifa Cobalt-blue Much purer than No.
5

7 Twelve miles from Cadiz . Yellow-green Very thick

8 Cadiz Harbour Yellow-green Exceedingly thick

9 Fourteen miles from Cadiz Yellow-green Thick, but less so

10 Fourteen miles from Cadiz Bright green Much less thick

11 Between Capes St. Mary and Deep indigo Very little matter,
Vincent. very pure

12 Off the Burlings. Strong green. Thick, with fine
matter

13 Beyond the Burlings . Indigo Very little matter,
pure

14 Off Cape Finisterre. Undecided. Less pure

15 Bay of Biscay Black-indigo. Very little matter,
very pure

16 Bay of Biscay Indigo Very fine matter.
Iridescent

17 Off Ushant Dark green. A good deal of
matter

18 Off St. Catherine's . Yellow-green Exceedingly thick

19 Spithead Green Exceedingly thick

Here we have three specimens of water, described as green, a clearer green,
and bright green, taken in Gibraltar Harbour, at a point two miles from the
harbour, and off Cabreta Point. The home examination showed the first to be
thick with suspended matter, the second less thick, and the third still less
thick. Thus the green brightened as the suspended matter diminished in
amount.

Previous to the fourth observation our excellent navigating lieutenant, Mr.
Brown, steered along the coast, thus avoiding the adverse current which sets
in, through the Strait, from the Atlantic to the Mediterranean. He was at
length forced to cross the boundary of the Atlantic current, which was
defined with extraordinary sharpness. On the one side of it the water was a
vivid green, on the other a deep blue. Standing at the bow of the ship, a
bottle could be filled with blue water, while at the same moment a bottle cast
from the stern could be filled with green water. Two bottles were secured,
one on each side of this remarkable boundary. In the distance the Atlantic
had the hue called ultra-marine; but looked fairly down upon, it was of
almost inky blackness — black qualified by a trace of indigo.

What change does the home examination here reveal? In passing to indigo,
the water becomes suddenly augmented in purity, the suspended matter
becoming suddenly less. Off Tarifa, the deep indigo disappears, and the sea
is undecided in colour. Accompanying this change, we have a rise in the

quantity of suspended matter. Beyond Tarifa, we change to cobalt-blue, the
suspended matter falling at the same time in quantity. This water is distinctly
purer than the green. We approach Cadiz, and at twelve miles from the city
get into yellow-green water; this the London examination shows to be thick
with suspended matter. The same is true of Cadiz harbour, and also of a point
fourteen miles from Cadiz in the homeward direction. Here there is a sudden
change from yellow-green to a bright emerald-green, and accompanying the
change a sudden fall in the quantity of suspended matter. Between Cape St.
Mary and Cape St: Vincent the water changes to the deepest indigo, a further
diminution of the suspended matter being the concomitant phenomenon.

We now reach the remarkable group of rocks called the Burlings, and find
the water between the shore and the rocks a strong green; the home
examination shows it to be thick with fine matter. Fifteen or twenty miles
beyond the Burlings we come again into indigo water, from which the
suspended matter has in great part disappeared. Off Cape Finisterre, about
the place where the 'Captain' went down, the water becomes green, and the
home examination pronounces it to be thicker. Then we enter the Bay of
Biscay, where the indigo resumes its power, and where the home
examination shows the greatly augmented purity of the water. A second
specimen of water, taken from the Bay of Biscay, held in suspension fine
particles of a peculiar kind; the size of them was such as to render the water
richly iridescent. It showed itself green, blue, or salmon-coloured, according
to the direction of the line of vision. Finally, we come to our last two bottles,
the one taken opposite St. Catherine's lighthouse, in the Isle of Wight, the
other at Spithead. The sea at both these places was green, and both
specimens, as might be expected, were pronounced by the home examination
to be thick with suspended matter.

Two distinct series of observations are here referred to — the one consisting
of direct observations of the colour of the sea, conducted during the voyage
from Gibraltar to Portsmouth: the other carried out in the laboratory of the
Royal Institution. And here it is to be noted that in the home examination I
never knew what water was placed in my hands. The labels, with the names
of the localities written upon them, had been tied up, all information
regarding the source of the water being thus held back. The bottles were
simply numbered, and not till all of them had been examined, and described,
were the labels opened, and the locality and sea-colour corresponding to the

various specimens ascertained. The home observations, therefore, must have
been perfectly unbiassed, and they clearly establish the association of the
green colour with fine suspended matter, and of the ultramarine colour, and
more especially of the black-indigo hue of the Atlantic, with the comparative
absence of such matter.

So much for mere observation; but what is the cause of the dark hue of the
deep ocean? [Footnote: A note, written to me on October 22, by my friend
Canon Kingsley, contains the following reference to this point: 'I have never
seen the Lake of Geneva, but I thought of the brilliant dazzling dark blue of
the mid-Atlantic under the sunlight, and its black-blue under cloud, both so
solid that one might leap off the sponson on to it without fear; this was to me
the most wonderful thing which I saw on my voyages to and from the West
Indies.']

A preliminary remark or two will clear our way towards an explanation.
Colour resides in white light, appearing when any constituent of the white
light is withdrawn. The hue of a purple liquid, for example, is immediately
accounted for by its action on a spectrum. It cuts out the yellow and green,
and allows the red and blue to pass through. The blending of these two
colours produces the purple. But while such a liquid attacks with special
energy the yellow and green, it enfeebles the whole spectrum. By increasing
the thickness of the stratum we may absorb the whole of the light. The
colour of a blue liquid is similarly accounted for. It first extinguishes the red;
then, as the thickness augments, it attacks the orange, yellow, and green in
succession; the blue alone finally remaining. But even it might be
extinguished by a sufficient depth of 'the liquid.

And now we are prepared for a brief, but tolerably complete, statement of
that action of sea-water upon light, to which it owes its darkness. The
spectrum embraces three classes of rays — the thermal, the visual, and the
chemical. These divisions overlap each other; the thermal rays are in part
visual, the visual rays in part chemical, and vice versa. The vast body of
thermal rays lie beyond the red, being invisible. These rays are attacked with
exceeding energy by water. They are absorbed close to the surface of the sea,
and are the great agents in evaporation. At the same time the whole spectrum
suffers enfeeblement; water attacks all its rays, but with different degrees of
energy. Of the visual rays, the red are first extinguished. As the solar beam

plunges deeper into the sea, orange follows red, yellow follows orange,
green follows yellow, and the various shades of blue, where the water is
deep enough, follow green. Absolute extinction of the solar beam would be
the consequence if the water were deep and uniform. If it contained no
suspended matter, such water would be as black as ink. A reflected glimmer
of ordinary light would reach us from its surface, as it would from the
surface of actual ink; but no light, hence no colour, would reach us from the
body of the water.

In very clear and deep sea-water this condition is approximately fulfilled,
and hence the extraordinary darkness of such water. The indigo, already
referred to, is, I believe, to be ascribed in part to the suspended matter, which
is never absent, even in the purest natural water; and in part to the slight
reflection of the light from the limiting surfaces of strata of different
densities. A modicum of light is thus thrown back to the eye, before the
depth necessary to absolute extinction has been attained. An effect precisely
similar occurs under the moraines of glaciers. The ice here is exceptionally
compact, and, owing to the absence of the internal scattering common in
bubbled ice, the light plunges into the mass, where it is extinguished, the
perfectly clear ice presenting an appearance of pitchy blackness. [Footnote:
I learn from a correspondent that certain Welsh tarns, which are reputed
bottomless, have this inky hue.]

The green colour of the sea has now to be accounted for; and here, again, let
us fall back upon the sure basis of experiment. A strong white dinner-plate
had a lead weight securely fastened to it. Fifty or sixty yards of strong
hempen line were attached to the plate.

My assistant, Thorogood, occupied a boat, fastened as usual to the davits of
the 'Urgent,' while I occupied a second boat nearer the stern of the ship. He
cast the plate as a mariner heaves the lead, and by the time it reached me it
had sunk a considerable depth in the water. In all cases the hue of this plate
was green. Even when the sea was of the darkest indigo, the green was vivid
and pronounced. I could notice the gradual deepening of the colour as the
plate sank, but at its greatest depth, even in indigo water, the colour was still
a blue-green. [Footnote: In no case, of course, is the green pure, but a
mixture of green and blue.]

Other observations confirmed this one. The 'Urgent' is a screw steamer, and
right over the blades of the screw was an orifice called the screw-well,
through which one could look from the poop down upon the screw. The
surface-glimmer, which so pesters the eye, was here in a great measure
removed. Midway down, a plank crossed the screw-well from side to side;
on this I placed myself and observed the action of the screw underneath. The
eye was rendered sensitive by the moderation of the light; and, to remove
still further all disturbing causes, Lieutenant Walton had a sail and tarpaulin
thrown over the mouth of the well. Underneath this I perched myself on the
plank and watched the screw. In an indigo sea the play of colour was
indescribably beautiful, and the contrast between the water, which had the
screw-blades, and that which had the bottom of the ocean, as a background,
was extraordinary. The one was of the most brilliant green, the other of the
deepest ultramarine. The surface of the water above the screw-blade was
always ruffled. Liquid lenses were thus formed, by which the coloured light
was withdrawn from some places and concentrated upon others, the water
flashing with metallic lustre. The screw-blades in this case played the part of
the dinner-plate in the former case, and there were other instances of a
similar kind. The white bellies of porpoises showed the green hue, varying in
intensity as the creatures swung to and fro between the surface and the
deeper water. Foam, at a certain depth below the surface, was also green. In
a rough sea the light which penetrated the summit of a wave sometimes
reached the eye, a beautiful green cap being thus placed upon the wave, even
in indigo water.

But how is this colour to be connected with the suspended particles? Thus.
Take the dinner-plate which showed so brilliant a green when thrown into
indigo water. Suppose it to diminish in size, until it reaches an almost
microscopic magnitude. It would still behave substantially as the larger
plate, sending to the eye its modicum of green light. If the plate, instead of
being a large coherent mass, were ground to a powder sufficiently fine, and
in this condition diffused through the clear sea-water, it would also send
green light to the eye. In fact, the suspended particles which the home
examination reveals, act in all essential particulars like the plate, or like the
screw-blades, or like the foam, or like the bellies of the porpoises. Thus I
think the greenness of the sea is physically connected with the matter which
it holds in suspension.

We reached Portsmouth on January 5, 1871. Then ended a voyage which,
though its main object was not realised, has left behind it pleasant memories,
both of the aspects of nature and the kindliness of men.

.

.

.

.

--------------------

.

.

VII. NIAGARA.

[Footnote: A Discourse delivered at the Royal Institution of Great Britain,
April 4, 1873.]

It is one of the disadvantages of reading books about natural scenery that
they fill the mind with pictures, often exaggerated, often distorted, often
blurred, and, even when well drawn, injurious to the freshness of first
impressions. Such has been the fate of most of us with regard to the Falls of
Niagara. There was little accuracy in the estimates of the first observers of
the cataract. Startled by an exhibition of power so novel and so grand,
emotion leaped beyond the control of the judgment, and gave currency to
notions which have often led to disappointment.

A record of a voyage in 1535 by a French mariner named Jacques Cartier,
contains, it is said, the first printed allusion to Niagara. In 1603 the first map
of the district was constructed by a Frenchman named Champlain. In 1648
the Jesuit Rageneau, in a letter to his superior at Paris, mentions Niagara as
'a cataract of frightful height.' [Footnote: From an interesting little book
presented to me at Brooklyn by its author, Mr. Holly, some of these data are
derived: Hennepin, Kalm, Bakewell, Lyell, Hall, and others I have myself

consulted.] In the winter of 1678 and 1679 the cataract was visited by Father
Hennepin, and described in a book dedicated 'to the King of Great Britain.'
He gives a drawing of the waterfall, which shows that serious changes have
taken place since his time. He describes it as 'a great and prodigious cadence
of water, to which the universe does not offer a parallel.' The height of the
fall, according to Hennepin, was more than 600 feet. 'The waters,' he says,
'which fall from this great precipice do foam and boil in the most astonishing
manner, making a noise more terrible than that of thunder. When the wind
blows to the south its frightful roaring may be heard for more than fifteen
leagues.' The Baron la Hontan, who visited Niagara in 1687, makes the
height 800 feet. In 1721 Charlevois, in a letter to Madame de Maintenon,
after referring to the exaggerations of his predecessors, thus states the result
of his own observations: 'For my part, after examining it on all sides, I am
inclined to think that we cannot allow it less than 140 or 150 feet,' — a
remarkably close estimate. At that time, viz. a hundred and fifty years ago, it
had the shape of a horseshoe, and reasons will subsequently be given for
holding that this has been always the form of the cataract, from its origin to
its present site.

As regards the noise of the fall, Charlevois declares the accounts of his
predecessors, which, I may say, are repeated to the present hour, to be
altogether extravagant. He is perfectly right. The thunders of Niagara are
formidable enough to those who really seek them at the base of the
Horseshoe Fall; but on the banks of the river, and particularly above the fall,
its silence, rather than its noise, is surprising. This arises, in part, from the
lack of resonance; the surrounding country being flat, and therefore
furnishing no echoing surfaces to reinforce the shock of the water. The
resonance from the surrounding rocks causes the Swiss Reuss at the Devil's
Bridge, when full, to thunder more loudly than the Niagara.

On Friday, November 1, 1872, just before reaching the village of Niagara
Falls, I caught, from the railway train, my first glimpse of the smoke of the
cataract. Immediately after my arrival I went with a friend to the northern
end of the American Fall. It may be that my mood at the time toned down
the impression produced by the first aspect of this grand cascade; but I felt
nothing like disappointment, knowing, from old experience, that time and
close acquaintanceship, the gradual interweaving of mind and nature, must
powerfully influence my final estimate of the scene. After dinner we crossed

to Goat Island, and, turning to the right, reached the southern end of the
American Fall. The river is here studded with small islands. Crossing a
wooden bridge to Luna Island, and clasping a tree which grows near its
edge, I looked long at the cataract, which here shoots down the precipice like
an avalanche of foam. It grew in power and beauty. The channel spanned by
the wooden bridge was deep, and the river there doubled over the edge of the
precipice, like the swell of a muscle, unbroken. The ledge here overhangs,
the water being poured out far beyond the base of the precipice. A space,
called the Cave of the Winds, is thus enclosed between the wall of rock and
the falling water.

Goat Island ends in a sheer dry precipice, which connects the American and
Horseshoe Falls. Midway between both is a wooden hut, the residence of the
guide to the Cave of the Winds, and from the hut a winding staircase, called
Biddle's Stair, descends to the base of the precipice. On the evening of my
arrival I went down this stair, and wandered along the bottom of the cliff.
One well-known factor in the formation and retreat of the cataract was
immediately observed. A thick layer of limestone formed the upper portion
of the cliff. This rested upon a bed of soft shale, which extended round the
base of the cataract. The violent recoil of the water against this yielding
substance crumbles it away, undermining the ledge above, which,
unsupported, eventually breaks off, and produces the observed recession.

At the southern extremity of the Horseshoe is a promontory, formed by the
doubling back of the gorge excavated by the cataract, and into which it
plunges. On the promontory stands a stone building, called the Terrapin
Tower, the door of which had been nailed up because of the decay of the
staircase within it. Through the kindness of Mr. Townsend, the
superintendent of Goat Island, the door was opened for me. From this tower,
at all hours of the day, and at some hours of the night, I watched and listened
to the Horseshoe Fall. The river here is evidently much deeper than the
American branch; and instead of bursting into foam where it quits the ledge,
it bends solidly over, and falls in a continuous layer of the most vivid green.
The tint is not uniform; long stripes of deeper hue alternating with bands of
brighter colour. Close to the ledge over which the water rolls, foam is
generated, the light falling upon which, and flashing back from it, is sifted in
its passage to and fro, and changed from white to emerald-green. Heaps of
superficial foam are also formed at intervals along the ledge, and are

immediately drawn into long white striae. [Footnote: The direction of the
wind with reference to the course of a ship may be inferred with accuracy
from the foam-streaks on the surface of the sea.] Lower down, the surface,
shaken by the reaction from below, incessantly rustles into whiteness. The
descent finally resolves itself into a rhythm, the water reaching the bottom of
the fall in periodic gushes. Nor is the spray uniformly diffused through the
air, but is wafted through it in successive veils of gauze-like texture. From
all this it is evident that beauty is not absent from the Horseshoe Fall, but
majesty is its chief attribute. The plunge of the water is not wild, but
deliberate, vast, and fascinating. From the Terrapin Tower, the adjacent arm
of the Horseshoe is seen projected against the opposite one, midway down;
to the imagination, therefore, is left the picturing of the gulf into which the
cataract plunges.

The delight which natural scenery produces in some minds is difficult to
explain, and the conduct which it prompts can hardly be fairly criticised by
those who have never experienced it. It seems to me a deduction from the
completeness of the celebrated Thomas Young, that he was unable to
appreciate natural scenery. 'He had really,' says Dean Peacock, 'no taste for
life in the country; he was one of those who thought that no one who was
able to live in London would be content to 'live elsewhere.' Well, Dr. Young,
like Dr. Johnson, had a right to his delights; but I can understand a,
hesitation to accept them, high as they were, to the exclusion of

That o'erflowing joy which Nature yields
To her true lovers.

To all who are of this mind, the strengthening of desire on my part to see and
know Niagara Falls, as far as it is possible for them to be seen and known,
will be intelligible.

On the first evening of my visit, I met, at the head of Biddle's Stair, the guide
to the Cave of the Winds. He was in the prime of manhood — large, well
built, firm and pleasant in mouth and eye. My interest in the scene stirred up
his, and made him communicative.

Turning to a photograph, he described, by reference to it, a feat which he had
accomplished some time previously, and which had brought him almost

under the green water of the Horseshoe Fall. 'Can you lead me there to-
morrow?' I asked. He eyed me enquiringly, weighing, perhaps, the chances
of a man of light build, and with grey in his whiskers, in such an
undertaking. 'I wish,' I added, 'to see as much of the fall as can be seen, and
where you lead I will endeavour to follow.' His scrutiny relaxed into a smile,
and he said, 'Very well; I shall be ready for you to-morrow.'

On the morrow, accordingly, I came. In the hut at the head of Biddle's Stair I
stripped wholly, and re-dressed according to instructions, — drawing on two
pairs of woollen pantaloons, three woollen jackets, two pairs of socks, and a
pair of felt shoes. Even if wet, my guide assured me that the clothes would
keep me from being chilled; and he was right. A suit and hood of yellow
oilcloth covered all. Most laudable precautions were taken by the young
assistant who helped to dress me to keep the water out; but his devices broke
down immediately when severely tested.

We descended the stair; the handle of a pitchfork doing, in my case, the duty
of an alpenstock. At the bottom, the guide enquired whether we should go
first to the Cave of the Winds, or to the Horseshoe, remarking that the latter
would try us most. I decided on getting the roughest done first, and he turned
to the left over the stones. They were sharp and trying. The base of the first
portion of the cataract is covered with huge boulders, obviously the ruins of
the limestone ledge above. The water does not distribute itself uniformly
among these, but seeks out channels through which it pours torrentially. We
passed some of these with wetted feet, but without difficulty. At length we
came to the side of a more formidable current. My guide walked along its
edge until he reached its least turbulent portion. Halting, he said, 'This is our
greatest difficulty; if we can cross here, we shall get far towards the
Horseshoe.'

He waded in. It evidently required all his strength to steady him. The water
rose above his loins, and it foamed still higher. He had to search for footing,
amid unseen boulders, against which the torrent rose violently. He struggled
and swayed, but he struggled successfully, and finally reached the shallower
water at the other side. Stretching out his arm, he said to me, 'Now come on.'
I looked down the torrent, as it' rushed to the river below, which was
seething with the tumult of the cataract. De Saussure recommended the
inspection of Alpine dangers, with the view of making them familiar to the

eye before they are encountered; and it is a wholesome custom in places of
difficulty to put the possibility of an accident clearly before the mind, and to
decide beforehand what ought to be done should the accident occur. Thus
wound up in the present instance, I entered the water. Even where it was not
more than knee-deep, its power was manifest. As it rose around me, I sought
to split the torrent by presenting a side to it; but the insecurity of the footing
enabled it to grasp my loins, twist me fairly round, and bring its impetus to
bear upon my back. Further struggle was impossible; and feeling my balance
hopelessly gone, I turned, flung myself towards the bank just quitted, and
was instantly, as expected, swept into shallower water.

The oilcloth covering was a great incumbrance; it had been made for a much
stouter man, and, standing upright after my submersion, my legs occupied
the centre of two bags of water. My guide exhorted me to try again.
Prudence was at my elbow, whispering dissuasion; but, taking everything
into account, it appeared more immoral to retreat than to proceed. Instructed
by the first misadventure, I once more entered the stream. Had the
alpenstock been of iron it might have helped me; but, as it was, the tendency
of the water to sweep it out of my hands rendered it worse than useless. I,
however, clung to it by habit. Again the torrent rose, and again I wavered;
but, by keeping the left hip well against it, I remained upright, and at length
grasped the hand of my leader at the other side. He laughed pleasantly. The
first victory was gained, and he enjoyed it. 'No traveller,' he said, 'was ever
here before.' Soon afterwards, by trusting to a piece of drift-wood which
seemed firm, I was again taken off my feet, but was immediately caught by a
protruding rock.

We clambered over the boulders towards the thickest spray, which soon
became so weighty as to cause us to stagger under its shock. For the most
part nothing could be seen; we were in the midst of bewildering tumult,
lashed by the water, which sounded at times like the cracking of innumerable
whips. Underneath this was the deep resonant roar of the cataract. I tried to
shield my eyes with my hands, and look upwards; but the defence was
useless. The guide continued to move on, but at a certain place he halted,
desiring me to take shelter in his lee, and observe the cataract. The spray did
not come so much from the upper ledge, as from the rebound of the shattered
water when it struck the bottom. Hence the eyes could be protected from the
blinding shock of the spray, while the line of vision to the upper ledges

remained to some extent clear. On looking upwards over the guide's shoulder
I could see the water bending over the ledge, while the Terrapin Tower
loomed fitfully through the intermittent spray-gusts. We were right under the
tower. A little farther on the cataract, after its first plunge, hit a protuberance
some way down, and flew from it in a prodigious burst of spray; through this
we staggered. We rounded the promontory on which the Terrapin Tower
stands, and moved, amid the wildest commotion, along the arm of the Horse-
hoe, until the boulders failed us, and the cataract fell into the profound gorge
of the Niagara River.

Here the guide sheltered me again, and desired me 'to look up; I did so, and
could see, as before, the green gleam of the mighty curve sweeping over the
"dipper ledge, and the fitful plunge of the water, as the spray between us and
it alternately gathered and disappeared. An eminent friend of mine often
speaks of the mistake of those physicians who regard man's ailments as
purely chemical, to be met by chemical remedies only. He contends for the
psychological element of cure. By agreeable emotions, he says, nervous
currents are liberated which stimulate blood, brain, and viscera. The
influence rained from ladies' eyes enables my friend to thrive on dishes
which would kill him if eaten alone. A sanative effect of the same order I
experienced amid the spray and thunder of Niagara. Quickened by the
emotions there aroused, the blood sped exultingly through the arteries,
abolishing introspection, clearing the heart of all bitterness, and enabling one
to think with tolerance, if not with tenderness, on the most relentless and
unreasonable foe. Apart from its scientific value, and purely as a moral
agent, the play was worth the candle. My companion knew no more of me
than that I enjoyed the wildness of the scene; but as I bent in the shelter of
his large frame he said, 'I should like to see you attempting to describe all
this.' He rightly thought it indescribable. The name of this gallant fellow was
Thomas Conroy.

We returned, clambering at intervals up and down, so as to catch glimpses of
the most impressive portions of the cataract. We passed under ledges formed
by tabular masses of limestone, and through some curious openings formed
by the falling together of the summits of the rocks. At length we found
ourselves beside our enemy of the morning. Conroy halted for a minute or
two, scanning the torrent thoughtfully. I said that, as a guide, he ought to
have a rope in such a place; but he retorted that, as no traveller had ever

thought of coming there, he did not see the necessity of keeping a rope. He
waded in. The struggle to keep himself erect was evident enough; he
swayed, but recovered himself again and again. At length he slipped, gave
way, did as I had done, threw himself towards the bank, and was swept into
the shallows. Standing in the stream near its edge, he stretched his arm
towards me. I retained the pitchfork handle, for it had been useful among the
boulders. By wading some way in, the staff could be made to reach him, and
I proposed his seizing it. 'If you are sure,' he replied, 'that, in case of giving
way, you can maintain your grasp, then I will certainly hold you.' Remarking
that he might count on this, I waded in, and stretched the staff to my
companion. It was firmly grasped by both of us. Thus helped, though its
onset was strong, I moved safely across the torrent. All danger ended here.
We afterwards roamed sociably among the torrents and boulders below the
Cave of the Winds. The rocks were covered with organic slime, which could
not have been walked over with bare feet, but the felt shoes effectually
prevented slipping. We reached the cave and entered it, first by a wooden
way carried over the boulders, and then along a narrow ledge, to the point
eaten deepest into the shale. When the wind is from the south, the falling
water, I am told, can be seen tranquilly from this spot; but when we were
there, a blinding hurricane of spray was whirled against us. On the evening
of the same day, I went behind the water on the Canada side, which, after the
experiences of the morning, struck me as an imposture.

Still even this latter is exciting to some nerves. Its effect upon himself is thus
vividly described by Bakewell, jun.: 'On turning a sharp angle of the rock, a
sudden gust of wind met us, coming from the hollow between the fall and
the rock, which drove the spray directly in our faces, with such force that in
an instant we were wet through. When in the midst of this shower-bath the
shock took away my breath: I turned back and scrambled over the loose
stones to escape the conflict. The guide soon followed, and told me that I
had passed the worst part. With that assurance I made a second attempt; but
so wild and disordered was my imagination that when I had reached half
way I could bear it no longer.' [Footnote: 'Mag. of Nat. Hist.,' 1830, pp. 121,
122.]

To complete my knowledge I desired to see the fall from the river below it,
and long negotiations were necessary to secure the means of doing so. The
only boat fit for the undertaking had been laid up for the winter; but this

difficulty, through the kind intervention of Mr. Townsend, was overcome.
The main one was to secure oarsmen sufficiently strong and skilful to urge
the boat where I wished it to be taken. The son of the owner of the boat, a
finely-built young fellow, but only twenty, and therefore not sufficiently
hardened, was willing to go; and up the river, it was stated, there lived
another man who could do anything with the boat which strength and daring
could accomplish. He came. His figure and expression of face certainly
indicated extraordinary firmness and power. On Tuesday, November 5, we
started, each of us being clad in oilcloth. The elder oarsman at once assumed
a tone of authority over his companion, and struck immediately in amid the
breakers below the American Fall. He hugged the cross freshets instead of
striking out into the smoother water. I asked him why he did so, and he
replied that they were directed outwards, not downwards. The struggle,
however, to prevent the bow of the boat from being turned by them, was
often very severe.

The spray was in general blinding, but at times it disappeared and yielded
noble views of the fall. The edge of the cataract is crimped by indentations
which exalt its beauty. Here and there, a little below the highest ledge, a
secondary one juts out; the water strikes it and bursts from it in huge
protuberant masses of foam and spray. We passed Goat Island, came to the
Horseshoe, and worked for a time along its base, the boulders over which
Conroy and myself had scrambled a few days previously lying between us
and the cataract. A rock was before us, concealed and revealed at intervals,
as the waves passed over it. Our leader tried to get above this rock, first on
the outside of it. The water, however, was here in violent motion. The men
struggled fiercely, the older one ringing out an incessant peal of command
and exhortation to the younger. As we were just clearing the rock, the bow
came obliquely to the surge; the boat was turned suddenly round and shot
with astonishing rapidity down the river. The men returned to the charge,
now trying to get up between the half-concealed rock and the boulders to the
left. But the torrent set in strongly through this channel. The tugging was
quick and violent, but we made little way. At length, seizing a rope, the
principal oarsman made a desperate attempt to get upon one of the boulders,
hoping to be able to drag the boat through the channel; but it bumped so
violently against the rock, that the man flung himself back and relinquished
the attempt.

We returned along the base of the American Fall, running in and out among
the currents which rushed from it laterally into the river. Seen from below
the American Fall is certainly exquisitely beautiful, but it is a mere frill of
adornment to its nobler neighbour the Horseshoe. At times we took to the
river, from the centre of which the Horseshoe Fall appeared especially
magnificent. A streak of cloud across the neck of Mont Blanc can double its
apparent height, so here the green summit of the cataract shining above the
smoke of spray appeared lifted to an extraordinary elevation. Had Hennepin
and La Hontan seen the fall from this position, their estimates of the height
would have been perfectly excusable.

-----

From a point a little way below the American Fall, a ferry crosses the river,
in summer, to the Canadian side. Below the ferry is a suspension bridge for
carriages and foot-passengers, and a mile or two lower down is the railway
suspension bridge. Between ferry and bridge the river Niagara flows
unruffled; but at the suspension bridge the bed steepens and the river
quickens its motion. Lower down the gorge narrows, and the rapidity and
turbulence increase. At the place called the 'Whirlpool Rapids' I estimated
the width of the river at 300 feet, an estimate confirmed by the dwellers on
the spot. When it is remembered that the drainage of nearly half a continent
is compressed into this space, the impetuosity of the river's rush may be
imagined. Had it not been for Mr. Bierstädt, the distinguished photographer
of Niagara, I should have quitted the place without seeing these rapids; for
this, and for his agreeable company to the spot, I have to thank him. From
the edge of the cliff above the rapids, we descended, a little, I confess, to a
climber's disgust, in an 'elevator,' because the effects are best seen from the
water level.

Two kinds of motion are here obviously active, a motion of translation and a
motion of undulation — the race of the river through its gorge, and the great
waves generated by its collision with, and rebound from, the obstacles in its
way. In the middle of the river the rush and tossing are most violent; at all
events, the impetuous force of the individual waves is here most strikingly
displayed. Vast pyramidal heaps leap incessantly from the river, some of
them with such energy as to jerk their summits into the air, where they hang
momentarily suspended in crowds of liquid spherules. The sun shone for a

few minutes. At times the wind, coming up the river, searched and sifted the
spray, carrying away the lighter drops, and leaving the heavier ones behind.
Wafted in the proper direction, rainbows appeared and disappeared fitfully in
the lighter mist. In other directions the common gleam of the sunshine from
the waves and their shattered crests was exquisitely beautiful. The
complexity of the action was still further illustrated by the fact, that in some
cases, as if by the exercise of a local explosive force, the drops were shot
radially from a particular centre, forming around it a kind of halo.

The first impression, and, indeed, the current explanation of these rapids is,
that the central bed of the river is cumbered with large boulders, and that the
jostling, tossing, and wild leaping of the water there, are due to its impact
against these obstacles. I doubt this explanation. At all events, there is
another sufficient reason to be taken into account. Boulders derived from the
adjacent cliffs visibly cumber the sides of the river. Against these the water
rises and sinks rhythmically but violently, large waves being thus produced.
On the generation of each wave, there is an immediate compounding of the
wave-motion with he river-motion. The ridges, which in still water would
proceed in circular curves round the centre of disturbance, cross the river
obliquely, and the result is that at the centre waves commingle, which have
really been generated at the sides. In the first instance, we had a composition
of wave-motion with river-motion; here we have the coalescence of waves
with waves. Where crest and furrow cross each other, the motion is annulled;
where furrow and furrow cross, the river is ploughed to a greater depth; and
where crest and crest aid each other, we have that astonishing leap of the
water which breaks the cohesion of the crests, and tosses them shattered into
the air. From the water level the cause of the action is not so easily seen; but
from the summit of the cliff the lateral generation of the waves, and their
propagation to the perfectly obvious. If this explanation be correct, the
phenomena observed at the Whirlpool Rapids form one of the grandest
illustrations of the principle of interference. The Nile 'cataract,' Mr. Huxley
informs me, offers more moderate examples of the same action.

At some distance below the Whirlpool Rapids we have the celebrated
whirlpool itself. Here the river makes a sudden bend to the north-east,
forming nearly a right angle with its previous direction. The water strikes the
concave bank with great force, and scoops it incessantly away. A vast basin
has been thus formed, in which the sweep of the river prolongs itself in

gyratory currents. Bodies and trees which have come over the falls, are
stated to circulate here for days without finding the outlet. From various
points of the cliffs above, this is curiously hidden. The rush of the river into
the whirlpool is obvious enough; and though you imagine the outlet must be
visible, if one existed, you cannot find it. Turning, however, round the bend
of the precipice to the north-east, the outlet comes into view.

The Niagara season was over; the chatter of sightseers had ceased, and the
scene presented itself as one of holy seclusion and beauty. I went down to
the river's edge, where the weird loneliness seemed to increase. The basin is
enclosed by high and almost precipitous banks — covered, at the time, with
russet woods. A kind of mystery attaches itself to gyrating water, due
perhaps to the fact that we are to some extent ignorant of the direction of its
force. It is said that at certain points of the whirlpool, pine-trees are sucked
down, to be ejected mysteriously elsewhere. The 'water is of the brightest
emerald-green. The gorge through which it escapes is narrow, and the
motion of the river swift though silent. The surface is steeply inclined, but it
is perfectly unbroken. There are no lateral waves, no ripples with their
breaking bubbles to raise a murmur; while the depth is here too great to
allow the inequality of the bed to ruffle the surface. Nothing can be more
beautiful than this sloping liquid mirror formed by the Niagara, in sliding
from the whirlpool.

The green colour is, I think, correctly accounted for in the last Fragment.
While crossing the Atlantic in 1872-73 I had frequent opportunities of
testing the explanation there given. Looked properly down upon, there are
portions of the ocean to which we should hardly ascribe a trace of blue; at
the most, a mere hint of indigo reaches the eye. The water, indeed, is
practically black, and this is an indication both of its depth and of its
freedom from mechanically suspended matter. In small thicknesses water is
sensibly transparent to all kinds of light; but, as the thickness increases, the
rays of low refrangibility are first absorbed, and after them the other rays.
Where, therefore, the water is very deep and very pure, all the colours are
absorbed, and such water ought to appear black, as no light is sent from its
interior to the eye. The approximation of the Atlantic Ocean to this condition
is an indication of its extreme purity.

Throw a white pebble into such water; as it sinks it becomes greener and
greener, and, before it disappears, it reaches a vivid blue-green. Break such a
pebble into fragments, each of these will behave like the unbroken mass;
grind the pebble to powder, every particle will yield its modicum of green;
and if the particles be so fine as to remain suspended in the water, the
scattered light will be a uniform green. Hence the greenness of shoal water.
You go to bed with the black Atlantic around you. You rise in the morning,
find it a vivid green, and correctly infer that you are crossing the bank of
Newfoundland. Such water is found charged with fine matter in a state of
mechanical suspension. The light from the bottom may sometimes come into
play, but it is not necessary. A storm can render the water muddy, by
rendering the particles too numerous and gross. Such a case occurred
towards the close of my visit to Niagara. There had been rain and storm in
the upper lake-regions, and the quantity of suspended matter brought down
quite extinguished the fascinating green of the Horseshoe.

Nothing can be more superb than the green of the Atlantic waves, when the
circumstances are favourable to the exhibition of the colour. As long as a
wave remains unbroken no colour appears; but when the foam just doubles
over the crest, like an Alpine snow-cornice, under the cornice we often see a
display of the most exquisite green. It is metallic in its brilliancy. But the
foam is necessary to its production. The foam is first illuminated, and it
scatters the light in all directions; the light which passes through the higher
portion of the wave alone reaches the eye, and gives to that portion its
matchless colour. The folding of the wave, producing as it does a series of
longitudinal protuberances and furrows which act like cylindrical lenses,
introduces variations in the intensity of the light, and materially enhances its
beauty.

-----

We have now to consider the genesis and proximate destiny of the Falls of
Niagara. We may open our way to this subject by a few preliminary remarks
upon erosion. Time and intensity are the main factors of geologic change,
and they are in a certain sense convertible. A feeble force acting through
long periods, and an intense force acting through short ones, may produce
approximately the same results. To Dr. Hooker I have been indebted for
some specimens of stones, the first examples of which were picked up by

Mr. Hackworth on the shores of Lyell's Bay, near Wellington, in New
Zealand. They were described by Mr. Travers in the 'Transactions of the
New Zealand Institute.' Unacquainted with their origin, you would certainly
ascribe their forms to human workmanship. They resemble knives and spear-
heads, being apparently chiselled off into facets, with as much attention to
symmetry as if a tool, guided by human intelligence, had passed over them.
But no human instrument has been brought to bear upon these stones. They
have been wrought into their present shape by the wind-blown sand of
Lyell's Bay. Two winds are, dominant here, and they in succession urged the
sand against opposite sides of the stone; every little particle of sand chipped
away its infinitesimal bit of stone, and in the end sculptured these singular
forms. [Footnote: 'These stones, which have a strong resemblance to works
of human art, occur in great abundance, and of various sizes, from half-an-
inch to several inches in length. A large number were exhibited showing the
various forms, which are those of wedges, knives, arrow-heads, &c., and all
with sharp cutting edges.

'Mr. Travers explained that, notwithstanding their artificial appearance,
these stones were formed by the cutting action of the wind-driven sand, as it
passed to and fro over an exposed boulder-bank. He gave a minute account
of the manner in which the varieties of form are produced, and referred to
the effect which the erosive action thus indicated would have on railway and
other works executed on sandy tracts.

'Dr. Hector stated that although, as a group, the specimens on the table
could not well be mistaken for artificial productions, still the forms are so
peculiar, and the edges, in a few of them, so perfect, that if they were
discovered associated with human works, there is no doubt that they would
have been referred to the so-called "stone period."' — Extracted from the
Minutes of the Wellington Philosophical Society, February 9, 1869.]

The Sphynx of Egypt is nearly covered up by the sand of the desert. The
neck of the Sphynx is partly cut across, not, as I am assured by Mr. Huxley,
by ordinary weathering, but by the eroding action of the fine sand blown
against it. In these cases Nature furnishes us with hints which may be taken
advantage of in art; and this action of sand has been recently turned to
extraordinary account in the United States. When in Boston, I was taken by
my courteous and helpful friend, Mr. Josiah Quincey, to see the action of the

sand-blast. A kind of hopper containing fine silicious sand was connected
with a reservoir of compressed air, the pressure being variable at pleasure.
The hopper ended in a long slit, from which the sand was blown. A plate of
glass was placed beneath this slit, and caused to pass slowly under it; it came
out perfectly depolished, with a bright opalescent glimmer, such as could
only be produced by the most careful grinding. Every little particle of sand
urged against the glass, having all its energy concentrated on the point of
impact, formed there a little pit, the depolished surface consisting of
innumerable hollows of this description.

But this was not all. By protecting certain portions of the surface, and
exposing others, figures and tracery of any required form could be etched
upon the glass. The figures of open iron-work could be thus copied; while
wire-gauze placed over the glass produced a reticulated pattern. But it
required no such resisting substance as iron to shelter the glass. The patterns
of the finest lace could be thus reproduced; the delicate filaments of the lace
itself offering a sufficient protection. All these effects have been obtained
with a simple model of the sand-blast devised by my assistant. A fraction of
a minute suffices to etch upon glass a rich and beautiful lace pattern. Any
yielding substance may be employed to protect the glass. By diffusing the
shock of the particle, such substances practically destroy the local erosive
power. The hand can bear, without inconvenience, a sand-shower which
would pulverise glass. Etchings executed on glass with suitable kinds of ink
are accurately worked out by the sandblast. In fact, within certain limits, the
harder the surface, the greater is the concentration of the shock, and the more
effectual is the erosion. It is not necessary that the sand should be the harder
substance of the two; corundum, for example, is much harder than quartz;
still, quartz-sand can not only depolish, but actually blow a hole through a
plate of corundum. Nay, glass may be depolished by the impact of fine shot;
the grains in this case bruising the glass, before they have time to flatten and
turn their energy into heat.

And here, in passing, we may tie together one or two apparently unrelated
facts. Supposing you turn on, at the lower part of a house, a cock which is
fed by a pipe from a cistern at the top of the house, the column of water,
from the cistern downwards, is set in motion. By turning off the cock, this
motion is stopped; and, when the turning off is very sudden, the pipe, if not
strong, may be burst by the internal impact of the water. By distributing the

turning of the cock over half a second of time, the shock and danger of
rupture may be entirely avoided. We have here an example of the
concentration of energy in time. The sand-blast illustrates the concentration
of energy in space. The action of flint and steel is an illustration of the same
principle. The heat required to generate the spark is intense; and the
mechanical action, being moderate, must, to produce fire, be in the highest
degree concentrated. This concentration is secured by the collision of hard
substances. Calc-spar will not supply the place of flint, nor lead the place of
steel, in the production of fire by collision. With the softer substances, the
total heat produced may be greater than with the hard ones, but, to produce
the spark, the heat must be intensely localised.

We can, however, go far beyond the mere depolishing of glass; indeed I have
already said that quartz-sand can wear a hole through corundum. This leads
me to express my acknowledgments to General Tilghman, [Footnote: The
absorbent power, if I may use the phrase, exerted by the industrial arts in the
United States, is forcibly illustrated by the rapid transfer of men like Mr.
Tilghman from the life of the soldier to that of the civilian. General
McClellan, now a civil engineer, whom I had the honour of frequently
meeting in New York, is a most eminent example of the same kind. At the end
of the war, indeed, a million and a half of men were thus drawn, in an
astonishingly short time, from military to civil life.] who is the inventor of
the sand-Blast. To his spontaneous kindness I am indebted for some
beautiful illustrations of his process. In one thick plate of glass a figure has
been worked out to a depth of three eighths of an inch. A second plate, seven
eighths of an inch thick, is entirely perforated. In a circular plate of marble,
nearly half an inch thick, open work of most intricate and elaborate
description has been executed. It would probably take many days to perform
this work by any ordinary process; with the sand-blast it was accomplished
in an hour. So much for the strength of the blast; its delicacy is illustrated by
this beautiful example of line engraving, etched on glass by means of the
Blast.

This power of erosion, so strikingly displayed when sand is urged by air,
renders us better able to conceive its action when urged by water. The
erosive power of a river is vastly augmented by the solid matter carried
along with it. Sand or pebbles, caught in a river vortex, can wear away the
hardest rock potholes' and deep cylindrical shafts being thus produced. An

extraordinary instance of this kind of erosion is to be seen in the Val
Tournanche, above the village of this name. The gorge at Handeck has been
thus cut out. Such waterfalls were once frequent in the valleys of
Switzerland; for hardly any valley is without one or more transverse barriers
of resisting material, over which the river flowing through the valley once
fell as a cataract. Near Pontresina, in the Engadin, there is such a case; a hard
gneiss being there worn away to form a gorge, through which the river from
the Morteratsch glacier rushes. The barrier of the Kirchet above Meyringen
is also a case in point. Behind it was a lake, derived from the glacier of the
Aar, and over the barrier the lake poured its excess of water. Here the rock,
being limestone, was in part dissolved; but added to this we had the action of
the sand and gravel carried along by the water, which, on striking the rock,
chipped it away like the particles of the sand-Blast. Thus, by solution and
mechanical erosion, the great chasm of the Finsteraarschlucht was formed. It
is demonstrable that the water which flows at the bottoms of such deep
fissures once flowed at the level of their present edges, and tumbled down
the lower faces of the barriers. Almost every valley in Switzerland furnishes
examples of this kind; the untenable hypothesis of earthquakes, once so
readily resorted to in accounting for these gorges, being now for the most
part abandoned. To produce the Canons of Western America, no other cause
is needed than the integration of effects individually infinitesimal.

And now we come to Niagara. Soon after Europeans had taken possession of
the country, the conviction appears to have arisen that the deep channel of
the river Niagara below the falls had been excavated by the cataract. In Mr.
Bakewell's 'Introduction to Geology,' the prevalence of this belief has been
referred to. It is expressed thus by Professor Joseph Henry in the
'Transactions of the Albany Institute:' [Footnote: Quoted by Bakewell.] 'In
viewing the position of the falls, and the features of the country round, it is
impossible not to be impressed with the idea that this great natural raceway
has been formed by the continued action of the irresistible Niagara, and that
the falls, beginning at Lewiston, have, in the course of ages, worn back the
rocky strata to their present site.' The same view is advocated by Sir Charles
Lyell, by Mr. Hall, by M. Agassiz, by Professor Ramsay, indeed by most of
those who have inspected the place.

A connected image of the origin and progress of the cataract is easily
obtained. Walking northward from the village of Niagara Falls by the side of

the river, we have to our left the deep and comparatively narrow gorge,
through which the Niagara flows. The bounding cliffs of this gorge are from
300 to 350 feet high. We reach the whirlpool, trend to the north-east, and
after a little time gradually resume our northward course. Finally, at about
seven miles from the present falls, we come to the edge of a declivity, which
informs us that we have been hitherto walking on table-land. At some
hundreds of feet below us is a comparatively level plain, which stretches to
Lake Ontario. The declivity marks the end of the precipitous gorge of the
Niagara. Here the river escapes from its steep mural boundaries, and in a
widened bed pursues its way to the lake which finally receives its waters.

The fact that in historic times, even within the memory of man, the fall has
sensibly receded, prompts the question, How far has this recession gone? At
what point did the ledge which thus continually creeps backwards begin its
retrograde course? To minds disciplined in such researches the answer has
been, and will be — At the precipitous declivity which crossed the Niagara
from Lewiston on the American to Queenston on the Canadian side. Over
this transverse barrier the united affluents of all the upper lakes once poured
their waters, and here the work of erosion began. The dam, moreover, was
demonstrably of sufficient height to cause the river above it to submerge
Goat Island; and this would perfectly account for the finding by Sir Charles
Lyell, Mr. Hall, and others, in the sand and gravel of the island, the same
fluviatile shells as are now found in the Niagara River higher up. It would
also account for those deposits along the sides of the river, the discovery of
which enabled Lyell, Hall, and Ramsay to reduce to demonstration the
popular belief that the Niagara once flowed through a shallow valley.

The physics of the problem of excavation, which I made clear to my mind
before quitting Niagara, are revealed by a close inspection of the present
Horseshoe Fall. We see evidently that the greatest weight of water bends
over the very apex of the Horseshoe. In a passage in his excellent chapter on
Niagara Falls, Mr. Hall alludes to this fact. Here we have the most copious
and the most violent whirling of the shattered liquid; here the most powerful
eddies recoil against the shale. From this portion of the fall, indeed, the spray
sometimes rises without solution of continuity to the region of clouds,
becoming gradually more attenuated, and passing finally through the
condition of true cloud into invisible vapour, which is sometimes
reprecipitated higher up. All the phenomena point distinctly to the centre of

the river as the place of greatest mechanical energy, and from the centre the
vigour of the fall gradually dies away towards the sides. The Horseshoe
form, with the concavity facing downwards, is an obvious and necessary
consequence of this action. Right along the middle of the river the apex of
the curve pushes its way backwards, cutting along the centre a deep and
comparatively narrow groove, and draining the sides as it passes them.
[Footnote: In the discourse the excavation of the centre and drainage of the
sides action was illustrated by a model devised by my assistant, Mr. John
Cottrell.] Hence the remarkable discrepancy between the widths of the
Niagara above and below the Horseshoe. All along its course, from Lewiston
Heights to its present position, the form of the fall was probably that of a
horseshoe; for this is merely the expression of the greater depth, and
consequently greater excavating power, of the centre of the river. The gorge,
moreover, varies in width, as the depth of the centre of the ancient river
varied, being narrowest where that depth was greatest.

The vast comparative erosive energy of the Horseshoe Fall comes strikingly
into view when it and the American Fall are compared together. The
American branch of the river is cut at a right angle by the gorge of the
Niagara. Here the Horseshoe Fall was the real excavator. It cut the rock, and
formed the precipice, over which the American Fall tumbles. But since its
formation, the erosive action of the American Fall has been almost nil, while
the Horseshoe has cut its way for 600 yards across the end of Goat Island,
and is now doubling back to excavate its channel parallel to the length of the
island. This point, which impressed me forcibly, has not, I have just learned,
escaped the acute observation of Professor Ramsay. [Footnote: His words
are: 'Where the body of water is small in the American Fall, the edge has
only receded a few yards (where most eroded) during the time that the
Canadian Fall has receded from the north corner of Goat Island to the
innermost curve of the Horseshoe Fall.' — Quarterly Journal of Geological
Society, May 1859.] The river bends; the Horseshoe immediately
accommodates itself to the bending, and will follow implicitly the direction
of the deepest water in the upper stream. The flexures of the gorge are
determined by those of the river channel above it. Were the Niagara centre
above the fall sinuous, the gorge would obediently follow its sinuosities.
Once suggested, no doubt geographers will be able to point out many
examples of this action. The Zambesi is thought to present a great difficulty
to the erosion theory, because of the sinuosity of the chasm below the

Victoria Falls. But, assuming the basalt to be of tolerably uniform texture,
had the river been examined before the formation of this sinuous channel,
the present zigzag course of the gorge below the fall could, I am persuaded,
have been predicted, while the sounding of the present river would enable us
to predict the course to be pursued by the erosion in the future.

But not only has the Niagara River cut the gorge; it has carried away the
chips of its own workshop. The shale, being probably crumbled, is easily
carried away. But at the base of the fall we find the huge boulders already
described, and by some means or other these are removed down the river.
The ice which fills the gorge in winter, and which grapples with the
boulders, has been regarded as the transporting agent. Probably it is so to
some extent. But erosion acts without ceasing on the abutting points of the
boulders, thus withdrawing their support and urging them gradually down
the river. Solution also does its portion of the work. That solid matter is
carried down is proved by the difference of depth between the Niagara River
and Lake Ontario, where the river enters it. The depth falls from 72 feet to
20 feet, in consequence of the deposition of solid matter caused by the
diminished motion of the river. [Footnote: Near the mouth of the gorge at
Queenston, the depth, according to the Admiralty Chart, is 180 feet; well
within the gorge it is 132 feet.]

.

The annexed highly instructive map has been reduced from one published in
Mr. Hall's 'Geology of New York.' It is based on surveys executed in 1842,
by Messrs. Gibson and Evershed. The ragged edge of the American Fall
north of Goat Island marks the amount of erosion which it has been able to
accomplish, while the Horseshoe Fall was cutting its way southward across
the end of Goat Island to its present position. The American Fall is 168 feet
high, a precipice cut down, not by itself, but by the Horseshoe Fall. The
latter in 1842 was 159 feet high, and, as shown by the map, is already
turning eastward, to excavate its gorge along the centre of the upper river. P
is the apex of the Horseshoe, and T marks the site of the Terrapin Tower,
with the promontory adjacent, round which I was conducted by Conroy.
Probably since 1842 the Horseshoe has worked back beyond the position
here assigned to it.

In conclusion, we may say a word regarding the proximate future of Niagara.
At the rate of excavation assigned to it by Sir Charles Lyell, namely, a foot a
year, five thousand years or so will carry the Horseshoe Fall far higher than
Goat Island. As the gorge recedes it will drain, as it has hitherto done, the
banks right and left of it, thus leaving a nearly level terrace between Goat
Island and the edge of the gorge. Higher up it will totally drain the American
branch of the river; the channel of which in due time will become cultivable
land. The American Fall will then be transformed into a dry precipice,
forming a simple continuation of the cliffy boundary of the Niagara gorge.
At the place occupied by the fall at this moment we shall have the gorge
enclosing a right angle, a second whirlpool being the consequence. To those
who visit Niagara a few millenniums hence I leave the verification of this
prediction. All that can be said is, that if the causes now in action continue to
act, it will prove itself literally true.

-----

Fig. 6.

POSTSCRIPT.

A year or so after I had quitted the United States, a man sixty years of age,
while engaged in painting one of the bridges which connect Goat Island with
the Three Sisters, slipped through the rails of the bridge into the rapids, and
was carried impetuously towards the Horseshoe Fall. He was urged against a
rock which rose above the water, and with the grasp of desperation he clung
to it. The population of the village of Niagara Falls was soon upon the
island, and ropes were brought, but there was none to use them. In the midst
of the excitement, a tall powerful young fellow was observed making his
way silently through the crowd. He reached a rope; selected from the
bystanders a number of men, and placed one end of the rope in their hands.

The other end he fastened round himself, and choosing a point considerably
above that to which the man clung, he plunged into the rapids. He was
carried violently downwards, but he caught the rock, secured the old painter
and saved him. Newspapers from all parts of the Union poured in upon me,
describing this gallant act of my guide Conroy.

.

.

.

.

--------------------

.

.

VIII. THE PARALLEL ROADS OF GLEN ROY.

[Footnote: A discourse delivered at the Royal Institution of Great Britain on
June 9, 1876.]

THE first published allusion to the Parallel Roads of Glen Roy occurs in the
appendix to the third volume of Pennant's 'Tour in Scotland,' a work
published in 1776. 'In the face of these hills,' says this writer, 'both sides of
the glen, there are three roads at small distances from each other and directly
opposite on each side. These roads have been measured in the complete parts
of them, and found to be 26 paces of a man 5 feet 10 inches high. The two
highest are pretty near each other, about 50 yards, and the lowest double that
distance from the nearest to it. They are carried along the sides of the glen
with the utmost regularity, nearly as exact as drawn with a line of rule and
compass.'

The correct heights of the three roads of Glen Roy are respectively 1150,
1070, and 860 feet above the sea. Hence a vertical distance of 80 feet

separates the two highest, while the lowest road is 210 feet below the middle
one.

These 'roads' are usually shelves or terraces formed in the yielding drift
which here covers the slopes of the mountains. They are all sensibly
horizontal and therefore parallel. Pennant accepted as reasonable the
explanation of them given by the country people in his time. They thought
that the roads 'were designed for the chase, and that the terraces were made
after the spots were cleared in lines from wood, in order to tempt the animals
into the open paths after they were rouzed, in order that they might come
within reach of the bowmen who might conceal themselves in the woods
above and below.'

In these attempts of 'the country people' we have an illustration of that
impulse to which all scientific knowledge is due — the desire to know the
causes of things; and it is a matter of surprise that in the case of the parallel
roads, with their weird appearance challenging enquiry, this impulse did not
make itself more rapidly and energetically felt. Their remoteness may
perhaps account for the fact that until the year 1817 no systematic
description of them, and no scientific attempt at an explanation of them,
appeared. In that year Dr. MacCulloch, who was then President of the
Geological Society, presented to that Society a memoir, in which the roads
were discussed, and pronounced to be the margins of lakes once embosomed
in Glen Roy. Why there should be three roads, or why the lakes should stand
at these particular levels, was left unexplained.

To Dr. MacCulloch succeeded a man, possibly not so learned as a geologist,
but obviously fitted by nature to grapple with her facts and to put them in
their proper setting. I refer to Sir Thomas Dick-Lauder, who presented to the
Royal Society of Edinburgh, on the 2nd of March, 1818, his paper on the
Parallel Roads of Glen. Roy. In looking over the literature of this subject,
which is now copious, it is interesting to observe the differentiation of
minds, and to single out those who went by a kind of instinct to the core of
the question, from those who erred in it, or who learnedly occupied
themselves with its analogies, adjuncts, and details. There is no man, in my
opinion, connected with the history of the subject, who has shown, in
relation to it, this spirit of penetration, this force of scientific insight, more
conspicuously than Sir Thomas Dick-Lauder. Two distinct mental processes

are involved in the treatment of such a question. Firstly, the faithful and
sufficient observation of the data; and secondly, that higher mental process
in which the constructive imagination comes into play, connecting the
separate facts of observation with their common cause, and weaving them
into an organic whole. In neither of these requirements did Sir Thomas Dick-
Lauder fail.

Adjacent to Glen Roy is a valley called Glen Gluoy, along the sides of which
ran a single shelf, or terrace, formed obviously in the same manner as the
parallel roads of Glen Roy. The two shelves on the opposing sides of the
glen were at precisely the same level, and Dick-Lauder wished to see
whether, and how, they became united at the head of the glen. He followed
the shelves into the recesses of the mountains. The bottom of the valley, as it
rose, came ever nearer to them, until finally, at the head of Glen Gluoy, he
reached a col, or watershed, of precisely the same elevation as the road
which swept round the glen.

The correct height of this col is 1170 feet above the sea; that is to say, 20 feet
above the highest road in Glen Roy.

From this col a lateral branch-valley — Glen Turrit — led down to Glen
Roy. Our explorer descended from the col to the highest road of the latter
glen, and pursued it exactly as he had pursued the road in Glen Gluoy. For a
time it belted the mountain sides at a considerable height above the bottom
of the valley; but this rose as he proceeded, coming ever nearer to the
highest shelf, until finally he reached a col, or watershed, looking into Glen
Spey, and of precisely the same elevation as the highest road of Glen Roy.

He then dropped down to the lowest of these roads, and followed it towards
the mouth of the glen. Its elevation above the bottom of the valley gradually
increased; not because the shelf rose, but because it remained level while the
valley sloped downwards. He found this lowest road doubling round the hills
at the mouth of Glen Roy, and running along the sides of the mountains
which flank Glen Spean. He followed it eastwards.

PARALLEL ROADS OF GLEN ROY.
After a Sketch by Sir Thomas Dick-Lauder.

The bottom of the Spean Valley, like the others, gradually rose, and therefore
gradually approached the road on the adjacent mountain-side. He came to
Loch Laggan, the surface of which rose almost to the level of the road, and
beyond the head of this lake he found, as in the other two cases, a col, or
watershed, at Makul, of exactly the same level as the single road in Glen
Spean, which, it will be remembered, is a continuation of the lowest road in
Glen Roy.

Here we have a series of facts of obvious significance as regards the solution
of this problem. The effort of the mind to form a coherent image from such
facts may be compared with the effort of the eyes to cause the pictures of a
stereoscope to coalesce. For a time we exercise a certain strain, the object
remaining vague and indistinct. Suddenly its various parts seem to run
together, the object starting forth in clear and definite relief. Such, I take it,

was the effect of his ponderings upon the mind of Sir Thomas Dick-Lauder.
His solution was this: Taking all their features into account, he was
convinced that water only could have produced the terraces. But how had the
water been collected? He saw clearly that, supposing the mouth of Glen
Gluoy to be stopped by a barrier sufficiently high, if the waters from the
mountains flanking the glen were allowed to collect, they would form behind
the barrier a lake, the surface of which would gradually rise until it reached
the level of the col at the head of the glen. The rising would then cease; the
superfluous water of Glen Gluoy discharging itself over the col into Glen
Roy. As long as the barrier stopping the mouth of Glen Gluoy continued high
enough, we should have in that glen a lake at the precise level of its shelf,
which lake, acting upon the loose drift of the flanking mountains, would
form the shelf revealed by observation.

So much for Glen Gluoy. But suppose the mouth of Glen Roy also stopped
by a similar barrier. Behind it also the water from the adjacent mountains
would collect. The surface of the lake thus formed would gradually rise,
until it had reached the level of the col which divides Glen Roy from Glen
Spey. Here the rising of the lake would cease; its superabundant water being
poured over the col into the valley of the Spey. This state of things would
continue as long as a sufficiently high barrier remained at the mouth of Glen
Roy. The lake thus dammed in, with its surface at the level of the highest
parallel road, would act, as in Glen Gluoy, upon the friable drift
overspreading the mountains, and would form the highest road or terrace of
Glen Roy.

And now let us suppose the barrier to be so far removed from the mouth of
Glen Roy as to establish a connection between it and the upper part of Glen
Spean, while the lower part of the latter glen still continued to be blocked up.
Upper Glen Spean and Glen Roy would then be occupied by a continuous
lake, the level of which would obviously be determined by the col at the
head of Loch Laggan. The water in Glen Roy would sink from the level it
had previously maintained, to the level of its new place of escape. This new
lake-surface would correspond exactly with the lowest parallel road, and it
would form that road by its action upon the drift of the adjacent mountains.

In presence of the observed facts, this solution commends itself strongly to
the scientific mind. The question next occurs, What was the character of the

assumed barrier which stopped the glens? There are at the present moment
vast masses of detritus in certain portions of Glen Spean, and of such detritus
Sir Thomas Dick-Lauder imagined his barriers to have been formed. By
some unknown convulsion, this detritus had been heaped up. But, once
given, and once granted that it was subsequently removed in the manner
indicated, the single road of Glen Gluoy and the highest and lowest roads of
Glen Roy would be explained in a satisfactory manner.

To account for the second or middle road of Glen Roy, Sir Thomas Dick-
Lauder invoked a new agency. He supposed that at a certain point in the
breaking down or waste of his dam, a halt occurred, the barrier holding its
ground at a particular level sufficiently long to dam a lake rising to the
height of, and forming the second road. This point of weakness was at once
detected by Mr. Darwin, and adduced by him as proving that the levels of the
cols did not constitute an essential feature in the phenomena of the parallel
roads. Though not destroyed, Sir Thomas Dick-Lauder's theory was
seriously shaken by this argument, and it became a point of capital
importance, if the facts permitted, to remove such source of weakness. This
was done in 1847 by Mr. David Milne, now Mr. Milne-Home. On walking
up Glen Roy from Roy Bridge, we pass the mouth of a lateral glen, called
Glen Glaster, running eastward from Glen Roy. There is nothing in this
lateral glen to attract attention, or to suggest that it could have any
conspicuous influence in the production of the parallel roads. Hence,
probably, the failure of Sir Thomas Dick-Lauder to notice it. But Mr. Milne-
Home entered this glen, on the northern side of which the middle and lowest
roads are fairly shown. The principal stream running through the glen turns
at a certain point northwards and loses itself among hills too high to offer
any outlet. But another branch of the glen turns to the south-east; and,
following up this branch, Mr. Milne-Home reached a col, or watershed, of
the precise level of the second Glen Roy road. When the barrier blocking the
glens had been so far removed as to open this col, the water in Glen Roy
would sink to the level of the second road. A new lake of diminished depth
would be thus formed, the surplus water of which would escape over the
Glen Glaster col into Glen Spean. The margin of this new lake, acting upon
the detrital matter, would form the second road. The theory of Sir Thomas
Dick-Lauder, as regards the part played by the cols, was re-riveted by this
new and unexpected discovery.

I have referred to Mr. Darwin, whose powerful mind swayed for a time the
convictions of the scientific world in relation to this question. His notion was
— and it is a notion which very naturally presents itself — that the parallel
roads were formed by the sea; that this whole region was once submerged
and subsequently upheaved; that there were pauses in the process of
upheaval, during which these glens constituted so many fiords, on the sides
of which the parallel terraces were formed. This theory will not bear close
criticism; nor is it now maintained by Mr. Darwin himself. It would not
account for the sea being 20 feet higher in Glen Gluoy than in Glen Roy. It
would not account for the absence of the second and third Glen Roy roads
from Glen Gluoy, where the mountain flanks are quite as impressionable as
in Glen Roy. It would not account for the absence of the shelves from the
other mountains in the neighbourhood, all of which 'would have been
clasped by the sea had the sea been there. Here then, and no doubt
elsewhere, Mr. Darwin has shown himself to be fallible; but here, as
elsewhere, he has shown himself equal to that discipline of surrender to
evidence which girds his intellect with such unassailable moral strength.

But, granting the significance of Sir Thomas Dick-Lauder's facts, and the
reasonableness, on the whole, of the views which he has founded on them,
they will not bear examination in detail. No such barriers of detritus as he
assumed could have existed without leaving traces behind them; but there is
no trace left. There is detritus enough in Glen Spean, but not where it is
wanted. The two highest parallel roads stop abruptly at different points near
the mouth of Glen Roy, but no remnant of the barrier against which they
abutted is to be seen. It might be urged that the subsequent invasion of the
valley by glaciers has swept the detritus away; but there have been no
glaciers in these valleys since the disappearance of the lakes. Professor
Geikie has favoured me with a drawing of the Glen Spean 'road' near the
entrance to Glen Trieg. The road forms a shelf round a great mound of
detritus which, had a glacier followed the formation of the shelf, must have
been cleared away. Taking all the circumstances into account, you may, I
think, with safety dismiss the detrital barrier as incompetent to account for
the present condition of Glen Gluoy and Glen Roy.

Hypotheses in science, though apparently transcending experience, are in
reality experience modified by scientific thought and pushed into an ultra
experiential region. At the time that he wrote, Sir Thomas Dick-Lauder