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Title: Familiar Letters on Chemistry, and Its Relation to Commerce,
Physiology, and Agriculture

Author: Freiherr von Justus Liebig

Editor: John Gardner

Release date: October 1, 2003 [eBook #4524]
Most recently updated: December 28, 2020

Language: English

Other information and formats: www.gutenberg.org/ebooks/4524

Credits: Produced by Steve Solomon. HTML version by Al Haines.

*** START OF THE PROJECT GUTENBERG EBOOK FAMILIAR
LETTERS ON CHEMISTRY, AND ITS RELATION TO COMMERCE,
PHYSIOLOGY, AND AGRICULTURE ***

FAMILIAR LETTERS ON
CHEMISTRY,
AND ITS RELATION TO COMMERCE, PHYSIOLOGY,
AND AGRICULTURE,

BY JUSTUS LIEBIG, M.D., PH. D., F.R.S.,
PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF
GIESSEN.

EDITED BY
JOHN GARDNER, M.D.,

MEMBER OF THE CHEMICAL SOCIETY.

Second Edition, Corrected.
LONDON:
MDCCCXLIV.

PREFACE

The Letters contained in this little Volume embrace some of the most
important points of the science of Chemistry, in their application to Natural
Philosophy, Physiology, Agriculture, and Commerce. Some of them treat of
subjects which have already been, or will hereafter be, more fully discussed
in my larger works. They were intended to be mere sketches, and were
written for the especial purpose of exciting the attention of governments,
and an enlightened public, to the necessity of establishing Schools of
Chemistry, and of promoting, by every means, the study of a science so

intimately connected with the arts, pursuits, and social well-being of
modern civilised nations.

For my own part I do not scruple to avow the conviction, that ere long, a
knowledge of the principal truths of Chemistry will be expected in every
educated man, and that it will be as necessary to the Statesman, the Political
Economist, and the Practical Agriculturist, as it is already indispensable to
the Physician, and the Manufacturer.

In Germany, such of these Letters as have been already published, have
not failed to produce some of the results anticipated. New professorships
have been established in the Universities of Goettingen and Wuertzburg, for
the express purpose of facilitating the application of chemical truths to the
practical arts of life, and of following up the new line of investigation and
research—the bearing of Chemistry upon Physiology, Medicine, and
Agriculture,—which may be said to be only just begun.

My friend, Dr. Ernest Dieffenbach, one of my first pupils, who is well
acquainted with all the branches of Chemistry, Physics, Natural History, and
Medicine, suggested to me that a collection of these Letters would be
acceptable to the English public, which has so favourably received my
former works.

I readily acquiesced in the publication of an English edition, and
undertook to write a few additional Letters, which should embrace some
conclusions I have arrived at, in my recent investigations, in connection
with the application of chemical science to the physiology of plants and
agriculture.

My esteemed friend, Dr. Gardner, has had the kindness to revise the
manuscript and the proof sheets for publication, for which I cannot refrain
expressing my best thanks.

It only remains for me to add a hope, that this little offering may serve to
make new friends to our beautiful and useful science, and be a
remembrancer to those old friends who have, for many years past, taken a
lively interest in all my labours.

JUSTUS LIEBIG
Giessen, Aug. 1843.

CONTENTS
LETTER I
The Subject proposed. Materials employed for Chemical Apparatus:— GLASS
—CAOUTCHOUC—CORK—PLATINUM. THE BALANCE. The "Elements"
of the Ancients, represent the forms of matter. Lavoisier and his successors.
Study of the materials composing the Earth. Synthetic production of Minerals—
LAPIS LAZULI. Organic Chemistry.

LETTER II
Changes of Form which every kind of Matter undergoes. Conversion of Gases
into Liquids and Solids. Carbonic Acid—its curious properties in a solid state.
Condensation of Gases by porous bodies. By Spongy Platinum. Importance of
this property in Nature.

LETTER III
The Manufacture of Soda from Culinary Salt; its importance in the Arts and in
Commerce. Glass—Soap—Sulphuric Acid. Silver Refining. Bleaching. TRADE
IN SULPHUR.

LETTER IV
Connection of Theory with Practice. Employment of MAGNETISM as a moving
power—its impracticability. Relation of Coals and Zinc as economic sources of
Force. Manufacture of Beet-root Sugar—its impolicy. Gas for illumination.

LETTER V

ISOMERISM, or identity of composition in bodies with different chemical and
physical properties. CRYSTALLISATION. AMORPHISM. ISOMORPHISM, or
similarity of properties in bodies totally different in composition.

LETTER VI
ALLIANCE OF CHEMISTRY WITH PHYSIOLOGY. Division of Food into
nourishment, and materials for combustion. Effects of Atmospheric Oxygen.
Balance of CARBON and OXYGEN.

LETTER VII
ANIMAL HEAT, its laws and influence on the Animal Functions. Loss and
SUPPLY. Influence of Climate. Fuel of Animal Heat. Agency of Oxygen in
Disease. Respiration.

LETTER VIII
ALIMENTS. Constituents of the Blood. Fibrine, Albumen. Inorganic
Substances. Isomerism of Fibrine, Albumen, and elements of nutrition. Relation
of animal and vegetable organisms.

LETTER IX
Growth of Animals. Uses of Butter and Milk. Metamorphoses of Tissues. Food
of Carnivora, and of the Horse.

LETTER X
Application of the preceding facts to Man. Division of human Food. Uses of
Gelatine.

LETTER XI
CIRCULATION OF MATTER IN THE ANIMAL AND VEGETABLE
KINGDOMS. The Ocean. AGRICULTURE. RESTITUTION OF AN
EQUILIBRIUM IN THE SOIL. Causes of the exhaustion of Land. Virginia.
England. Relief gained by importation of bones. Empirical farming
unsatisfactory. Necessity for scientific principles. Influence of the atmosphere.
Of Saline and Earthy matters of the soil.

LETTER XII
SCIENCE AND ART OF AGRICULTURE. NECESSITY OF CHEMISTRY.
Rationale of agricultural processes. Washing for gold.

LETTER XIII
ILLUSTRATION OF THE NECESSITY OF CHEMISTRY TO ADVANCE
AND PERFECT AGRICULTURE. Manner in which FALLOW ameliorates the
soil. Uses of Lime. Effects of Burning. Of Marl.

LETTER XIV
NATURE AND EFFECTS OF MANURES. Animal bodies subject to constant
waste. Parts separating—exuviae—waste vegetable matters—together contain all
the elements of the soil and of food. Various value of excrements of different
animals as manure.

LETTER XV
SOURCE OF THE CARBON AND NITROGEN OF PLANTS. Produce of
Carbon in Forests and Meadows supplied only with mineral aliments prove it to
be from the atmosphere. Relations between Mineral constituents, and Carbon
and Nitrogen. Effects of the Carbonic Acid and Ammonia of Manures. Necessity
of inorganic constituents to the formation of aliments, of blood, and therefore of
nutrition. NECESSITY OF INQUIRIES by ANALYSIS to advance
AGRICULTURE.

LETTER XVI
RESULTS OF THE AUTHOR'S LATEST INQUIRIES. Superlative importance
of the PHOSPHATES OF LIME and ALKALIES to the cultivation of the
CEREALIA. Sources of a SUPPLY of these MATERIALS.

LETTERS ON CHEMISTRY

LETTER I

My dear Sir,

The influence which the science of chemistry exercises upon human
industry, agriculture, and commerce; upon physiology, medicine, and other
sciences, is now so interesting a topic of conversation everywhere, that it
may be no unacceptable present to you if I trace in a few familiar letters
some of the relations it bears to these various sciences, and exhibit for you
its actual effect upon the present social condition of mankind.

In speaking of the present state of chemistry, its rise and progress, I shall
need no apology if, as a preliminary step, I call your attention to the
implements which the chemist employs—the means which are
indispensable to his labours and to his success.

These consist, generally, of materials furnished to us by nature, endowed
with many most remarkable properties fitting them for our purposes; if one
of them is a production of art, yet its adaptation to the use of mankind,—the
qualities which render it available to us,—must be referred to the same
source as those derived immediately from nature.

Cork, Platinum, Glass, and Caoutchouc, are the substances to which I
allude, and which minister so essentially to modern chemical investigations.
Without them, indeed, we might have made some progress, but it would
have been slow; we might have accomplished much, but it would have been
far less than has been done with their aid. Some persons, by the
employment of expensive substances, might have successfully pursued the
science; but incalculably fewer minds would have been engaged in its
advancement. These materials have only been duly appreciated and fully
adopted within a very recent period. In the time of Lavoisier, the rich alone
could make chemical researches; the necessary apparatus could only be
procured at a very great expense.

And first, of Glass: every one is familiar with most of the properties of
this curious substance; its transparency, hardness, destitution of colour, and
stability under ordinary circumstances: to these obvious qualities we may
add those which especially adapt it to the use of the chemist, namely, that it
is unaffected by most acids or other fluids contained within it. At certain
temperatures it becomes more ductile and plastic than wax, and may be
made to assume in our hands, before the flame of a common lamp, the form
of every vessel we need to contain our materials, and of every apparatus
required to pursue our experiments.

Then, how admirable and valuable are the properties of Cork! How little
do men reflect upon the inestimable worth of so common a substance! How
few rightly esteem the importance of it to the progress of science, and the
moral advancement of mankind!—There is no production of nature or art
equally adapted to the purposes to which the chemist applies it. Cork
consists of a soft, highly elastic substance, as a basis, having diffused
throughout a matter with properties resembling wax, tallow, and resin, yet
dissimilar to all of these, and termed suberin. This renders it perfectly
impermeable to fluids, and, in a great measure, even to gases. It is thus the
fittest material we possess for closing our bottles, and retaining their
contents. By its means, and with the aid of Caoutchouc, we connect our
vessels and tubes of glass, and construct the most complicated apparatus.
We form joints and links of connexion, adapt large apertures to small, and
thus dispense altogether with the aid of the brassfounder and the mechanist.

Thus the implements of the chemist are cheaply and easily procured,
immediately adapted to any purpose, and readily repaired or altered.

Again, in investigating the composition of solid bodies,—of minerals,—
we are under the necessity of bringing them into a liquid state, either by
solution or fusion. Now vessels of glass, of porcelain, and of all non-
metallic substances, are destroyed by the means we employ for that
purpose,—are acted upon by many acids, by alkalies and the alkaline
carbonates. Crucibles of gold and silver would melt at high temperatures.
But we have a combination of all the qualities we can desire in Platinum.
This metal was only first adapted to these uses about fifty years since. It is
cheaper than gold, harder and more durable than silver, infusible at all
temperatures of our furnaces, and is left intact by acids and alkaline
carbonates. Platinum unites all the valuable properties of gold and of
porcelain, resisting the action of heat, and of almost all chemical agents.

As no mineral analysis could be made perfectly without platinum
vessels, had we not possessed this metal, the composition of minerals would
have yet remained unknown; without cork and caoutchouc we should have
required the costly aid of the mechanician at every step. Even without the
latter of these adjuncts our instruments would have been far more costly
and fragile. Possessing all these gifts of nature, we economise incalculably
our time—to us more precious than money!

Such are our instruments. An equal improvement has been accomplished
in our laboratory. This is no longer the damp, cold, fireproof vault of the
metallurgist, nor the manufactory of the druggist, fitted up with stills and
retorts. On the contrary, a light, warm, comfortable room, where beautifully
constructed lamps supply the place of furnaces, and the pure and odourless
flame of gas, or of spirits of wine, supersedes coal and other fuel, and gives
us all the fire we need; where health is not invaded, nor the free exercise of
thought impeded: there we pursue our inquiries, and interrogate Nature to
reveal her secrets.

To these simple means must be added "The Balance," and then we
possess everything which is required for the most extensive researches.

The great distinction between the manner of proceeding in chemistry and
natural philosophy is, that one weighs, the other measures. The natural
philosopher has applied his measures to nature for many centuries, but only
for fifty years have we attempted to advance our philosophy by weighing.

For all great discoveries chemists are indebted to the "balance"—that
incomparable instrument which gives permanence to every observation,
dispels all ambiguity, establishes truth, detects error, and guides us in the
true path of inductive science.

The balance, once adopted as a means of investigating nature, put an end
to the school of Aristotle in physics. The explanation of natural phenomena
by mere fanciful speculations, gave place to a true natural philosophy. Fire,
air, earth, and water, could no longer be regarded as elements. Three of
them could henceforth be considered only as significative of the forms in
which all matter exists. Everything with which we are conversant upon the
surface of the earth is solid, liquid, or aeriform; but the notion of the
elementary nature of air, earth, and water, so universally held, was now
discovered to belong to the errors of the past.

Fire was found to be but the visible and otherwise perceptible indication
of changes proceeding within the, so called, elements.

Lavoisier investigated the composition of the atmosphere and of water,
and studied the many wonderful offices performed by an element common
to both in the scheme of nature, namely, oxygen: and he discovered many of
the properties of this elementary gas.

After his time, the principal problem of chemical philosophers was to
determine the composition of the solid matters composing the earth. To the
eighteen metals previously known were soon added twenty-four discovered
to be constituents of minerals. The great mass of the earth was shown to be
composed of metals in combination with oxygen, to which they are united
in one, two, or more definite and unalterable proportions, forming
compounds which are termed metallic oxides, and these, again, combined
with oxides of other bodies, essentially different to metals, namely, carbon
and silicium. If to these we add certain compounds of sulphur with metals,
in which the sulphur takes the place of oxygen, and forms sulphurets, and

one other body,—common salt,—(which is a compound of sodium and
chlorine), we have every substance which exists in a solid form upon our
globe in any very considerable mass. Other compounds, innumerably
various, are found only in small scattered quantities.

The chemist, however, did not remain satisfied with the separation of
minerals into their component elements, i.e. their analysis; but he sought by
synthesis, i.e. by combining the separate elements and forming substances
similar to those constructed by nature, to prove the accuracy of his
processes and the correctness of his conclusions. Thus he formed, for
instance, pumice-stone, feldspar, mica, iron pyrites, &c. artificially.

But of all the achievements of inorganic chemistry, the artificial
formation of lapis lazuli was the most brilliant and the most conclusive.
This mineral, as presented to us by nature, is calculated powerfully to arrest
our attention by its beautiful azure-blue colour, its remaining unchanged by
exposure to air or to fire, and furnishing us with a most valuable pigment,
Ultramarine, more precious than gold!

The analysis of lapis lazuli represented it to be composed of silica,
alumina, and soda, three colourless bodies, with sulphur and a trace of iron.
Nothing could be discovered in it of the nature of a pigment, nothing to
which its blue colour could be referred, the cause of which was searched for
in vain. It might therefore have been supposed that the analyst was here
altogether at fault, and that at any rate its artificial production must be
impossible. Nevertheless, this has been accomplished, and simply by
combining in the proper proportions, as determined by analysis, silica,
alumina, soda, iron, and sulphur. Thousands of pounds weight are now
manufactured from these ingredients, and this artificial ultramarine is as
beautiful as the natural, while for the price of a single ounce of the latter we
may obtain many pounds of the former.

With the production of artificial lapis lazuli, the formation of mineral
bodies by synthesis ceased to be a scientific problem to the chemist; he has
no longer sufficient interest in it to pursue the subject. He may now be
satisfied that analysis will reveal to him the true constitution of minerals.
But to the mineralogist and geologist it is still in a great measure an

unexplored field, offering inquiries of the highest interest and importance to
their pursuits.

After becoming acquainted with the constituent elements of all the
substances within our reach and the mutual relations of these elements, the
remarkable transmutations to which the bodies are subject under the
influence of the vital powers of plants and animals, became the principal
object of chemical investigations, and the highest point of interest. A new
science, inexhaustible as life itself, is here presented us, standing upon the
sound and solid foundation of a well established inorganic chemistry. Thus
the progress of science is, like the development of nature's works, gradual
and expansive. After the buds and branches spring forth the leaves and
blossoms, after the blossoms the fruit.

Chemistry, in its application to animals and vegetables, endeavours
jointly with physiology to enlighten us respecting the mysterious processes
and sources of organic life.

LETTER II

My dear Sir,

In my former letter I reminded you that three of the supposed elements
of the ancients represent the forms or state in which all the ponderable
matter of our globe exists; I would now observe, that no substance
possesses absolutely any one of those conditions; that modern chemistry
recognises nothing unchangeably solid, liquid, or aeriform: means have
been devised for effecting a change of state in almost every known
substance. Platinum, alumina, and rock crystal, it is true, cannot be liquified
by the most intense heat of our furnaces, but they melt like wax before the
flame of the oxy-hydrogen blowpipe. On the other hand, of the twenty-eight
gaseous bodies with which we are acquainted, twenty-five may be reduced

to a liquid state, and one into a solid. Probably, ere long, similar changes of
condition will be extended to every form of matter.

There are many things relating to this condensation of the gases worthy
of your attention. Most aeriform bodies, when subjected to compression, are
made to occupy a space which diminishes in the exact ratio of the increase
of the compressing force. Very generally, under a force double or triple of
the ordinary atmospheric pressure, they become one half or one third their
former volume. This was a long time considered to be a law, and known as
the law of Marriotte; but a more accurate study of the subject has
demonstrated that this law is by no means of general application. The
volume of certain gases does not decrease in the ratio of the increase of the
force used to compress them, but in some, a diminution of their bulk takes
place in a far greater degree as the pressure increases.

Again, if ammoniacal gas is reduced by a compressing force to one-sixth
of its volume, or carbonic acid is reduced to one thirty-sixth, a portion of
them loses entirely the form of a gas, and becomes a liquid, which, when
the pressure is withdrawn, assumes again in an instant its gaseous state—
another deviation from the law of Marriotte.

Our process for reducing gases into fluids is of admirable simplicity. A
simple bent tube, or a reduction of temperature by artificial means, have
superseded the powerful compressing machines of the early experimenters.

The cyanuret of mercury, when heated in an open glass tube, is resolved
into cyanogen gas and metallic mercury; if this substance is heated in a tube
hermetically sealed, the decomposition occurs as before, but the gas, unable
to escape, and shut up in a space several hundred times smaller than it
would occupy as gas under the ordinary atmospheric pressure, becomes a
fluid in that part of the tube which is kept cool.

When sulphuric acid is poured upon limestone in an open vessel,
carbonic acid escapes with effervescence as a gas, but if the decomposition
is effected in a strong, close, and suitable vessel of iron, we obtain the
carbonic acid in the state of liquid. In this manner it may be obtained in
considerable quantities, even many pounds weight. Carbonic acid is

separated from other bodies with which it is combined as a fluid under a
pressure of thirty-six atmospheres.

The curious properties of fluid carbonic acid are now generally known.
When a small quantity is permitted to escape into the atmosphere, it
assumes its gaseous state with extraordinary rapidity, and deprives the
remaining fluid of caloric so rapidly that it congeals into a white crystalline
mass like snow: at first, indeed, it was thought to be really snow, but upon
examination it proved to be pure frozen carbonic acid. This solid, contrary
to expectation, exercises only a feeble pressure upon the surrounding
medium. The fluid acid inclosed in a glass tube rushes at once, when
opened, into a gaseous state, with an explosion which shatters the tube into
fragments; but solid carbonic acid can be handled without producing any
other effect than a feeling of intense cold. The particles of the carbonic acid
being so closely approximated in the solid, the whole force of cohesive
attraction (which in the fluid is weak) becomes exerted, and opposes its
tendency to assume its gaseous state; but as it receives heat from
surrounding bodies, it passes into gas gradually and without violence. The
transition of solid carbonic acid into gas deprives all around it of caloric so
rapidly and to so great an extent, that a degree of cold is produced
immeasurably great, the greatest indeed known. Ten, twenty, or more
pounds weight of mercury, brought into contact with a mixture of ether and
solid carbonic acid, becomes in a few moments firm and malleable. This,
however, cannot be accomplished without considerable danger. A
melancholy accident occurred at Paris, which will probably prevent for the
future the formation of solid carbonic acid in these large quantities, and
deprive the next generation of the gratification of witnessing these curious
experiments. Just before the commencement of the lecture in the Laboratory
of the Polytechnic School, an iron cylinder, two feet and a half long and one
foot in diameter, in which carbonic acid had been developed for experiment
before the class, burst, and its fragments were scattered about with the most
tremendous force; it cut off both the legs of the assistant and killed him on
the spot. This vessel, formed of the strongest cast-iron, and shaped like a
cannon, had often been employed to exhibit experiments in the presence of
the students. We can scarcely think, without shuddering, of the dreadful
calamity such an explosion would have occasioned in a hall filled with
spectators.

When we had ascertained the fact of gases becoming fluid under the
influence of cold or pressure, a curious property possessed by charcoal, that
of absorbing gas to the extent of many times its volume,—ten, twenty, or
even as in the case of ammoniacal gas or muriatic acid gas, eighty or ninety
fold,—which had been long known, no longer remained a mystery. Some
gases are absorbed and condensed within the pores of the charcoal, into a
space several hundred times smaller than they before occupied; and there is
now no doubt they there become fluid, or assume a solid state. As in a
thousand other instances, chemical action here supplants mechanical forces.
Adhesion or heterogeneous attraction, as it is termed, acquired by this
discovery a more extended meaning; it had never before been thought of as
a cause of change of state in matter; but it is now evident that a gas adheres
to the surface of a solid body by the same force which condenses it into a
liquid.

The smallest amount of a gas,—atmospheric air for instance,—can be
compressed into a space a thousand times smaller by mere mechanical
pressure, and then its bulk must be to the least measurable surface of a solid
body, as a grain of sand to a mountain. By the mere effect of mass,—the
force of gravity,—gaseous molecules are attracted by solids and adhere to
their surfaces; and when to this physical force is added the feeblest
chemical affinity, the liquifiable gases cannot retain their gaseous state. The
amount of air condensed by these forces upon a square inch of surface is
certainly not measurable; but when a solid body, presenting several hundred
square feet of surface within the space of a cubic inch, is brought into a
limited volume of gas, we may understand why that volume is diminished,
why all gases without exception are absorbed. A cubic inch of charcoal
must have, at the lowest computation, a surface of one hundred square feet.
This property of absorbing gases varies with different kinds of charcoal: it
is possessed in a higher degree by those containing the most pores, i.e.
where the pores are finer; and in a lower degree in the more spongy kinds,
i.e. where the pores are larger.

In this manner every porous body—rocks, stones, the clods of the fields,
&c.,—imbibe air, and therefore oxygen; the smallest solid molecule is thus
surrounded by its own atmosphere of condensed oxygen; and if in their
vicinity other bodies exist which have an affinity for oxygen, a combination

is effected. When, for instance, carbon and hydrogen are thus present, they
are converted into nourishment for vegetables,—into carbonic acid and
water. The development of heat when air is imbibed, and the production of
steam when the earth is moistened by rain, are acknowledged to be
consequences of this condensation by the action of surfaces.

But the most remarkable and interesting case of this kind of action is the
imbibition of oxygen by metallic platinum. This metal, when massive, is of
a lustrous white colour, but it may be brought, by separating it from its
solutions, into so finely divided a state, that its particles no longer reflect
light, and it forms a powder as black as soot. In this condition it absorbs
eight hundred times its volume of oxygen gas, and this oxygen must be
contained within it in a state of condensation very like that of fluid water.

When gases are thus condensed, i.e. their particles made to approximate
in this extraordinary manner, their properties can be palpably shown. Their
chemical actions become apparent as their physical characteristic
disappears. The latter consists in the continual tendency of their particles to
separate from each other; and it is easy to imagine that this elasticity of
gaseous bodies is the principal impediment to the operation of their
chemical force; for this becomes more energetic as their particles
approximate. In that state in which they exist within the pores or upon the
surface of solid bodies, their repulsion ceases, and their whole chemical
action is exerted. Thus combinations which oxygen cannot enter into,
decompositions which it cannot effect while in the state of gas, take place
with the greatest facility in the pores of platinum containing condensed
oxygen. When a jet of hydrogen gas, for instance, is thrown upon spongy
platinum, it combines with the oxygen condensed in the interior of the
mass; at their point of contact water is formed, and as the immediate
consequence heat is evolved; the platinum becomes red hot and the gas is
inflamed. If we interrupt the current of the gas, the pores of the platinum
become instantaneously filled again with oxygen; and the same
phenomenon can be repeated a second time, and so on interminably.

In finely pulverised platinum, and even in spongy platinum, we therefore
possess a perpetuum mobile—a mechanism like a watch which runs out and

winds itself up—a force which is never exhausted—competent to produce
effects of the most powerful kind, and self-renewed ad infinitum.

Many phenomena, formerly inexplicable, are satisfactorily explained by
these recently discovered properties of porous bodies. The metamorphosis
of alcohol into acetic acid, by the process known as the quick vinegar
manufacture, depends upon principles, at a knowledge of which we have
arrived by a careful study of these properties.

LETTER III

My dear Sir,

The manufacture of soda from common culinary salt, may be regarded
as the foundation of all our modern improvements in the domestic arts; and
we may take it as affording an excellent illustration of the dependence of
the various branches of human industry and commerce upon each other, and
their relation to chemistry.

Soda has been used from time immemorial in the manufacture of soap
and glass, two chemical productions which employ and keep in circulation
an immense amount of capital. The quantity of soap consumed by a nation
would be no inaccurate measure whereby to estimate its wealth and
civilisation. Of two countries, with an equal amount of population, the
wealthiest and most highly civilised will consume the greatest weight of
soap. This consumption does not subserve sensual gratification, nor depend
upon fashion, but upon the feeling of the beauty, comfort, and welfare,
attendant upon cleanliness; and a regard to this feeling is coincident with
wealth and civilisation. The rich in the middle ages concealed a want of
cleanliness in their clothes and persons under a profusion of costly scents
and essences, whilst they were more luxurious in eating and drinking, in
apparel and horses. With us a want of cleanliness is equivalent to
insupportable misery and misfortune.

Soap belongs to those manufactured products, the money value of which
continually disappears from circulation, and requires to be continually
renewed. It is one of the few substances which are entirely consumed by
use, leaving no product of any worth. Broken glass and bottles are by no
means absolutely worthless; for rags we may purchase new cloth, but soap-
water has no value whatever. It would be interesting to know accurately the
amount of capital involved in the manufacture of soap; it is certainly as
large as that employed in the coffee trade, with this important difference as
respects Germany, that it is entirely derived from our own soil.

France formerly imported soda from Spain,—Spanish sodas being of the
best quality—at an annual expenditure of twenty to thirty millions of francs.
During the war with England the price of soda, and consequently of soap
and glass, rose continually; and all manufactures suffered in consequence.

The present method of making soda from common salt was discovered
by Le Blanc at the end of the last century. It was a rich boon for France, and
became of the highest importance during the wars of Napoleon. In a very
short time it was manufactured to an extraordinary extent, especially at the
seat of the soap manufactories. Marseilles possessed for a time a monopoly
of soda and soap. The policy of Napoleon deprived that city of the
advantages derived from this great source of commerce, and thus excited
the hostility of the population to his dynasty, which became favourable to
the restoration of the Bourbons. A curious result of an improvement in a
chemical manufacture! It was not long, however, in reaching England.

In order to prepare the soda of commerce (which is the carbonate) from
common salt, it is first converted into Glauber's salt (sulphate of soda). For
this purpose 80 pounds weight of concentrated sulphuric acid (oil of vitriol)
are required to 100 pounds of common salt. The duty upon salt checked, for
a short time, the full advantage of this discovery; but when the Government
repealed the duty, and its price was reduced to its minimum, the cost of
soda depended upon that of sulphuric acid.

The demand for sulphuric acid now increased to an immense extent; and,
to supply it, capital was embarked abundantly, as it afforded an excellent
remuneration. The origin and formation of sulphuric acid was studied most
carefully; and from year to year, better, simpler, and cheaper methods of

making it were discovered. With every improvement in the mode of
manufacture, its price fell; and its sale increased in an equal ratio.

Sulphuric acid is now manufactured in leaden chambers, of such
magnitude that they would contain the whole of an ordinary-sized house. As
regards the process and the apparatus, this manufacture has reached its
acme—scarcely is either susceptible of improvement. The leaden plates of
which the chambers are constructed, requiring to be joined together with
lead (since tin or solder would be acted on by the acid), this process was,
until lately, as expensive as the plates themselves; but now, by means of the
oxy-hydrogen blowpipe, the plates are cemented together at their edges by
mere fusion, without the intervention of any kind of solder.

And then, as to the process: according to theory, 100 pounds weight of
sulphur ought to produce 306 pounds of sulphuric acid; in practice 300
pounds are actually obtained; the amount of loss is therefore too
insignificant for consideration.

Again; saltpetre being indispensable in making sulphuric acid, the
commercial value of that salt had formerly an important influence upon its
price. It is true that 100 pounds of saltpetre only are required to 1000
pounds of sulphur; but its cost was four times greater than an equal weight
of the latter.

Travellers had observed near the small seaport of Yquiqui, in the district
of Atacama, in Peru, an efflorescence covering the ground over extensive
districts. This was found to consist principally of nitrate of soda. Advantage
was quickly taken of this discovery. The quantity of this valuable salt
proved to be inexhaustible, as it exists in beds extending over more than
200 square miles. It was brought to England at less than half the freight of
the East India saltpetre (nitrate of potassa); and as, in the chemical
manufacture neither the potash nor the soda were required, but only the
nitric acid, in combination with the alkali, the soda-saltpetre of South
America soon supplanted the potash-nitre of the East. The manufacture of
sulphuric acid received a new impulse; its price was much diminished
without injury to the manufacturer; and, with the exception of fluctuations
caused by the impediments thrown in the way of the export of sulphur from
Sicily, it soon became reduced to a minimum, and remained stationary.

Potash-saltpetre is now only employed in the manufacture of
gunpowder; it is no longer in demand for other purposes; and thus, if
Government effect a saving of many hundred thousand pounds annually in
gunpowder, this economy must be attributed to the increased manufacture
of sulphuric acid.

We may form an idea of the amount of sulphuric acid consumed, when
we find that 50,000 pounds weight are made by a small manufactory, and
from 200,000 to 600,000 pounds by a large one annually. This manufacture
causes immense sums to flow annually into Sicily. It has introduced
industry and wealth into the arid and desolate districts of Atacama. It has
enabled us to obtain platina from its ores at a moderate and yet
remunerating price; since the vats employed for concentrating this acid are
constructed of this metal, and cost from 1000l. to 2000l. sterling. It leads to
frequent improvements in the manufacture of glass, which continually
becomes cheaper and more beautiful. It enables us to return to our fields all
their potash—a most valuable and important manure—in the form of ashes,
by substituting soda in the manufacture of glass and soap.

It is impossible to trace, within the compass of a letter, all the
ramifications of this tissue of changes and improvements resulting from one
chemical manufacture; but I must still claim your attention to a few more of
its most important and immediate results. I have already told you, that in the
manufacture of soda from culinary salt, it is first converted into sulphate of
soda. In this first part of the process, the action of sulphuric acid produces
muriatic acid to the extent of one-and-a-half the amount of the sulphuric
acid employed. At first, the profit upon the soda was so great, that no one
took the trouble to collect the muriatic acid: indeed it had no commercial
value. A profitable application of it was, however, soon discovered: it is a
compound of chlorine, and this substance may be obtained from it purer
than from any other source. The bleaching power of chlorine has long been
known; but it was only employed upon a large scale after it was obtained
from this residuary muriatic acid, and it was found that in combination with
lime it could be transported to distances without inconvenience.
Thenceforth it was used for bleaching cotton; and, but for this new
bleaching process, it would scarcely have been possible for the cotton
manufacture of Great Britain to have attained its present enormous extent,

—it could not have competed in price with France and Germany. In the old
process of bleaching, every piece must be exposed to the air and light
during several weeks in the summer, and kept continually moist by manual
labour. For this purpose, meadow land, eligibly situated, was essential. Now
a single establishment near Glasgow bleaches 1400 pieces of cotton daily,
throughout the year. What an enormous capital would be required to
purchase land for this purpose! How greatly would it increase the cost of
bleaching to pay interest upon this capital, or to hire so much land in
England! This expense would scarcely have been felt in Germany. Besides
the diminished expense, the cotton stuffs bleached with chlorine suffer less
in the hands of skilful workmen than those bleached in the sun; and already
the peasantry in some parts of Germany have adopted it, and find it
advantageous.

Another use to which cheap muriatic acid is applied, is the manufacture
of glue from bones. Bone contains from 30 to 36 per cent. of earthy matter
—chiefly phosphate of lime, and the remainder is gelatine. When bones are
digested in muriatic acid they become transparent and flexible like leather,
the earthy matter is dissolved, and after the acid is all carefully washed
away, pieces of glue of the same shape as the bones remain, which are
soluble in hot water and adapted to all the purposes of ordinary glue,
without further preparation.

Another important application of sulphuric acid may be adduced;
namely, to the refining of silver and the separation of gold, which is always
present in some proportion in native silver. Silver, as it is usually obtained
from mines in Europe, contains in 16 ounces, 6 to 8 ounces of copper.
When used by the silversmith, or in coining, 16 ounces must contain in
Germany 13 ounces of silver, in England about 14 1/2. But this alloy is
always made artificially by mixing pure silver with the due proportion of
the copper; and for this purpose the silver must be obtained pure by the
refiner. This he formerly effected by amalgamation, or by roasting it with
lead; and the cost of this process was about 2l. for every hundred-weight of
silver. In the silver so prepared, about 1/1200 to 1/2000th part of gold
remained; to effect the separation of this by nitrio-hydrochloric acid was
more expensive than the value of the gold; it was therefore left in utensils,
or circulated in coin, valueless. The copper, too, of the native silver was no

use whatever. But the 1/1000th part of gold, being about one and a half per
cent. of the value of the silver, now covers the cost of refining, and affords
an adequate profit to the refiner; so that he effects the separation of the
copper, and returns to his employer the whole amount of the pure silver, as
well as the copper, without demanding any payment: he is amply
remunerated by that minute portion of gold. The new process of refining is
a most beautiful chemical operation: the granulated metal is boiled in
concentrated sulphuric acid, which dissolves both the silver and the copper,
leaving the gold nearly pure, in the form of a black powder. The solution is
then placed in a leaden vessel containing metallic copper; this is gradually
dissolved, and the silver precipitated in a pure metallic state. The sulphate
of copper thus formed is also a valuable product, being employed in the
manufacture of green and blue pigments.

Other immediate results of the economical production of sulphuric acid,
are the general employment of phosphorus matches, and of stearine candles,
that beautiful substitute for tallow and wax. Twenty-five years ago, the
present prices and extensive applications of sulphuric and muriatic acids, of
soda, phosphorus, &c., would have been considered utterly impossible.
Who is able to foresee what new and unthought-of chemical productions,
ministering to the service and comforts of mankind, the next twenty-five
years may produce?

After these remarks you will perceive that it is no exaggeration to say,
we may fairly judge of the commercial prosperity of a country from the
amount of sulphuric acid it consumes. Reflecting upon the important
influence which the price of sulphur exercises upon the cost of production
of bleached and printed cotton stuffs, soap, glass, &c., and remembering
that Great Britain supplies America, Spain, Portugal, and the East, with
these, exchanging them for raw cotton, silk, wine, raisins, indigo, &c., &c.,
we can understand why the English Government should have resolved to
resort to war with Naples, in order to abolish the sulphur monopoly, which
the latter power attempted recently to establish. Nothing could be more
opposed to the true interests of Sicily than such a monopoly; indeed, had it
been maintained a few years, it is highly probable that sulphur, the source of
her wealth, would have been rendered perfectly valueless to her. Science
and industry form a power to which it is dangerous to present impediments.

It was not difficult to perceive that the issue would be the entire cessation of
the exportation of sulphur from Sicily. In the short period the sulphur
monopoly lasted, fifteen patents were taken out for methods to obtain back
the sulphuric acid used in making soda. Admitting that these fifteen
experiments were not perfectly successful, there can be no doubt it would
ere long have been accomplished. But then, in gypsum, (sulphate of lime),
and in heavy-spar, (sulphate of barytes), we possess mountains of sulphuric
acid; in galena, (sulphate of lead), and in iron pyrites, we have no less
abundance of sulphur. The problem is, how to separate the sulphuric acid,
or the sulphur, from these native stores. Hundreds of thousands of pounds
weight of sulphuric acid were prepared from iron pyrites, while the high
price of sulphur consequent upon the monopoly lasted. We should probably
ere long have triumphed over all difficulties, and have separated it from
gypsum. The impulse has been given, the possibility of the process proved,
and it may happen in a few years that the inconsiderate financial
speculation of Naples may deprive her of that lucrative commerce. In like
manner Russia, by her prohibitory system, has lost much of her trade in
tallow and potash. One country purchases only from absolute necessity
from another, which excludes her own productions from her markets.
Instead of the tallow and linseed oil of Russia, Great Britain now uses palm
oil and cocoa-nut oil of other countries. Precisely analogous is the
combination of workmen against their employers, which has led to the
construction of many admirable machines for superseding manual labour. In
commerce and industry every imprudence carries with it its own
punishment; every oppression immediately and sensibly recoils upon the
head of those from whom it emanates.

LETTER IV

My dear Sir,

One of the most influential causes of improvement in the social
condition of mankind is that spirit of enterprise which induces men of

capital to adopt and carry out suggestions for the improvement of
machinery, the creation of new articles of commerce, or the cheaper
production of those already in demand; and we cannot but admire the
energy with which such men devote their talents, their time, and their
wealth, to realise the benefits of the discoveries and inventions of science.
For even when these are expended upon objects wholly incapable of
realisation,—nay, even when the idea which first gave the impulse proves in
the end to be altogether impracticable or absurd, immediate good to the
community generally ensues; some useful and perhaps unlooked-for result
flows directly, or springs ultimately, from exertions frustrated in their main
design. Thus it is also in the pursuit of science. Theories lead to
experiments and investigations; and he who investigates will scarcely ever
fail of being rewarded by discoveries. It may be, indeed, the theory sought
to be established is entirely unfounded in nature; but while searching in a
right spirit for one thing, the inquirer may be rewarded by finding others far
more valuable than those which he sought.

At the present moment, electro-magnetism, as a moving power, is
engaging great attention and study; wonders are expected from its
application to this purpose. According to the sanguine expectations of many
persons, it will shortly be employed to put into motion every kind of
machinery, and amongst other things it will be applied to impel the
carriages of railroads, and this at so small a cost, that expense will no longer
be matter of consideration. England is to lose her superiority as a
manufacturing country, inasmuch as her vast store of coals will no longer
avail her as an economical source of motive power. "We," say the German
cultivators of this science, "have cheap zinc, and, how small a quantity of
this metal is required to turn a lathe, and consequently to give motion to any
kind of machinery!"

Such expectations may be very attractive, and yet they are altogether
illusory! they will not bear the test of a few simple calculations; and these
our friends have not troubled themselves to institute.

With a simple flame of spirits of wine, under a proper vessel containing
boiling water, a small carriage of 200 to 300 pounds weight can be put into
motion, or a weight of 80 to 100 pounds may be raised to a height of 20

feet. The same effects may be produced by dissolving zinc in dilute
sulphuric acid in a certain apparatus. This is certainly an astonishing and
highly interesting discovery; but the question to be determined is, which of
the two processes is the least expensive?

In order to answer this question, and to judge correctly of the hopes
entertained from this discovery, let me remind you of what chemists
denominate "equivalents." These are certain unalterable ratios of effects
which are proportionate to each other, and may therefore be expressed in
numbers. Thus, if we require 8 pounds of oxygen to produce a certain
effect, and we wish to employ chlorine for the same effect, we must employ
neither more nor less than 35 1/2 pounds weight. In the same manner, 6
pounds weight of coal are equivalent to 32 pounds weight of zinc. The
numbers representing chemical equivalents express very general ratios of
effects, comprehending for all bodies all the actions they are capable of
producing.

If zinc be combined in a certain manner with another metal, and
submitted to the action of dilute sulphuric acid, it is dissolved in the form of
an oxide; it is in fact burned at the expense of the oxygen contained in the
fluid. A consequence of this action is the production of an electric current,
which, if conducted through a wire, renders it magnetic. In thus effecting
the solution of a pound weight, for example, of zinc, we obtain a definite
amount of force adequate to raise a given weight one inch, and to keep it
suspended; and the amount of weight it will be capable of suspending will
be the greater the more rapidly the zinc is dissolved.

By alternately interrupting and renewing the contact of the zinc with the
acid, and by very simple mechanical arrangements, we can give to the iron
an upward and downward or a horizontal motion, thus producing the
conditions essential to the motion of any machinery.

This moving force is produced by the oxidation of the zinc; and, setting
aside the name given to the force in this case, we know that it can be
produced in another manner. If we burn the zinc under the boiler of a steam-
engine, consequently in the oxygen of the air instead of the galvanic pile,
we should produce steam, and by it a certain amount of force. If we should
assume, (which, however, is not proved,) that the quantity of force is

unequal in these cases,—that, for instance, we had obtained double or triple
the amount in the galvanic pile, or that in this mode of generating force less
loss is sustained,—we must still recollect the equivalents of zinc and coal,
and make these elements of our calculation. According to the experiments
of Despretz, 6 pounds weight of zinc, in combining with oxygen, develops
no more heat than 1 pound of coal; consequently, under equal conditions,
we can produce six times the amount of force with a pound of coal as with a
pound of zinc. It is therefore obvious that it would be more advantageous to
employ coal instead of zinc, even if the latter produced four times as much
force in a galvanic pile, as an equal weight of coal by its combustion under
a boiler. Indeed it is highly probable, that if we burn under the boiler of a
steam-engine the quantity of coal required for smelting the zinc from its
ores, we shall produce far more force than the whole of the zinc so obtained
could originate in any form of apparatus whatever.

Heat, electricity, and magnetism, have a similar relation to each other as
the chemical equivalents of coal, zinc, and oxygen. By a certain measure of
electricity we produce a corresponding proportion of heat or of magnetic
power; we obtain that electricity by chemical affinity, which in one shape
produces heat, in another electricity or magnetism. A certain amount of
affinity produces an equivalent of electricity in the same manner as, on the
other hand, we decompose equivalents of chemical compounds by a definite
measure of electricity. The magnetic force of the pile is therefore limited to
the extent of the chemical affinity, and in the case before us is obtained by
the combination of the zinc and sulphuric acid. In the combustion of coal,
the heat results from, and is measured by, the affinity of the oxygen of the
atmosphere for that substance.

It is true that with a very small expense of zinc, we can make an iron
wire a magnet capable of sustaining a thousand pounds weight of iron; let
us not allow ourselves to be misled by this. Such a magnet could not raise a
single pound weight of iron two inches, and therefore could not impart
motion. The magnet acts like a rock, which while at rest presses with a
weight of a thousand pounds upon a basis; it is like an inclosed lake,
without an outlet and without a fall. But it may be said, we have, by
mechanical arrangements, given it an outlet and a fall. True; and this must
be regarded as a great triumph of mechanics; and I believe it is susceptible

of further improvements, by which greater force may be obtained. But with
every conceivable advantage of mechanism, no one will dispute that one
pound of coal, under the boiler of a steam-engine, will give motion to a
mass several hundred times greater than a pound of zinc in the galvanic
pile.

Our experience of the employment of electro-magnetism as a motory
power is, however, too recent to enable us to foresee the ultimate results of
contrivances to apply it; and, therefore, those who have devoted themselves
to solve the problem of its application should not be discouraged, inasmuch
as it would undoubtedly be a most important achievement to supersede the
steam-engine, and thus escape the danger of railroads, even at double their
expense.

Professor Weber of Gottingen has thrown out a suggestion, that if a
contrivance could be devised to enable us to convert at will the wheels of
the steam-carriage into magnets, we should be enabled to ascend and
descend acclivities with great facility. This notion may ultimately be, to a
certain extent, realised.

The employment of the galvanic pile as a motory power, however, must,
like every other contrivance, depend upon the question of its relative
economy: probably some time hence it may so far succeed as to be adopted
in certain favourable localities; it may stand in the same relation to steam
power as the manufacture of beet sugar bears to that of cane, or as the
production of gas from oils and resins to that from mineral coal.

The history of beet-root sugar affords us an excellent illustration of the
effect of prices upon commercial productions. This branch of industry
seems at length, as to its processes, to be perfected. The most beautiful
white sugar is now manufactured from the beet-root, in the place of the
treacle-like sugar, having the taste of the root, which was first obtained; and
instead of 3 or 4 per cent., the proportion obtained by Achard, double or
even treble that amount is now produced. And notwithstanding the
perfection of the manufacture, it is probable it will ere long be in most
places entirely discontinued. In the years 1824 to 1827, the prices of
agricultural produce were much lower than at present, while the price of
sugar was the same. At that time one malter [1] of wheat was 10s., and one

klafter [2] of wood 18s., and land was falling in price. Thus, food and fuel
were cheap, and the demand for sugar unlimited; it was, therefore,
advantageous to grow beet-root, and to dispose of the produce of land as
sugar. All these circumstances are now different. A malter of wheat costs
18s.; a klafter of wood, 30s. to 36s. Wages have risen, but not in proportion,
whilst the price of colonial sugar has fallen. Within the limits of the German
commercial league, as, for instance, at Frankfort-on-the-Maine, a pound of
the whitest and best loaf sugar is 7d.; the import duty is 31/d., or 30s. per
cwt., leaving 31/d. as the price of the sugar. In the year 1827, then, one
malter of wheat was equal to 40 lbs. weight of sugar, whilst at present that
quantity of wheat is worth 70 lbs. of sugar. If indeed fuel were the same in
price as formerly, and 70 lbs. of sugar could be obtained from the same
quantity of the root as then yielded 40 lbs., it might still be advantageously
produced; but the amount, if now obtained by the most approved methods
of extraction, falls far short of this; and as fuel is double the price, and
labour dearer, it follows that, at present, it is far more advantageous to
cultivate wheat and to purchase sugar.

There are, however, other elements which must enter into our
calculations; but these serve to confirm our conclusion that the manufacture
of beet-root sugar as a commercial speculation must cease. The leaves and
residue of the root, after the juice was expressed, were used as food for
cattle, and their value naturally increased with the price of grain. By the
process formerly pursued, 75 lbs. weight of juice were obtained from 100
lbs. of beet-root, and gave 5 lbs. of sugar. The method of Schutzenbach,
which was eagerly adopted by the manufacturers, produced from the same
quantity of root 8 lbs. of sugar; but it was attended with more expense to
produce, and the loss of the residue as food for cattle. The increased
expense in this process arises from the larger quantity of fuel required to
evaporate the water; for instead of merely evaporating the juice, the dry
residue is treated with water, and we require fuel sufficient to evaporate 106
lbs. of fluid instead of 75 lbs., and the residue is only fit for manure. The
additional 3 lbs. of sugar are purchased at the expense of much fuel, and the
loss of the residue as an article of food.

If the valley of the Rhine possessed mines of diamonds as rich as those
of Golconda, Visiapoor, or the Brazils, they would probably not be worth

the working: at those places the cost of extraction is 28s. to 30s. the carat.
With us it amounts to three or four times as much—to more, in fact, than
diamonds are worth in the market. The sand of the Rhine contains gold; and
in the Grand Duchy of Baden many persons are occupied in gold-washing
when wages are low; but as soon as they rise, this employment ceases. The
manufacture of sugar from beet-root, in the like manner, twelve to fourteen
years ago offered advantages which are now lost: instead, therefore, of
maintaining it at a great sacrifice, it would be more reasonable, more in
accordance with true natural economy, to cultivate other and more valuable
productions, and with them purchase sugar. Not only would the state be the
gainer, but every member of the community. This argument does not apply,
perhaps, to France and Bohemia, where the prices of fuel and of colonial
sugar are very different to those in Germany.

The manufacture of gas for lighting, from coal, resin, and oils, stands
with us on the same barren ground.

The price of the materials from which gas is manufactured in England
bears a direct proportion to the price of corn: there the cost of tallow and oil
is twice as great as in Germany, but iron and coal are two-thirds cheaper;
and even in England the manufacture of gas is only advantageous when the
other products of the distillation of coal, the coke, &c., can be sold.

It would certainly be esteemed one of the greatest discoveries of the age
if any one could succeed in condensing coal gas into a white, dry, solid,
odourless substance, portable, and capable of being placed upon a
candlestick, or burned in a lamp. Wax, tallow, and oil, are combustible
gases in a solid or fluid form, which offer many advantages for lighting, not
possessed by gas: they furnish, in well-constructed lamps, as much light,
without requiring the expensive apparatus necessary for the combustion of
gas, and they are generally more economical. In large towns, or such
establishments as hotels, where coke is in demand, and where losses in
stolen tallow or oil must be considered, together with the labour of snuffing
candles and cleaning lamps, the higher price of gas is compensated. In
places where gas can be manufactured from resin, oil of turpentine, and
other cheap oils, as at Frankfort, this is advantageous so long as it is
pursued on small scale only. If large towns were lighted in the same

manner, the materials would rise in price: the whole amount at present
produced would scarcely suffice for two such towns as Berlin and Munich.
But no just calculation can be made from the present prices of turpentine,
resin, &c., which are not produced upon any large scale.
[Footnote 1: Malter—a measure containing several bushels, but varying in
different countries.]

[Footnote 2: Klafter—a cord, a stack, measuring six feet every way.]

LETTER V

My dear Sir,

Until very recently it was supposed that the physical qualities of bodies,
i.e. hardness, colour, density, transparency, &c., and still more their
chemical properties, must depend upon the nature of their elements, or upon
their composition. It was tacitly received as a principle, that two bodies
containing the same elements in the same proportion, must of necessity
possess the same properties. We could not imagine an exact identity of
composition giving rise to two bodies entirely different in their sensible
appearance and chemical relations. The most ingenious philosophers
entertained the opinion that chemical combination is an inter-penetration of
the particles of different kinds of matter, and that all matter is susceptible of
infinite division. This has proved to be altogether a mistake. If matter were
infinitely divisible in this sense, its particles must be imponderable, and a
million of such molecules could not weigh more than an infinitely small
one. But the particles of that imponderable matter, which, striking upon the
retina, give us the sensation of light, are not in a mathematical sense
infinitely small.

Inter-penetration of elements in the production of a chemical compound,
supposes two distinct bodies, A and B, to occupy one and the same space at
the same time. If this were so, different properties could not consist with an
equal and identical composition.

That hypothesis, however, has shared the fate of innumerable
imaginative explanations of natural phenomena, in which our predecessors
indulged. They have now no advocate. The force of truth, dependent upon
observation, is irresistible. A great many substances have been discovered
amongst organic bodies, composed of the same elements in the same
relative proportions, and yet exhibiting physical and chemical properties
perfectly distinct one from another. To such substances the term Isomeric
(from 1/ao1/ equal and aei1/o1/ part) is applied. A great class of bodies,
known as the volatile oils, oil of turpentine, essence of lemons, oil of
balsam of copaiba, oil of rosemary, oil of juniper, and many others,
differing widely from each other in their odour, in their medicinal effects, in
their boiling point, in their specific gravity, &c., are exactly identical in
composition,—they contain the same elements, carbon and hydrogen, in the
same proportions.

How admirably simple does the chemistry of organic nature present
itself to us from this point of view! An extraordinary variety of compound
bodies produced with equal weights of two elements! and how wide their
dissimilarity! The crystallised part of the oil of roses, the delicious
fragrance of which is so well known, a solid at ordinary temperatures,
although readily volatile, is a compound body containing exactly the same
elements, and in the same proportions, as the gas we employ for lighting
our streets; and, in short, the same elements, in the same relative quantities,
are found in a dozen other compounds, all differing essentially in their
physical and chemical properties.

These remarkable truths, so highly important in their applications, were
not received and admitted as sufficiently established, without abundant
proofs. Many examples have long been known where the analysis of two
different bodies gave the same composition; but such cases were regarded
as doubtful: at any rate, they were isolated observations, homeless in the
realms of science: until, at length, examples were discovered of two or
more bodies whose absolute identity of composition, with totally distinct
properties, could be demonstrated in a more obvious and conclusive manner
than by mere analysis; that is, they can be converted and reconverted into
each other without addition and without subtraction.

In cyanuric acid, hydrated cyanic acid, and cyamelide, we have three
such isomeric compounds.

Cyanuric acid is crystalline, soluble in water, and capable of forming
salts with metallic oxides.

Hydrated cyanic acid is a volatile and highly blistering fluid, which
cannot be brought into contact with water without being instantaneously
decomposed.

Cyamelide is a white substance very like porcelain, absolutely insoluble
in water.

Now if we place the first,—cyanuric acid,—in a vessel hermetically
sealed, and apply a high degree of heat, it is converted by its influence into
hydrated cyanic acid; and, then, if this is kept for some time at the common
temperature, it passes into cyamelide, no other element being present. And,
again inversely, cyamelide can be converted into cyanuric acid and hydrated
cyanic acid.

We have three other bodies which pass through similar changes, in
aldehyde, metaldehyde, and etaldehyde; and, again two, in urea and
cyanuret of ammonia. Further, 100 parts of aldehyde hydrated butyric acid
and acetic ether contain the same elements in the same proportion. Thus one
substance may be converted into another without addition or subtraction,
and without the participation of any foreign bodies in the change.

The doctrine that matter is not infinitely divisible, but on the contrary,
consists of atoms incapable of further division, alone furnishes us with a
satisfactory explanation of these phenomena. In chemical combinations, the
ultimate atoms of bodies do not penetrate each other, they are only arranged
side by side in a certain order, and the properties of the compound depend
entirely upon this order. If they are made to change their place—their mode
of arrangement—by an impulse from without, they combine again in a
different manner, and another compound is formed with totally different
properties. We may suppose that one atom combines with one atom of
another element to form a compound atom, while in other bodies two and
two, four and four, eight and eight, are united; so that in all such

compounds the amount per cent. of the elements is absolutely equal; and yet
their physical and chemical properties must be totally different, the
constitution of each atom being peculiar, in one body consisting of two, in
another of four, in a third of eight, and in a fourth of sixteen simple atoms.

The discovery of these facts immediately led to many most beautiful and
interesting results; they furnished us with a satisfactory explanation of
observations which were before veiled in mystery,—a key to many of
Nature's most curious recesses.

Again; solid bodies, whether simple or compound, are capable of
existing in two states, which are known by the terms amorphous and
crystalline.

When matter is passing from a gaseous or liquid state slowly into a solid,
an incessant motion is observed, as if the molecules were minute magnets;
they are seen to repel each other in one direction, and to attract and cohere
together in another, and in the end become arranged into a regular form,
which under equal circumstances is always the same for any given kind of
matter; that is, crystals are formed.

Time and freedom of motion for the particles of bodies are necessary to
the formation of crystals. If we force a fluid or a gas to become suddenly
solid, leaving no time for its particles to arrange themselves, and cohere in
that direction in which the cohesive attraction is strongest, no crystals will
be formed, but the resulting solid will have a different colour, a different
degree of hardness and cohesion, and will refract light differently; in one
word, will be amorphous. Thus we have cinnabar as a red and a jet-black
substance; sulphur a fixed and brittle body, and soft, semitransparent, and
ductile; glass as a milk-white opaque substance, so hard that it strikes fire
with steel, and in its ordinary and well-known state. These dissimilar states
and properties of the same body are occasioned in one case by a regular, in
the other by an irregular, arrangement of its atoms; one is crystalline, the
other amorphous.

Applying these facts to natural productions, we have reason to believe
that clay-slate, and many kinds of greywacke, are amorphous feldspar, as
transition limestone is amorphous marble, basalt and lava mixtures of

amorphous zeolite and augite. Anything that influences the cohesion, must
also in a certain degree alter the properties of bodies. Carbonate of lime, if
crystallised at ordinary temperatures, possesses the crystalline form,
hardness, and refracting power of common spar; if crystallised at a higher
temperature, it has the form and properties of arragonite.

Finally, Isomorphism, or the equality of form of many chemical
compounds having a different composition, tends to prove that matter
consists of atoms the mere arrangement of which produces all the properties
of bodies. But when we find that a different arrangement of the same
elements gives rise to various physical and chemical properties, and a
similar arrangement of different elements produces properties very much
the same, may we not inquire whether some of those bodies which we
regard as elements may not be merely modifications of the same substance?
—whether they are not the same matter in a different state of arrangement?
We know in fact the existence of iron in two states, so dissimilar, that in the
one, it is to the electric chain like platinum, and in the other it is like zinc;
so that powerful galvanic machines have been constructed of this one metal.

Among the elements are several instances of remarkable similarity of
properties. Thus there is a strong resemblance between platinum and
iridium; bromine and iodine; iron, manganese, and magnesium; cobalt and
nickel; phosphorus and arsenic; but this resemblance consists mainly in
their forming isomorphous compounds in which these elements exist in the
same relative proportion. These compounds are similar, because the atoms
of which they are composed are arranged in the same manner. The converse
of this is also true: nitrate of strontia becomes quite dissimilar to its
common state if a certain proportion of water is taken into its composition.

If we suppose selenium to be merely modified sulphur, and phosphorus
modified arsenic, how does it happen, we must inquire, that sulphuric acid
and selenic acid, phosphoric and arsenic acid, respectively form compounds
which it is impossible to distinguish by their form and solubility? Were
these merely isomeric, they ought to exhibit properties quite dissimilar!

We have not, I believe, at present the remotest ground to suppose that
any one of those substances which chemists regard as elements can be
converted into another. Such a conversion, indeed, would presuppose that

the element was composed of two or more ingredients, and was in fact not
an element; and until the decomposition of these bodies is accomplished,
and their constituents discovered, all pretensions to such conversions
deserve no notice.

Dr. Brown of Edinburgh thought he had converted iron into rhodium,
and carbon or paracyanogen into silicon. His paper upon this subject was
published in the Transactions of the Royal Society of Edinburgh, and
contained internal evidence, without a repetition of his experiments, that he
was totally unacquainted with the principles of chemical analysis. But his
experiments have been carefully repeated by qualified persons, and they
have completely proved his ignorance: his rhodium is iron, and his silicon
an impure incombustible coal.

LETTER VI

My dear Sir,

One of the most remarkable effects of the recent progress of science is
the alliance of chemistry with physiology, by which a new and unexpected
light has been thrown upon the vital processes of plants and animals. We
have now no longer any difficulty in understanding the different actions of
aliments, poisons, and remedial agents—we have a clear conception of the
causes of hunger, of the exact nature of death; and we are not, as formerly,
obliged to content ourselves with a mere description of their symptoms. It is
now ascertained with positive certainty, that all the substances which
constitute the food of man must be divided into two great classes, one of
which serves for the nutrition and reproduction of the animal body, whilst
the other ministers to quite different purposes. Thus starch, gum, sugar,
beer, wine, spirits, &c., furnish no element capable of entering into the
composition of blood, muscular fibre, or any part which is the seat of the
vital principle. It must surely be universally interesting to trace the great
change our views have undergone upon these subjects, as well as to become

acquainted with the researches from which our present knowledge is
derived.

The primary conditions of the maintenance of animal life, are a constant
supply of certain matters, animal food, and of oxygen, in the shape of
atmospheric air. During every moment of life, oxygen is absorbed from the
atmosphere in the organs of respiration, and the act of breathing cannot
cease while life continues.

The observations of physiologists have demonstrated that the body of an
adult man supplied abundantly with food, neither increases nor diminishes
in weight during twenty-four hours, and yet the quantity of oxygen
absorbed into his system, in that period, is very considerable. According to
the experiments of Lavoisier, an adult man takes into his system from the
atmosphere, in one year, no less than 746 pounds weight of oxygen; the
calculations of Menzies make the quantity amount even to 837 pounds; but
we find his weight at the end of the year either exactly the same or different
one way or the other by at most a few pounds. What, it may be asked, has
become of the enormous amount of oxygen thus introduced into the human
system in the course of one year? We can answer this question satisfactorily.
No part of the oxygen remains in the body, but is given out again, combined
with carbon and hydrogen. The carbon and hydrogen of certain parts of the
animal body combine with the oxygen introduced through the lungs and
skin, and pass off in the forms of carbonic acid and vapour of water. At
every expiration and every moment of life, a certain amount of its elements
are separated from the animal organism, having entered into combination
with the oxygen of the atmosphere.

In order to obtain a basis for the approximate calculation, we may
assume, with Lavoisier and Seguin, that an adult man absorbs into his
system 32 1/2 ounces of oxygen daily,—that is, 46,037 cubic inches =
15,661 grains, French weight; and further, that the weight of the whole
mass of his blood is 24 pounds, of which 80 per cent. is water. Now, from
the known composition of the blood, we know that in order to convert its
whole amount of carbon and hydrogen into carbonic acid and water, 64.102
grains of oxygen are required. This quantity will be taken into the system in
four days and five hours. Whether the oxygen enters into combination

directly with the elements of the blood, or with the carbon and hydrogen of
other parts of the body, it follows inevitably—the weight of the body
remaining unchanged and in a normal condition—that as much of these
elements as will suffice to supply 24 pounds of blood, must be taken into
the system in four days and five hours; and this necessary amount is
furnished by the food.

We have not, however, remained satisfied with mere approximation: we
have determined accurately, in certain cases, the quantity of carbon taken
daily in the food, and of that which passes out of the body in the faeces and
urine combined—that is, uncombined with oxygen; and from these
investigations it appears that an adult man taking moderate exercise
consumes 13.9 ounces of carbon, which pass off through the skin and lungs
as carbonic acid gas. [1]

It requires 37 ounces of oxygen to convert 13 9/10 of carbon into
carbonic acid. Again; according to the analysis of Boussingault, (Annales
de Chim. et de Phys., lxx. i. p.136), a horse consumes 79 1/10 ounces of
carbon in twenty-four hours, a milch cow 70 3/4 ounces; so that the horse
requires 13 pounds 3 1/2 ounces, and the cow 11 pounds 10 3/4 ounces of
oxygen. [2]

As no part of the oxygen taken into the system of an animal is given off
in any other form than combined with carbon or hydrogen, and as in a
normal condition, or state of health, the carbon and hydrogen so given off
are replaced by those elements in the food, it is evident that the amount of
nourishment required by an animal for its support must be in a direct ratio
with the quantity of oxygen taken in to its system. Two animals which in
equal times take up by means of the lungs and skin unequal quantities of
oxygen, consume an amount of food unequal in the same ratio. The
consumption of oxygen in a given time may be expressed by the number of
respirations; it is, therefore, obvious that in the same animal the quantity of
nourishment required must vary with the force and number of respirations.
A child breathes quicker than an adult, and, consequently, requires food
more frequently and proportionably in larger quantity, and bears hunger less
easily. A bird deprived of food dies on the third day, while a serpent,
confined under a bell, respires so slowly that the quantity of carbonic acid

generated in an hour can scarcely be observed, and it will live three months,
or longer, without food. The number of respirations is fewer in a state of
rest than during labour or exercise: the quantity of food necessary in both
cases must be in the same ratio. An excess of food, a want of a due amount
of respired oxygen, or of exercise, as also great exercise (which obliges us
to take an increased supply of food), together with weak organs of
digestion, are incompatible with health.

But the quantity of oxygen received by an animal through the lungs not
only depends upon the number of respirations, but also upon the
temperature of the respired air. The size of the thorax of an animal is
unchangeable; we may therefore regard the volume of air which enters at
every inspiration as uniform. But its weight, and consequently the amount
of oxygen it contains, is not constant. Air is expanded by heat, and
contracted by cold—an equal volume of hot and cold air contains, therefore,
an unequal amount of oxygen. In summer atmospheric air contains water in
the form of vapour, it is nearly deprived of it in winter; the volume of
oxygen in the same volume of air is smaller in summer than in winter. In
summer and winter, at the pole and at the equator, we inspire an equal
volume of air; the cold air is warmed during respiration and acquires the
temperature of the body. In order, therefore, to introduce into the lungs a
given amount of oxygen, less expenditure of force is necessary in winter
than in summer, and for the same expenditure of force more oxygen is
inspired in winter. It is also obvious that in an equal number of respirations
we consume more oxygen at the level of the sea than on a mountain.

The oxygen taken into the system is given out again in the same form,
both in summer and winter: we expire more carbon at a low than at a high
temperature, and require more or less carbon in our food in the same
proportion; and, consequently, more is respired in Sweden than in Sicily,
and in our own country and eighth more in winter than in summer. Even if
an equal weight of food is consumed in hot and cold climates, Infinite
Wisdom has ordained that very unequal proportions of carbon shall be taken
in it. The food prepared for the inhabitants of southern climes does not
contain in a fresh state more than 12 per cent. of carbon, while the blubber
and train oil which feed the inhabitants of Polar regions contain 66 to 80 per
cent. of that element.

From the same cause it is comparatively easy to be temperate in warm
climates, or to bear hunger for a long time under the equator; but cold and
hunger united very soon produce exhaustion.

The oxygen of the atmosphere received into the blood in the lungs, and
circulated throughout every part of the animal body, acting upon the
elements of the food, is the source of animal heat.
[Footnote 1: This account is deduced from observations made upon the average
daily consumption of about 30 soldiers in barracks. The food of these men,
consisting of meat, bread, potatoes, lentils, peas, beans, butter, salt, pepper, &c.,
was accurately weighed during a month, and each article subjected to ultimate
analysis. Of the quantity of food, beer, and spirits, taken by the men when out of
barracks, we have a close approximation from the report of the sergeant; and
from the weight and analysis of the faeces and urine, it appears that the carbon
which passes off through these channels may be considered equivalent to the
amount taken in that portion of the food, and of sour-crout, which was not
included in the estimate.]

[Footnote 2: 17.5 ounces = 0.5 kilogramme.]

LETTER VII

My dear Sir,

The source of animal heat, its laws, and the influence it exerts upon the
functions of the animal body, constitute a curious and highly interesting
subject, to which I would now direct your attention.

All living creatures, whose existence depends upon the absorption of
oxygen, possess within themselves a source of heat, independent of
surrounding objects.

This general truth applies to all animals, and extends to the seed of
plants in the act of germination, to flower-buds when developing, and fruits
during their maturation.

In the animal body, heat is produced only in those parts to which arterial
blood, and with it the oxygen absorbed in respiration, is conveyed. Hair,
wool, and feathers, receive no arterial blood, and, therefore, in them no heat
is developed. The combination of a combustible substance with oxygen is,
under all circumstances, the only source of animal heat. In whatever way
carbon may combine with oxygen, the act of combination is accompanied
by the disengagement of heat. It is indifferent whether this combination
takes place rapidly or slowly, at a high or at a low temperature: the amount
of heat liberated is a constant quantity.

The carbon of the food, being converted into carbonic acid within the
body, must give out exactly as much heat as if it had been directly burnt in
oxygen gas or in common air; the only difference is, the production of the
heat is diffused over unequal times. In oxygen gas the combustion of carbon
is rapid and the heat intense; in atmospheric air it burns slower and for a
longer time, the temperature being lower; in the animal body the
combination is still more gradual, and the heat is lower in proportion.

It is obvious that the amount of heat liberated must increase or diminish
with the quantity of oxygen introduced in equal times by respiration. Those
animals, therefore, which respire frequently, and consequently consume
much oxygen, possess a higher temperature than others, which, with a body
of equal size to be heated, take into the system less oxygen. The
temperature of a child (102 deg) is higher than that of an adult (99 1/2 deg).
That of birds (104 deg to 105.4 deg) is higher than that of quadrupeds (98
1/2 deg to 100.4 deg) or than that of fishes or amphibia, whose proper
temperature is from 2.7 to 3.6 deg higher than that of the medium in which
they live. All animals, strictly speaking, are warm-blooded; but in those
only which possess lungs is the temperature of the body quite independent
of the surrounding medium.

The most trustworthy observations prove that in all climates, in the
temperate zones as well as at the equator or the poles, the temperature of the
body in man, and in what are commonly called warm-blooded animals, is
invariably the same; yet how different are the circumstances under which
they live!

The animal body is a heated mass, which bears the same relation to
surrounding objects as any other heated mass. It receives heat when the
surrounding objects are hotter, it loses heat when they are colder, than itself.

We know that the rapidity of cooling increases with the difference
between the temperature of the heated body and that of the surrounding
medium; that is, the colder the surrounding medium the shorter the time
required for the cooling of the heated body.

How unequal, then, must be the loss of heat in a man at Palermo, where
the external temperature is nearly equal to that of the body, and in the polar
regions, where the external temperature is from 70 deg to 90 deg lower!

Yet, notwithstanding this extremely unequal loss of heat, experience has
shown that the blood of the inhabitant of the arctic circle has a temperature
as high as that of the native of the south, who lives in so different a
medium.

This fact, when its true significance is perceived, proves that the heat
given off to the surrounding medium is restored within the body with great
rapidity. This compensation must consequently take place more rapidly in
winter than in summer, at the pole than at the equator.

Now, in different climates the quantity of oxygen introduced into the
system by respiration, as has been already shown, varies according to the
temperature of the external air; the quantity of inspired oxygen increases
with the loss of heat by external cooling, and the quantity of carbon or
hydrogen necessary to combine with this oxygen must be increased in the
same ratio.

It is evident that the supply of the heat lost by cooling is effected by the
mutual action of the elements of the food and the inspired oxygen, which
combine together. To make use of a familiar, but not on that account a less
just illustration, the animal body acts, in this respect, as a furnace, which we
supply with fuel. It signifies nothing what intermediate forms food may
assume, what changes it may undergo in the body; the last change is
uniformly the conversion of its carbon into carbonic acid, and of its
hydrogen into water. The unassimilated nitrogen of the food, along with the

unburned or unoxidised carbon, is expelled in the urine or in the solid
excrements. In order to keep up in the furnace a constant temperature, we
must vary the supply of fuel according to the external temperature, that is,
according to the supply of oxygen.

In the animal body the food is the fuel; with a proper supply of oxygen
we obtain the heat given out during its oxidation or combustion. In winter,
when we take exercise in a cold atmosphere, and when consequently the
amount of inspired oxygen increases, the necessity for food containing
carbon and hydrogen increases in the same ratio; and by gratifying the
appetite thus excited, we obtain the most efficient protection against the
most piercing cold. A starving man is soon frozen to death. The animals of
prey in the arctic regions, as every one knows, far exceed in voracity those
of the torrid zone.

In cold and temperate climates, the air, which incessantly strives to
consume the body, urges man to laborious efforts in order to furnish the
means of resistance to its action, while, in hot climates, the necessity of
labour to provide food is far less urgent.

Our clothing is merely an equivalent for a certain amount of food. The
more warmly we are clothed the less urgent becomes the appetite for food,
because the loss of heat by cooling, and consequently the amount of heat to
be supplied by the food, is diminished.

If we were to go naked, like certain savage tribes, or if in hunting or
fishing we were exposed to the same degree of cold as the Samoyedes, we
should be able with ease to consume 10 lbs. of flesh, and perhaps a dozen of
tallow candles into the bargain, daily, as warmly clad travellers have related
with astonishment of these people. We should then also be able to take the
same quantity of brandy or train oil without bad effects, because the carbon
and hydrogen of these substances would only suffice to keep up the
equilibrium between the external temperature and that of our bodies.

According to the preceding expositions, the quantity of food is regulated
by the number of respirations, by the temperature of the air, and by the
amount of heat given off to the surrounding medium.

No isolated fact, apparently opposed to this statement, can affect the
truth of this natural law. Without temporary or permanent injury to health,
the Neapolitan cannot take more carbon and hydrogen in the shape of food
than he expires as carbonic acid and water; and the Esquimaux cannot
expire more carbon and hydrogen than he takes in the system as food,
unless in a state of disease or of starvation. Let us examine these states a
little more closely.

The Englishman in Jamaica perceives with regret the disappearance of
his appetite, previously a source of frequently recurring enjoyment; and he
succeeds, by the use of cayenne pepper, and the most powerful stimulants,
in enabling himself to take as much food as he was accustomed to eat at
home. But the whole of the carbon thus introduced into the system is not
consumed; the temperature of the air is too high, and the oppressive heat
does not allow him to increase the number of respirations by active
exercise, and thus to proportion the waste to the amount of food taken;
disease of some kind, therefore, ensues.

On the other hand, England sends her sick to southern regions, where the
amount of the oxygen inspired is diminished in a very large proportion.
Those whose diseased digestive organs have in a greater or less degree lost
the power of bringing the food into the state best adapted for oxidation, and
therefore are less able to resist the oxidising influence of the atmosphere of
their native climate, obtain a great improvement in health. The diseased
organs of digestion have power to place the diminished amount of food in
equilibrium with the inspired oxygen, in the mild climate; whilst in a colder
region the organs of respiration themselves would have been consumed in
furnishing the necessary resistance to the action of the atmospheric oxygen.

In our climate, hepatic diseases, or those arising from excess of carbon,
prevail in summer; in winter, pulmonary diseases, or those arising from
excess of oxygen, are more frequent.

The cooling of the body, by whatever cause it may be produced,
increases the amount of food necessary. The mere exposure to the open air,
in a carriage or on the deck of a ship, by increasing radiation and
vaporisation, increases the loss of heat, and compels us to eat more than
usual. The same is true of those who are accustomed to drink large

quantities of cold water, which is given off at the temperature of the body,
98 1/2 deg. It increases the appetite, and persons of weak constitution find it
necessary, by continued exercise, to supply to the system the oxygen
required to restore the heat abstracted by the cold water. Loud and long
continued speaking, the crying of infants, moist air, all exert a decided and
appreciable influence on the amount of food which is taken.

We have assumed that carbon and hydrogen especially, by combining
with oxygen, serve to produce animal heat. In fact, observation proves that
the hydrogen of the food plays a no less important part than the carbon.

The whole process of respiration appears most clearly developed, when
we consider the state of a man, or other animal, totally deprived of food.

The first effect of starvation is the disappearance of fat, and this fat
cannot be traced either in the urine or in the scanty faeces. Its carbon and
hydrogen have been given off through the skin and lungs in the form of
oxidised products; it is obvious that they have served to support respiration.

In the case of a starving man, 32 1/2 oz. of oxygen enter the system
daily, and are given out again in combination with a part of his body. Currie
mentions the case of an individual who was unable to swallow, and whose
body lost 100 lbs. in weight during a month; and, according to Martell
(Trans. Linn. Soc., vol. xi. p.411), a fat pig, overwhelmed in a slip of earth,
lived 160 days without food, and was found to have diminished in weight,
in that time, more than 120 lbs. The whole history of hybernating animals,
and the well-established facts of the periodical accumulation, in various
animals, of fat, which, at other periods, entirely disappears, prove that the
oxygen, in the respiratory process, consumes, without exception, all such
substances as are capable of entering into combination with it. It combines
with whatever is presented to it; and the deficiency of hydrogen is the only
reason why carbonic acid is the chief product; for, at the temperature of the
body, the affinity of hydrogen for oxygen far surpasses that of carbon for
the same element.

We know, in fact, that the graminivora expire a volume of carbonic acid
equal to that of the oxygen inspired, while the carnivora, the only class of
animals whose food contains fat, inspire more oxygen than is equal in

volume to the carbonic acid expired. Exact experiments have shown, that in
many cases only half the volume of oxygen is expired in the form of
carbonic acid. These observations cannot be gainsaid, and are far more
convincing than those arbitrary and artificially produced phenomena,
sometimes called experiments; experiments which, made as too often they
are, without regard to the necessary and natural conditions, possess no
value, and may be entirely dispensed with; especially when, as in the
present case, Nature affords the opportunity for observation, and when we
make a rational use of that opportunity.

In the progress of starvation, however, it is not only the fat which
disappears, but also, by degrees all such of the solids as are capable of
being dissolved. In the wasted bodies of those who have suffered starvation,
the muscles are shrunk and unnaturally soft, and have lost their
contractibility; all those parts of the body which were capable of entering
into the state of motion have served to protect the remainder of the frame
from the destructive influence of the atmosphere. Towards the end, the
particles of the brain begin to undergo the process of oxidation, and
delirium, mania, and death close the scene; that is to say, all resistance to
the oxidising power of the atmospheric oxygen ceases, and the chemical
process of eremacausis, or decay, commences, in which every part of the
body, the bones excepted, enters into combination with oxygen.

The time which is required to cause death by starvation depends on the
amount of fat in the body, on the degree of exercise, as in labour or exertion
of any kind, on the temperature of the air, and finally, on the presence or
absence of water. Through the skin and lungs there escapes a certain
quantity of water, and as the presence of water is essential to the
continuance of the vital motions, its dissipation hastens death. Cases have
occurred, in which a full supply of water being accessible to the sufferer,
death has not occurred till after the lapse of twenty days. In one case, life
was sustained in this way for the period of sixty days.

In all chronic diseases death is produced by the same cause, namely, the
chemical action of the atmosphere. When those substances are wanting,
whose function in the organism is to support the process of respiration,
when the diseased organs are incapable of performing their proper function

of producing these substances, when they have lost the power of
transforming the food into that shape in which it may, by entering into
combination with the oxygen of the air, protect the system from its
influence, then, the substance of the organs themselves, the fat of the body,
the substance of the muscles, the nerves, and the brain, are unavoidably
consumed.

The true cause of death in these cases is the respiratory process, that is,
the action of the atmosphere.

A deficiency of food, and a want of power to convert the food into a part
of the organism, are both, equally, a want of resistance; and this is the
negative cause of the cessation of the vital process. The flame is
extinguished, because the oil is consumed; and it is the oxygen of the air
which has consumed it.

In many diseases substances are produced which are incapable of
assimilation. By the mere deprivation of food, these substances are removed
from the body without leaving a trace behind; their elements have entered
into combination with the oxygen of the air.

From the first moment that the function of the lungs or of the skin is
interrupted or disturbed, compounds, rich in carbon, appear in the urine,
which acquires a brown colour. Over the whole surface of the body oxygen
is absorbed, and combines with all the substances which offer no resistance
to it. In those parts of the body where the access of oxygen is impeded; for
example, in the arm-pits, or in the soles of the feet, peculiar compounds are
given out, recognisable by their appearance, or by their odour. These
compounds contain much carbon.

Respiration is the falling weight—the bent spring, which keeps the clock
in motion; the inspirations and expirations are the strokes of the pendulum
which regulate it. In our ordinary time-pieces, we know with mathematical
accuracy the effect produced on their rate of going, by changes in the length
of the pendulum, or in the external temperature. Few, however, have a clear
conception of the influence of air and temperature on the health of the
human body; and yet the research into the conditions necessary to keep it in
the normal state is not more difficult than in the case of a clock.

LETTER VIII

My dear Sir,

Having attempted in my last letter to explain to you the simple and
admirable office subserved by the oxygen of the atmosphere in its
combination with carbon in the animal body, I will now proceed to present
you with some remarks upon those materials which sustain its mechanisms
in motion, and keep up their various functions,—namely, the Aliments.

If the increase in mass in an animal body, the development and
reproduction of its organs depend upon the blood, then those substances
only which are capable of being converted into blood can be properly
regarded as nourishment. In order then to ascertain what parts of our food
are nutritious, we must compare the composition of the blood with the
composition of the various articles taken as food.

Two substances require especial consideration as the chief ingredients of
the blood; one of these separates immediately from the blood when it is
withdrawn from the circulation.

It is well known that in this case blood coagulates, and separates into a
yellowish liquid, the serum of the blood, and a gelatinous mass, which
adheres to a rod or stick in soft, elastic fibres, when coagulating blood is
briskly stirred. This is the fibrine of the blood, which is identical in all its
properties with muscular fibre, when the latter is purified from all foreign
matters.

The second principal ingredient of the blood is contained in the serum,
and gives to this liquid all the properties of the white of eggs, with which it
is indeed identical. When heated, it coagulates into a white elastic mass, and
the coagulating substance is called albumen.

Fibrine and albumen, the chief ingredients of blood, contain, in all, seven
chemical elements, among which nitrogen, phosphorus, and sulphur are
found. They contain also the earth of bones. The serum retains in solution
sea salt and other salts of potash and soda, in which the acids are carbonic,
phosphoric, and sulphuric acids. The globules of the blood contain fibrine
and albumen, along with a red colouring matter, in which iron is a constant
element. Besides these, the blood contains certain fatty bodies in small
quantity, which differ from ordinary fats in several of their properties.

Chemical analysis has led to the remarkable result, that fibrine and
albumen contain the same organic elements united in the same proportion,
—i.e., that they are isomeric, their chemical composition—the proportion of
their ultimate elements—being identical. But the difference of their external
properties shows that the particles of which they are composed are arranged
in a different order. (See Letter V).

This conclusion has lately been beautifully confirmed by a distinguished
physiologist (Denis), who has succeeded in converting fibrine into albumen,
that is, in giving it the solubility, and coagulability by heat, which
characterise the white of egg.

Fibrine and albumen, besides having the same composition, agree also in
this, that both dissolve in concentrated muriatic acid, yielding a solution of
an intense purple colour. This solution, whether made with fibrine or
albumen, has the very same re-actions with all substances yet tried.

Both albumen and fibrine, in the process of nutrition, are capable of
being converted into muscular fibre, and muscular fibre is capable of being
reconverted into blood. These facts have long been established by
physiologists, and chemistry has merely proved that these metamorphoses
can be accomplished under the influence of a certain force, without the aid
of a third substance, or of its elements, and without the addition of any
foreign element, or the separation of any element previously present in
these substances.

If we now compare the composition of all organised parts with that of
fibrine and albumen, the following relations present themselves:—

All parts of the animal body which have a decided shape, which form
parts of organs, contain nitrogen. No part of an organ which possesses
motion and life is destitute of nitrogen; all of them contain likewise carbon
and the elements of water; the latter, however, in no case in the proportion
to form water.

The chief ingredients of the blood contain nearly 17 per cent. of
nitrogen, and from numerous analyses it appears that no part of an organ
contains less than 17 per cent. of nitrogen.

The most convincing experiments and observations have proved that the
animal body is absolutely incapable of producing an elementary body, such
as carbon or nitrogen, out of substances which do not contain it; and it
obviously follows, that all kinds of food fit for the production either of
blood, or of cellular tissue, membranes, skin, hair, muscular fibre, &c., must
contain a certain amount of nitrogen, because that element is essential to the
composition of the above-named organs; because the organs cannot create it
from the other elements presented to them; and, finally, because no nitrogen
is absorbed from the atmosphere in the vital process.

The substance of the brain and nerves contains a large quantity of
albumen, and, in addition to this, two peculiar fatty acids, distinguished
from other fats by containing phosphorus (phosphoric acid?). One of these
contains nitrogen (Fremy).

Finally, water and common fat are those ingredients of the body which
are destitute of nitrogen. Both are amorphous or unorganised, and only so
far take part in the vital process as that their presence is required for the due
performance of the vital functions. The inorganic constituents of the body
are, iron, lime, magnesia, common salt, and the alkalies.

The nutritive process is seen in its simplest form in carnivorous animals.
This class of animals lives on the blood and flesh of the graminivora; but
this blood and flesh are, in all their properties, identical with their own.
Neither chemical nor physiological differences can be discovered.

The nutriment of carnivorous animals is derived originally from blood;
in their stomach it becomes dissolved, and capable of reaching all other

parts of the body; in its passage it is again converted into blood, and from
this blood are reproduced all those parts of their organisation which have
undergone change or metamorphosis.

With the exception of hoofs, hair, feathers, and the earth of bones, every
part of the food of carnivorous animals is capable of assimilation.

In a chemical sense, therefore, it may be said that a carnivorous animal,
in supporting the vital process, consumes itself. That which serves for its
nutrition is identical with those parts of its organisation which are to be
renewed.

The process of nutrition in graminivorous animals appears at first sight
altogether different. Their digestive organs are less simple, and their food
consists of vegetables, the great mass of which contains but little nitrogen.

From what substances, it may be asked, is the blood formed, by means
of which of their organs are developed? This question may be answered
with certainty.

Chemical researches have shown, that all such parts of vegetables as can
afford nutriment to animals contain certain constituents which are rich in
nitrogen; and the most ordinary experience proves that animals require for
their support and nutrition less of these parts of plants in proportion as they
abound in the nitrogenised constituents. Animals cannot be fed on matters
destitute of these nitrogenised constituents.

These important products of vegetation are especially abundant in the
seeds of the different kinds of grain, and of peas, beans, and lentils; in the
roots and the juices of what are commonly called vegetables. They exist,
however, in all plants, without exception, and in every part of plants in
larger or smaller quantity.

These nitrogenised forms of nutriment in the vegetable kingdom may be
reduced to three substances, which are easily distinguished by their external
characters. Two of them are soluble in water, the third is insoluble.

When the newly-expressed juices of vegetables are allowed to stand, a
separation takes place in a few minutes. A gelatinous precipitate, commonly
of a green tinge, is deposited, and this, when acted on by liquids which
remove the colouring matter, leaves a grayish white substance, well known
to druggists as the deposite from vegetable juices. This is one of the
nitrogenised compounds which serves for the nutrition of animals, and has
been named vegetable fibrine. The juice of grapes is especially rich in this
constituent, but it is most abundant in the seeds of wheat, and of the cerealia
generally. It may be obtained from wheat flour by a mechanical operation,
and in a state of tolerable purity; it is then called gluten, but the glutinous
property belongs, not to vegetable fibrine, but to a foreign substance,
present in small quantity, which is not found in the other cerealia.

The method by which it is obtained sufficiently proves that it is insoluble
in water; although we cannot doubt that it was originally dissolved in the
vegetable juice, from which it afterwards separated, exactly as fibrine does
from blood.

The second nitrogenised compound remains dissolved in the juice after
the separation of the fibrine. It does not separate from the juice at the
ordinary temperature, but is instantly coagulated when the liquid containing
it is heated to the boiling point.

When the clarified juice of nutritious vegetables, such as cauliflower,
asparagus, mangelwurzel, or turnips, is made to boil, a coagulum is formed,
which it is absolutely impossible to distinguish from the substance which
separates as a coagulum, when the serum of blood, or the white of an egg,
diluted with water, are heated to the boiling point. This is vegetable
albumen. It is found in the greatest abundance in certain seeds, in nuts,
almonds, and others, in which the starch of the gramineae is replaced by oil.

The third nitrogenised constituent of the vegetable food of animals is
vegetable caseine. It is chiefly found in the seeds of peas, beans, lentils, and
similar leguminous seeds. Like vegetable albumen, it is soluble in water,
but differs from it in this, that its solution is not coagulated by heat. When
the solution is heated or evaporated, a skin forms on its surface, and the
addition of an acid causes a coagulum, just as in animal milk.

These three nitrogenised compounds, vegetable fibrine, albumen, and
caseine, are the true nitrogenised constituents of the food of graminivorous
animals; all other nitrogenised compounds occurring in plants, are either
rejected by animals, as in the case of the characteristic principles of
poisonous and medicinal plants, or else they occur in the food in such very
small proportion, that they cannot possibly contribute to the increase of
mass in the animal body.

The chemical analysis of these three substances has led to the very
interesting result that they contain the same organic elements, united in the
same proportion by weight; and, what is still more remarkable, that they are
identical in composition with the chief constituents of blood, animal fibrine,
and albumen. They all three dissolve in concentrated muriatic acid with the
same deep purple colour, and even in their physical characters, animal
fibrine and albumen are in no respect different from vegetable fibrine and
albumen. It is especially to be noticed, that by the phrase, identity of
composition, we do not here intend mere similarity, but that even in regard
to the presence and relative amount of sulphur, phosphorus, and phosphate
of lime, no difference can be observed.

How beautifully and admirably simple, with the aid of these discoveries,
appears the process of nutrition in animals, the formation of their organs, in
which vitality chiefly resides! Those vegetable principles, which in animals
are used to form blood, contain the chief constituents of blood, fibrine and
albumen, ready formed, as far as regards their composition. All plants,
besides, contain a certain quantity of iron, which reappears in the colouring
matter of the blood. Vegetable fibrine and animal fibrine, vegetable
albumen and animal albumen, hardly differ, even in form; if these principles
be wanting in the food, the nutrition of the animal is arrested; and when
they are present, the graminivorous animal obtains in its food the very same
principles on the presence of which the nutrition of the carnivora entirely
depends.

Vegetables produce in their organism the blood of all animals, for the
carnivora, in consuming the blood and flesh of the graminivora, consume,
strictly speaking, only the vegetable principles which have served for the
nutrition of the latter. Vegetable fibrine and albumen take the form in the

stomach of the graminivorous animal as animal fibrine and albumen do in
that of the carnivorous animal.

From what has been said, it follows that the development of the animal
organism and its growth are dependent on the reception of certain principles
identical with the chief constituents of blood.

In this sense we may say that the animal organism gives to the blood
only its form; that it is incapable of creating blood out of other substances
which do not already contain the chief constituents of that fluid. We cannot,
indeed, maintain that the animal organism has no power to form other
compounds, for we know that it is capable of producing an extensive series
of compounds, differing in composition from the chief constituents of
blood; but these last, which form the starting-point of the series, it cannot
produce.

The animal organism is a higher kind of vegetable, the development of
which begins with those substances with the production of which the life of
an ordinary vegetable ends. As soon as the latter has borne seed, it dies, or a
period of its life comes to a termination.

In that endless series of compounds, which begins with carbonic acid,
ammonia, and water, the sources of the nutrition of vegetables, and includes
the most complex constituents of the animal brain, there is no blank, no
interruption. The first substance capable of affording nutriment to animals
is the last product of the creative energy of vegetables.

The substance of cellular tissue and of membranes, of the brain and
nerves, these the vegetable cannot produce.

The seemingly miraculous in the productive agency of vegetables
disappears in a great degree, when we reflect that the production of the
constituents of blood cannot appear more surprising than the occurrence of
the fat of beef and mutton in cocoa beans, of human fat in olive-oil, of the
principal ingredient of butter in palm-oil, and of horse fat and train-oil in
certain oily seeds.

LETTER IX

My dear Sir,

The facts detailed in my last letter will satisfy you as to the manner in
which the increase of mass in an animal, that is, its growth, is
accomplished; we have still to consider a most important question, namely,
the function performed in the animal system by substances destitute of
nitrogen; such as sugar, starch, gum, pectine, &c.

The most extensive class of animals, the graminivora, cannot live
without these substances; their food must contain a certain amount of one or
more of them, and if these compounds are not supplied, death quickly
ensues.

This important inquiry extends also to the constituents of the food of
carnivorous animals in the earliest periods of life; for this food also contains
substances, which are not necessary for their support in the adult state. The
nutrition of the young of carnivora is obviously accomplished by means
similar to those by which the graminivora are nourished; their development
is dependent on the supply of a fluid, which the body of the mother secretes
in the shape of milk.

Milk contains only one nitrogenised constituent, known under the name
of caseine; besides this, its chief ingredients are butter (fat), and sugar of
milk. The blood of the young animal, its muscular fibre, cellular tissue,
nervous matter, and bones, must have derived their origin from the
nitrogenised constituent of milk—the caseine; for butter and sugar of milk
contain no nitrogen.

Now, the analysis of caseine has led to the result, which, after the details
I have given, can hardly excite your surprise, that this substance also is
identical in composition with the chief constituents of blood, fibrine and
albumen. Nay more—a comparison of its properties with those of vegetable

caseine has shown—that these two substances are identical in all their
properties; insomuch, that certain plants, such as peas, beans, and lentils,
are capable of producing the same substance which is formed from the
blood of the mother, and employed in yielding the blood of the young
animal.

The young animal, therefore, receives in the form of caseine,—which is
distinguished from fibrine and albumen by its great solubility, and by not
coagulating when heated,—the chief constituent of the mother's blood. To
convert caseine into blood no foreign substance is required, and in the
conversion of the mother's blood into caseine, no elements of the
constituents of the blood have been separated. When chemically examined,
caseine is found to contain a much larger proportion of the earth of bones
than blood does, and that in a very soluble form, capable of reaching every
part of the body. Thus, even in the earliest period of its life, the
development of the organs, in which vitality resides, is, in the carnivorous
animal, dependent on the supply of a substance, identical in organic
composition with the chief constituents of its blood.

What, then, is the use of the butter and the sugar of milk? How does it
happen that these substances are indispensable to life?

Butter and sugar of milk contain no fixed bases, no soda nor potash.
Sugar of milk has a composition closely allied to that of the other kinds of
sugar, of starch, and of gum; all of them contain carbon and the elements of
water, the latter precisely in the proportion to form water.

There is added, therefore, by means of these compounds, to the
nitrogenised constituents of food, a certain amount of carbon; or, as in the
case of butter, of carbon and hydrogen; that is, an excess of elements, which
cannot possibly be employed in the production of blood, because the
nitrogenised substances contained in the food already contain exactly the
amount of carbon which is required for the production of fibrine and
albumen.

In an adult carnivorous animal, which neither gains nor loses weight,
perceptibly, from day to day, its nourishment, the waste of organised tissue,

and its consumption of oxygen, stand to each other in a well-defined and
fixed relation.

The carbon of the carbonic acid given off, with that of the urine; the
nitrogen of the urine, and the hydrogen given off as ammonia and water;
these elements, taken together, must be exactly equal in weight to the
carbon, nitrogen, and hydrogen of the metamorphosed tissues, and since
these last are exactly replaced by the food, to the carbon, nitrogen, and
hydrogen of the food. Were this not the case, the weight of the animal could
not possibly remain unchanged.

But, in the young of the carnivora, the weight does not remain
unchanged; on the contrary, it increases from day to day by an appreciable
quantity.

This fact presupposes, that the assimilative process in the young animal
is more energetic, more intense, than the process of transformation in the
existing tissues. If both processes were equally active, the weight of the
body could not increase; and were the waste by transformation greater, the
weight of the body would decrease.

Now, the circulation in the young animal is not weaker, but, on the
contrary, more rapid; the respirations are more frequent; and, for equal
bulks, the consumption of oxygen must be greater rather than smaller in the
young than in the adult animal. But, since the metamorphosis of organised
parts goes on more slowly, there would ensue a deficiency of those
substances, the carbon and hydrogen of which are adapted for combination
with oxygen; because, in the carnivora, nature has destined the new
compounds, produced by the metamorphosis of organised parts, to furnish
the necessary resistance to the action of the oxygen, and to produce animal
heat. What is wanting for these purposes an Infinite Wisdom has supplied to
the young in its natural food.

The carbon and hydrogen of butter, and the carbon of the sugar of milk,
no part of either of which can yield blood, fibrine, or albumen, are destined
for the support of the respiratory process, at an age when a greater
resistance is opposed to the metamorphosis of existing organisms; or, in

other words, to the production of compounds, which, in the adult state, are
produced in quantity amply sufficient for the purpose of respiration.

The young animal receives the constituents of its blood in the caseine of
the milk. A metamorphosis of existing organs goes on, for bile and urine are
secreted; the materials of the metamorphosed parts are given off in the form
of urine, of carbonic acid, and of water; but the butter and sugar of milk
also disappear; they cannot be detected in the faeces.

The butter and sugar of milk are given out in the form of carbonic acid
and water, and their conversion into oxidised products furnishes the clearest
proof that far more oxygen is absorbed than is required to convert the
carbon and hydrogen of the metamorphosed tissues into carbonic acid and
water.

The change and metamorphosis of organised tissues going on in the vital
process in the young animal, consequently yield, in a given time, much less
carbon and hydrogen in the form adapted for the respiratory process than
correspond to the oxygen taken up in the lungs. The substance of its
organised parts would undergo a more rapid consumption, and would
necessarily yield to the action of the oxygen, were not the deficiency of
carbon and hydrogen supplied from another source.

The continued increase of mass, or growth, and the free and unimpeded
development of the organs in the young animal, are dependent on the
presence of foreign substances, which, in the nutritive process, have no
other function than to protect the newly-formed organs from the action of
the oxygen. The elements of these substances unite with the oxygen; the
organs themselves could not do so without being consumed; that is, growth,
or increase of mass in the body,—the consumption of oxygen remaining the
same,—would be utterly impossible.

The preceding considerations leave no doubt as to the purpose for which
Nature has added to the food of the young of carnivorous mammalia
substances devoid of nitrogen, which their organism cannot employ for
nutrition, strictly so called, that is, for the production of blood; substances
which may be entirely dispensed with in their nourishment in the adult
state. In the young of carnivorous birds, the want of all motion is an

obvious cause of diminished waste in the organised parts; hence, milk is not
provided for them.

The nutritive process in the carnivora thus presents itself under two
distinct forms; one of which we again meet with in the graminivora.

In graminivorous animals, we observe, that during their whole life, their
existence depends on a supply of substances having a composition identical
with that of sugar of milk, or closely resembling it. Everything that they
consume as food contains a certain quantity of starch, gum, or sugar, mixed
with other matters.

The function performed in the vital process of the graminivora by these
substances is indicated in a very clear and convincing manner, when we
take into consideration the very small relative amount of the carbon which
these animals consume in the nitrogenised constituents of their food, which
bears no proportion whatever to the oxygen absorbed through the skin and
lungs.

A horse, for example, can be kept in perfectly good condition, if he
obtain as food 15 lbs. of hay and 4 1/2 lbs. of oats daily. If we now calculate
the whole amount of nitrogen in these matters, as ascertained by analysis (1
1/2 per cent. in the hay, 2.2 per cent. in the oats), in the form of blood, that
is, as fibrine and albumen, with the due proportion of water in blood (80 per
cent.), the horse receives daily no more than 4 1/2 oz. of nitrogen,
corresponding to about 8 lbs. of blood. But along with this nitrogen, that is,
combined with it in the form of fibrine or albumen, the animal receives only
about 14 1/2 oz. of carbon.

Without going further into the calculation, it will readily be admitted,
that the volume of air inspired and expired by a horse, the quantity of
oxygen consumed, and, as a necessary consequence, the amount of carbonic
acid given out by the animal, are much greater than in the respiratory
process in man. But an adult man consumes daily abut 14 oz. of carbon, and
the determination of Boussingault, according to which a horse expires 79
oz. daily, cannot be very far from the truth.

In the nitrogenised constituents of his food, therefore, the horse receives
rather less than the fifth part of the carbon which his organism requires for
the support of the respiratory process; and we see that the wisdom of the
Creator has added to his food the four-fifths which are wanting, in various
forms, as starch, sugar, &c. with which the animal must be supplied, or his
organism will be destroyed by the action of the oxygen.

It is obvious, that in the system of the graminivora, whose food contains
so small a portion, relatively, of the constituents of the blood, the process of
metamorphosis in existing tissues, and consequently their restoration or
reproduction, must go on far less rapidly than in the carnivora. Were this not
the case, a vegetation a thousand times more luxuriant than the actual one
would not suffice for their nourishment. Sugar, gum, and starch, would no
longer be necessary to support life in these animals, because, in that case,
the products of the waste, or metamorphosis of the organised tissues, would
contain enough carbon to support the respiratory process.

LETTER X

My dear Sir,

Let me now apply the principles announced in the preceding letters to
the circumstances of our own species. Man, when confined to animal food,
requires for his support and nourishment extensive sources of food, even
more widely extended than the lion and tiger, because, when he has the
opportunity, he kills without eating.

A nation of hunters, on a limited space, is utterly incapable of increasing
its numbers beyond a certain point, which is soon attained. The carbon
necessary for respiration must be obtained from the animals, of which only
a limited number can live on the space supposed. These animals collect
from plants the constituents of their organs and of their blood, and yield
them, in turn, to the savages who live by the chase alone. They, again,

receive this food unaccompanied by those compounds, destitute of nitrogen,
which, during the life of the animals, served to support the respiratory
process. In such men, confined to an animal diet, it is the carbon of the flesh
and of the blood which must take the place of starch and sugar.

But 15 lbs. of flesh contain no more carbon than 4 lbs. of starch, and
while the savage with one animal and an equal weight of starch should
maintain life and health for a certain number of days, he would be
compelled, if confined to flesh alone, in order to procure the carbon
necessary for respiration, during the same time, to consume five such
animals.

It is easy to see, from these considerations, how close the connection is
between agriculture and the multiplication of the human species. The
cultivation of our crops has ultimately no other object than the production
of a maximum of those substances which are adapted for assimilation and
respiration, in the smallest possible space. Grain and other nutritious
vegetables yield us, not only in starch, sugar, and gum, the carbon which
protects our organs from the action of oxygen, and produces in the
organism the heat which is essential to life, but also in the form of vegetable
fibrine, albumen, and caseine, our blood, from which the other parts of our
body are developed.

Man, when confined to animal food, respires, like the carnivora, at the
expense of the matters produced by the metamorphosis of organised tissues;
and, just as the lion, tiger, hyaena, in the cages of a menagerie, are
compelled to accelerate the waste of the organised tissues by incessant
motion, in order to furnish the matter necessary for respiration, so, the
savage, for the very same object, is forced to make the most laborious
exertions, and go through a vast amount of muscular exercise. He is
compelled to consume force merely in order to supply matter for
respiration.

Cultivation is the economy of force. Science teaches us the simplest
means of obtaining the greatest effect with the smallest expenditure of
power, and with given means to produce a maximum of force. The
unprofitable exertion of power, the waste of force in agriculture, in other

branches of industry, in science, or in social economy, is characteristic of
the savage state, or of the want of knowledge.

In accordance with what I have already stated, you will perceive that the
substances of which the food of man is composed may be divided into two
classes; into nitrogenised and non-nitrogenised. The former are capable of
conversion into blood; the latter are incapable of this transformation.

Out of those substances which are adapted to the formation of blood, are
formed all the organised tissues. The other class of substances, in the
normal state of health, serve to support the process of respiration. The
former may be called the plastic elements of nutrition; the latter, elements
of respiration.

Among the former we reckon—

Vegetable fibrine.
Vegetable albumen.
Vegetable caseine.
Animal flesh.
Animal blood.

Among the elements of respiration in our food, are—

Fat. Pectine.
Starch. Bassorine.
Gum. Wine.
Cane sugar. Beer.
Grape sugar. Spirits.
Sugar of milk.

The most recent and exact researches have established as a universal
fact, to which nothing yet known is opposed, that the nitrogenised
constituents of vegetable food have a composition identical with that of the
constituents of the blood.

No nitrogenised compound, the composition of which differs from that
of fibrine, albumen, and caseine, is capable of supporting the vital process

in animals.

The animal organism unquestionably possesses the power of forming,
from the constituents of its blood, the substance of its membranes and
cellular tissue, of the nerves and brain, and of the organic part of cartilages
and bones. But the blood must be supplied to it perfect in everything but its
form—that is, in its chemical composition. If this be not done, a period is
rapidly put to the formation of blood, and consequently to life.

This consideration enables us easily to explain how it happens that the
tissues yielding gelatine or chondrine, as, for example, the gelatine of skin
or of bones, are not adapted for the support of the vital process; for their
composition is different from that of fibrine or albumen. It is obvious that
this means nothing more than that those parts of the animal organism which
form the blood do not possess the power of effecting a transformation in the
arrangement of the elements of gelatine, or of those tissues which contain it.
The gelatinous tissues, the gelatine of the bones, the membranes, the cells
and the skin suffer, in the animal body, under the influence of oxygen and
moisture, a progressive alteration; a part of these tissues is separated, and
must be restored from the blood; but this alteration and restoration are
obviously confined within very narrow limits.

While, in the body of a starving or sick individual, the fat disappears and
the muscular tissue takes once more the form of blood, we find that the
tendons and membranes retain their natural condition, and the limbs of the
dead body their connections, which depend on the gelatinous tissues.

On the other hand, we see that the gelatine of bones devoured by a dog
entirely disappears, while only the bone earth is found in his excrements.
The same is true of man, when fed on food rich in gelatine, as, for example,
strong soup. The gelatine is not to be found either in the urine or in the
faeces, and consequently must have undergone a change, and must have
served some purpose in the animal economy. It is clear that the gelatine
must be expelled from the body in a form different from that in which it
was introduced as food.

When we consider the transformation of the albumen of the blood into a
part of an organ composed of fibrine, the identity in composition of the two

substances renders the change easily conceivable. Indeed we find the
change of a dissolved substance into an insoluble organ of vitality,
chemically speaking, natural and easily explained, on account of this very
identity of composition. Hence the opinion is not unworthy of a closer
investigation, that gelatine, when taken in the dissolved state, is again
converted, in the body, into cellular tissue, membrane and cartilage; that it
may serve for the reproduction of such parts of these tissues as have been
wasted, and for their growth.

And when the powers of nutrition in the whole body are affected by a
change of the health, then, even should the power of forming blood remain
the same, the organic force by which the constituents of the blood are
transformed into cellular tissue and membranes must necessarily be
enfeebled by sickness. In the sick man, the intensity of the vital force, its
power to produce metamorphoses, must be diminished as well in the
stomach as in all other parts of the body. In this condition, the uniform
experience of practical physicians shows that gelatinous matters in a
dissolved state exercise a most decided influence on the state of the health.
Given in a form adapted for assimilation, they serve to husband the vital
force, just as may be done, in the case of the stomach, by due preparation of
the food in general.

Brittleness in the bones of graminivorous animals is clearly owing to a
weakness in those parts of the organism whose function it is to convert the
constituents of the blood into cellular tissue and membrane; and if we can
trust to the reports of physicians who have resided in the East, the Turkish
women, in their diet of rice, and in the frequent use of enemata of strong
soup, have united the conditions necessary for the formation both of cellular
tissue and of fat.

LETTER XI

My dear Sir,

In the immense, yet limited expanse of the ocean, the animal and
vegetable kingdoms are mutually dependent upon, and successive to each
other. The animals obtain their constituent elements from the plants, and
restore them to the water in their original form, when they again serve as
nourishment to a new generation of plants.

The oxygen which marine animals withdraw in their respiration from the
air, dissolved in sea water, is returned to the water by the vital processes of
sea plants; that air is richer in oxygen than atmospheric air, containing 32 to
33 per cent. Oxygen, also, combines with the products of the putrefaction of
dead animal bodies, changes their carbon into carbonic acid, their hydrogen
into water, and their nitrogen assumes again the form of ammonia.

Thus we observe in the ocean a circulation takes place without the
addition or subtraction of any element, unlimited in duration, although
limited in extent, inasmuch as in a confined space the nourishment of plants
exists in a limited quantity.

We well know that marine plants cannot derive a supply of humus for
their nourishment through their roots. Look at the great sea-tang, the Fucus
giganteus: this plant, according to Cook, reaches a height of 360 feet, and a
single specimen, with its immense ramifications, nourishes thousands of
marine animals, yet its root is a small body, no larger than the fist. What
nourishment can this draw from a naked rock, upon the surface of which
there is no perceptible change? It is quite obvious that these plants require
only a hold,—a fastening to prevent a change of place,—as a counterpoise
to their specific gravity, which is less than that of the medium in which they
float. That medium provides the necessary nourishment, and presents it to
the surface of every part of the plant. Sea-water contains not only carbonic

acid and ammonia, but the alkaline and earthy phosphates and carbonates
required by these plants for their growth, and which we always find as
constant constituents of their ashes.

All experience demonstrates that the conditions of the existence of
marine plants are the same which are essential to terrestrial plants. But the
latter do not live like sea-plants, in a medium which contains all their
elements and surrounds with appropriate nourishment every part of their
organs; on the contrary, they require two media, of which one, namely the
soil, contains those essential elements which are absent from the medium
surrounding them, i.e. the atmosphere.

Is it possible that we could ever be in doubt respecting the office which
the soil and its component parts subserve in the existence and growth of
vegetables?—that there should have been a time when the mineral elements
of plants were not regarded as absolutely essential to their vitality? Has not
the same circulation been observed on the surface of the earth which we
have just contemplated in the ocean,—the same incessant change,
disturbance and restitution of equilibrium?

Experience in agriculture shows that the production of vegetables on a
given surface increases with the supply of certain matters, originally parts
of the soil which had been taken up from it by plants—the excrements of
man and animals. These are nothing more than matters derived from
vegetable food, which in the vital processes of animals, or after their death,
assume again the form under which they originally existed, as parts of the
soil. Now, we know that the atmosphere contains none of these substances,
and therefore can replace none; and we know that their removal from a soil
destroys its fertility, which may be restored and increased by a new supply.

Is it possible, after so many decisive investigations into the origin of the
elements of animals and vegetables, the use of the alkalies, of lime and the
phosphates, any doubt can exist as to the principles upon which a rational
agriculture depends? Can the art of agriculture be based upon anything but
the restitution of a disturbed equilibrium? Can it be imagined that any
country, however rich and fertile, with a flourishing commerce, which for
centuries exports its produce in the shape of grain and cattle, will maintain
its fertility, if the same commerce does not restore, in some form of manure,

those elements which have been removed from the soil, and which cannot
be replaced by the atmosphere? Must not the same fate await every such
country which has actually befallen the once prolific soil of Virginia, now in
many parts no longer able to grow its former staple productions—wheat and
tobacco?

In the large towns of England the produce both of English and foreign
agriculture is largely consumed; elements of the soil indispensable to plants
do not return to the fields,—contrivances resulting from the manners and
customs of English people, and peculiar to them, render it difficult, perhaps
impossible, to collect the enormous quantity of the phosphates which are
daily, as solid and liquid excrements, carried into the rivers. These
phosphates, although present in the soil in the smallest quantity, are its most
important mineral constituents. It was observed that many English fields
exhausted in that manner immediately doubled their produce, as if by a
miracle, when dressed with bone earth imported from the Continent. But if
the export of bones from Germany is continued to the extent it has hitherto
reached, our soil must be gradually exhausted, and the extent of our loss
may be estimated, by considering that one pound of bones contains as much
phosphoric acid as a hundred-weight of grain.

The imperfect knowledge of Nature and the properties and relations of
matter possessed by the alchemists gave rise, in their time, to an opinion
that metals as well as plants could be produced from a seed. The regular
forms and ramifications seen in crystals, they imagined to be the leaves and
branches of metal plants; and as they saw the seed of plants grow,
producing root, stem and leaves, and again blossoms, fruit and seeds,
apparently without receiving any supply of appropriate material, they
deemed it worthy of zealous inquiry to discover the seed of gold, and the
earth necessary for its development. If the metal seeds were once obtained,
might they not entertain hopes of their growth?

Such ideas could only be entertained when nothing was known of the
atmosphere, and its participation with the earth, in administering to the vital
processes of plants and animals. Modern chemistry indeed produces the
elements of water, and, combining them, forms water anew; but it does not

create those elements—it derives them from water; the new-formed
artificial water has been water before.

Many of our farmers are like the alchemists of old,—they are searching
for the miraculous seed,—the means, which, without any further supply of
nourishment to a soil scarcely rich enough to be sprinkled with indigenous
plants, shall produce crops of grain a hundred-fold.

The experience of centuries, nay, of thousands of years, is insufficient to
guard men against these fallacies; our only security from these and similar
absurdities must be derived from a correct knowledge of scientific
principles.

In the first period of natural philosophy, organic life was supposed to be
derived from water only; afterwards, it was admitted that certain elements
derived from the air must be superadded to the water; but we now know
that other elements must be supplied by the earth, if plants are to thrive and
multiply.

The amount of materials contained in the atmosphere, suited to the
nourishment of plants, is limited; but it must be abundantly sufficient to
cover the whole surface of the earth with a rich vegetation. Under the
tropics, and in those parts of our globe where the most genial conditions of
fertility exist,—a suitable soil, a moist atmosphere, and a high temperature,
—vegetation is scarcely limited by space; and, where the soil is wanting, it
is gradually supplied by the decaying leaves, bark and branches of plants. It
is obvious there is no deficiency of atmospheric nourishment for plants in
those regions, nor are these wanting in our own cultivated fields: all that
plants require for their development is conveyed to them by the incessant
motions of the atmosphere. The air between the tropics contains no more
than that of the arctic zones; and yet how different is the amount of produce
of an equal surface of land in the two situations!

This is easily explicable. All the plants of tropical climates, the oil and
wax palms, the sugar cane, &c., contain only a small quantity of the
elements of the blood necessary to the nutrition of animals, as compared
with our cultivated plants. The tubers of the potato in Chili, its native
country, where the plant resembles a shrub, if collected from an acre of

land, would scarcely suffice to maintain an Irish family for a single day
(Darwin). The result of cultivation in those plants which serve as food, is to
produce in them those constituents of the blood. In the absence of the
elements essential to these in the soil, starch, sugar and woody fibre, are
perhaps formed; but no vegetable fibrine, albumen, or caseine. If we intend
to produce on a given surface of soil more of these latter matters than the
plants can obtain from the atmosphere or receive from the soil of the same
surface in its uncultivated and normal state, we must create an artificial
atmosphere, and add the needed elements to the soil.

The nourishment which must be supplied in a given time to different
plants, in order to admit a free and unimpeded growth, is very unequal.

On pure sand, on calcareous soil, on naked rocks, only a few genera of
plants prosper, and these are, for the most part, perennial plants. They
require, for their slow growth, only such minute quantities of mineral
substances as the soil can furnish, which may be totally barren for other
species. Annual, and especially summer plants, grow and attain their
perfection in a comparatively short time; they therefore do not prosper on a
soil which is poor in those mineral substances necessary to their
development. To attain a maximum in height in the short period of their
existence, the nourishment contained in the atmosphere is not sufficient. If
the end of cultivation is to be obtained, we must create in the soil an
artificial atmosphere of carbonic acid and ammonia; and this surplus of
nourishment, which the leaves cannot appropriate from the air, must be
taken up by the corresponding organs, i.e. the roots, from the soil. But the
ammonia, together with the carbonic acid, are alone insufficient to become
part of a plant destined to the nourishment of animals. In the absence of the
alkalies, the phosphates and other earthy salts, no vegetable fibrine, no
vegetable caseine, can be formed. The phosphoric acid of the phosphate of
lime, indispensable to the cerealia and other vegetables in the formation of
their seeds, is separated as an excrement, in great quantities, by the rind and
barks of ligneous plants.

How different are the evergreen plants, the cacti, the mosses, the ferns,
and the pines, from our annual grasses, the cerealia and leguminous
vegetables! The former, at every time of the day during winter and summer,

obtain carbon through their leaves by absorbing carbonic acid which is not
furnished by the barren soil on which they grow; water is also absorbed and
retained by their coriaceous or fleshy leaves with great force. They lose
very little by evaporation, compared with other plants. On the other hand,
how very small is the quantity of mineral substances which they withdraw
from the soil during their almost constant growth in one year, in comparison
with the quantity which one crop of wheat of an equal weight receives in
three months!

It is by means of moisture that plants receive the necessary alkalies and
salts from the soil. In dry summers a phenomenon is observed, which, when
the importance of mineral elements to the life of a plant was unknown,
could not be explained. The leaves of plants first developed and perfected,
and therefore nearer the surface of the soil, shrivel up and become yellow,
lose their vitality, and fall off while the plant is in an active state of growth,
without any visible cause. This phenomenon is not seen in moist years, nor
in evergreen plants, and but rarely in plants which have long and deep roots,
nor is it seen in perennials in autumn and winter.

The cause of this premature decay is now obvious. The perfectly-
developed leaves absorb continually carbonic acid and ammonia from the
atmosphere, which are converted into elements of new leaves, buds, and
shoots; but this metamorphosis cannot be effected without the aid of the
alkalies, and other mineral substances. If the soil is moist, the latter are
continually supplied to an adequate amount, and the plant retains its lively
green colour; but if this supply ceases from a want of moisture to dissolve
the mineral elements, a separation takes place in the plant itself. The
mineral constituents of the juice are withdrawn from the leaves already
formed, and are used for the formation of the young shoots; and as soon as
the seeds are developed, the vitality of the leaves completely ceases. These
withered leaves contain only minute traces of soluble salts, while the buds
and shoots are very rich in them.

On the other hand, it has been observed, that where a soil is too highly
impregnated with soluble saline materials, these are separated upon the
surface of the leaves. This happens to culinary vegetables especially, whose
leaves become covered with a white crust. In consequence of these

exudations the plant sickens, its organic activity decreases, its growth is
disturbed; and if this state continues long, the plant dies. This is most
frequently seen in foliaceous plants, the large surfaces of which evaporate
considerable quantities of water. Carrots, pumpkins, peas, &c., are
frequently thus diseased, when, after dry weather, the plant being near its
full growth, the soil is moistened by short showers, followed again by dry
weather. The rapid evaporation carries off the water absorbed by the root,
and this leaves the salts in the plant in a far greater quantity than it can
assimilate. These salts effloresce upon the surface of the leaves, and if they
are herbaceous and juicy, produce an effect upon them as if they had been
watered with a solution containing a greater quantity of salts than their
organism can bear.

Of two plants of the same species, this disease befalls that which is
nearest its perfection; if one should have been planted later, or be more
backward in its development, the same external cause which destroys the
one will contribute to the growth of the other.

LETTER XII

My dear Sir,

Having now occupied several letters with the attempt to unravel, by
means of chemistry, some of the most curious functions of the animal body,
and, as I hope, made clear to you the distinctions between the two kinds of
constituent elements in food, and the purposes they severally subserve in
sustaining life, let me now direct your attention to a scarcely less interesting
and equally important subject—the means of obtaining from a given surface
of the earth the largest amount of produce adapted to the food of man and
animals.

Agriculture is both a science and an art. The knowledge of all the
conditions of the life of vegetables, the origin of their elements, and the

sources of their nourishment, forms its scientific basis.

From this knowledge we derive certain rules for the exercise of the ART,
the principles upon which the mechanical operations of farming depend, the
usefulness or necessity of these for preparing the soil to support the growth
of plants, and for removing every obnoxious influence. No experience,
drawn from the exercise of the art, can be opposed to true scientific
principles, because the latter should include all the results of practical
operations, and are in some instances solely derived therefrom. Theory must
correspond with experience, because it is nothing more than the reduction
of a series of phenomena to their last causes.

A field in which we cultivate the same plant for several successive years
becomes barren for that plant in a period varying with the nature of the soil:
in one field it will be in three, in another in seven, in a third in twenty, in a
fourth in a hundred years. One field bears wheat, and no peas; another
beans or turnips, but no tobacco; a third gives a plentiful crop of turnips, but
will not bear clover. What is the reason that a field loses its fertility for one
plant, the same which at first flourished there? What is the reason one kind
of plant succeeds in a field where another fails?

These questions belong to Science.

What means are necessary to preserve to a field its fertility for one and
the same plant?—what to render one field fertile for two, for three, for all
plants?

These last questions are put by Art, but they cannot be answered by Art.

If a farmer, without the guidance of just scientific principles, is trying
experiments to render a field fertile for a plant which it otherwise will not
bear, his prospect of success is very small. Thousands of farmers try such
experiments in various directions, the result of which is a mass of practical
experience forming a method of cultivation which accomplishes the desired
end for certain places; but the same method frequently does not succeed, it
indeed ceases to be applicable to a second or third place in the immediate
neighbourhood. How large a capital, and how much power, are wasted in
these experiments! Very different, and far more secure, is the path indicated

by SCIENCE; it exposes us to no danger of failing, but, on the contrary, it
furnishes us with every guarantee of success. If the cause of failure—of
barrenness in the soil for one or two plants—has been discovered, means to
remedy it may readily be found.

The most exact observations prove that the method of cultivation must
vary with the geognostical condition of the subsoil. In basalt, graywacke,
porphyry, sandstone, limestone, &c., are certain elements indispensable to
the growth of plants, and the presence of which renders them fertile. This
fully explains the difference in the necessary methods of culture for
different places; since it is obvious that the essential elements of the soil
must vary with the varieties of composition of the rocks, from the
disintegration of which they originated.

Wheat, clover, turnips, for example, each require certain elements from
the soil; they will not flourish where the appropriate elements are absent.
Science teaches us what elements are essential to every species of plants by
an analysis of their ashes. If therefore a soil is found wanting in any of
those elements, we discover at once the cause of its barrenness, and its
removal may now be readily accomplished.

The empiric attributes all his success to the mechanical operations of
agriculture; he experiences and recognises their value, without inquiring
what are the causes of their utility, their mode of action: and yet this
scientific knowledge is of the highest importance for regulating the
application of power and the expenditure of capital,—for insuring its
economical expenditure and the prevention of waste. Can it be imagined
that the mere passing of the ploughshare or the harrow through the soil—
the mere contact of the iron—can impart fertility miraculously? Nobody,
perhaps, seriously entertains such an opinion. Nevertheless, the modus
operandi of these mechanical operations is by no means generally
understood. The fact is quite certain, that careful ploughing exerts the most
favourable influence: the surface is thus mechanically divided, changed,
increased, and renovated; but the ploughing is only auxiliary to the end
sought.

In the effects of time, in what in Agriculture are technically called
fallows—the repose of the fields—we recognise by science certain

chemical actions, which are continually exercised by the elements of the
atmosphere upon the whole surface of our globe. By the action of its
oxygen and its carbonic acid, aided by water, rain, changes of temperature,
&c., certain elementary constituents of rocks, or of their ruins, which form
the soil capable of cultivation, are rendered soluble in water, and
consequently become separable from all their insoluble parts.

These chemical actions, poetically denominates the "tooth of time,"
destroy all the works of man, and gradually reduce the hardest rocks to the
condition of dust. By their influence the necessary elements of the soil
become fitted for assimilation by plants; and it is precisely the end which is
obtained by the mechanical operations of farming. They accelerate the
decomposition of the soil, in order to provide a new generation of plants
with the necessary elements in a condition favourable to their assimilation.
It is obvious that the rapidity of the decomposition of a solid body must
increase with the extension of its surface; the more points of contact we
offer in a given time to the external chemical agent, the more rapid will be
its action.

The chemist, in order to prepare a mineral for analysis, to decompose it,
or to increase the solubility of its elements, proceeds in the same way as the
farmer deals with his fields—he spares no labour in order to reduce it to the
finest powder; he separates the impalpable from the coarser parts by
washing, and repeats his mechanical bruising and trituration, being assured
his whole process will fail if he is inattentive to this essential and
preliminary part of it.

The influence which the increase of surface exercises upon the
disintegration of rocks, and upon the chemical action of air and moisture, is
strikingly illustrated upon a large scale in the operations pursued in the
gold-mines of Yaquil, in Chili. These are described in a very interesting
manner by Darwin. The rock containing the gold ore is pounded by mills
into the finest powder; this is subjected to washing, which separates the
lighter particles from the metallic; the gold sinks to the bottom, while a
stream of water carries away the lighter earthy parts into ponds, where it
subsides to the bottom as mud. When this deposit has gradually filled up the
pond, this mud is taken out and piled in heaps, and left exposed to the

action of the atmosphere and moisture. The washing completely removes all
the soluble part of the disintegrated rock; the insoluble part, moreover,
cannot undergo any further change while it is covered with water, and so
excluded from the influence of the atmosphere at the bottom of the pond.
But being exposed at once to the air and moisture, a powerful chemical
action takes place in the whole mass, which becomes indicated by an
efflorescence of salts covering the whole surface of the heaps in
considerable quantity. After being exposed for two or three years, the mud
is again subjected to the same process of washing, and a considerable
quantity of gold is obtained, this having been separated by the chemical
process of decomposition in the mass. The exposure and washing of the
same mud is repeated six or seven times, and at every washing it furnishes a
new quantity of gold, although its amount diminishes every time.

Precisely similar is the chemical action which takes place in the soil of
our fields; and we accelerate and increase it by the mechanical operations of
our agriculture. By these we sever and extend the surface, and endeavour to
make every atom of the soil accessible to the action of the carbonic acid and
oxygen of the atmosphere. We thus produce a stock of soluble mineral
substances, which serves as nourishment to a new generation of plants,
materials which are indispensable to their growth and prosperity.

LETTER XIII

My dear Sir,

Having in my last letter spoken of the general principles upon which the
science and art of agriculture must be based, let me now direct your
attention to some of those particulars between chemistry and agriculture,
and demonstrate the impossibility of perfecting the important art of rearing
food for man and animals, without a profound knowledge of our science.

All plants cultivated as food require for their healthy sustenance the
alkalies and alkaline earths, each in a certain proportion; and in addition to
these, the cerealia do not succeed in a soil destitute of silica in a soluble
condition. The combinations of this substance found as natural productions,
namely, the silicates, differ greatly in the degree of facility with which they
undergo decomposition, in consequence of the unequal resistance opposed
by their integral parts to the dissolving power of the atmospheric agencies.
Thus the granite of Corsica degenerates into a powder in a time which
scarcely suffices to deprive the polished granite of Heidelberg of its lustre.

Some soils abound in silicates so readily decomposable, that in every
one or two years, as much silicate of potash becomes soluble and fitted for
assimilation as is required by the leaves and straw of a crop of wheat. In
Hungary, extensive districts are not uncommon where wheat and tobacco
have been grown alternately upon the same soil for centuries, the land never
receiving back any of those mineral elements which were withdrawn in the
grain and straw. On the other hand, there are fields in which the necessary
amount of soluble silicate of potash for a single crop of wheat is not
separated from the insoluble masses in the soil in less than two, three, or
even more years.

The term fallow, in Agriculture, designates that period in which the soil,
left to the influence of the atmosphere, becomes enriched with those soluble
mineral constituents. Fallow, however, does not generally imply an entire
cessation of cultivation, but only an interval in the growth of the cerealia.
That store of silicates and alkalies which is the principal condition of their
success is obtained, if potatoes or turnips are grown upon the same fields in
the intermediate periods, since these crops do not abstract a particle of
silica, and therefore leave the field equally fertile for the following crop of
wheat.

The preceding remarks will render it obvious to you, that the mechanical
working of the soil is the simplest and cheapest method of rendering the
elements of nutrition contained in it accessible to plants.

But it may be asked, Are there not other means of decomposing the soil
besides its mechanical subdivision?—are there not substances, which by
their chemical operation will equally well or better render its constituents

suitable for entering into vegetable organisms? Yes: we certainly possess
such substances, and one of them, namely, quick-lime, has been employed
for the last century past in England for this purpose; and it would be
difficult to find a substance better adapted to this service, as it is simple, and
in almost all localities cheap and easily accessible.

In order to obtain correct views respecting the effect of quick-lime upon
the soil, let me remind you of the first process employed by the chemist
when he is desirous of analysing a mineral, and for this purpose wishes to
bring its elements into a soluble state. Let the mineral to be examined be,
for instance, feldspar; this substance, taken alone, even when reduced to the
finest powder, requires for its solution to be treated with an acid for weeks
or months; but if we first mix it with quick-lime, and expose the mixture to
a moderately strong heat, the lime enters into chemical combination with
certain elements of the feldspar, and its alkali (potass) is set free. And now
the acid, even without heat, dissolves not only the lime, but also so much of
the silica of the feldspar as to form a transparent jelly. The same effect
which the lime in this process, with the aid of heat, exerts upon the feldspar,
it produces when it is mixed with the alkaline argillaceous silicates, and
they are for a long time kept together in a moist state.

Common potters' clay, or pipe-clay, diffused through water, and added to
milk of lime, thickens immediately upon mixing; and if the mixture is kept
for some months, and then treated with acid, the clay becomes gelatinous,
which would not occur without the admixture with the lime. The lime, in
combining with the elements of the clay, liquifies it; and, what is more
remarkable, liberates the greater part of its alkalies. These interesting facts
were first observed by Fuchs, at Munich: they have not only led to a more
intimate knowledge of the nature and properties of the hydraulic cements,
but, what is far more important, they explain the effects of caustic lime
upon the soil, and guide the agriculturist in the application of an invaluable
means of opening it, and setting free its alkalies—substances so important,
nay, so indispensable to his crops.

In the month of October the fields of Yorkshire and Oxfordshire look as
it they were covered with snow. Whole square miles are seen whitened over

with quicklime, which during the moist winter months, exercises its
beneficial influence upon the stiff, clayey soil, of those counties.

According to the humus theory, quick-lime ought to exert the most
noxious influence upon the soil, because all organic matters contained in it
are destroyed by it, and rendered incapable of yielding their humus to a new
vegetation. The facts are indeed directly contrary to this now abandoned
theory: the fertility of the soil is increased by the lime. The cerealia require
the alkalies and alkaline silicates, which the action of the lime renders fit
for assimilation by the plants. If, in addition to these, there is any decaying
organic matter present in the soil supplying carbonic acid, it may facilitate
their development; but it is not essential to their growth. If we furnish the
soil with ammonia, and the phosphates, which are indispensable to the
cerealia, with the alkaline silicates, we have all the conditions necessary to
ensure an abundant harvest. The atmosphere is an inexhaustible store of
carbonic acid.

A no less favourable influence than that of lime is exercised upon the
soil of peaty land by the mere act of burning it: this greatly enhances its
fertility. We have not long been acquainted with the remarkable change
which the properties of clay undergo by burning. The observation was first
made in the process of analysing the clay silicates. Many of these, in their
natural state, are not acted on by acids, but they become perfectly soluble if
heated to redness before the application of the acid. This property belongs
to potters' clay, pipe-clay, loam, and many different modifications of clay in
soils. In their natural state they may be boiled in concentrated sulphuric
acid, without sensible change; but if feebly burned, as is done with the pipe-
clay in many alum manufactories, they dissolve in the acid with the greatest
facility, the contained silica being separated like jelly in a soluble state.
Potters' clay belongs to the most sterile kinds of soil, and yet it contains
within itself all the constituent elements essential to a most luxurious
growth of plants; but their mere presence is insufficient to secure this end.
The soil must be accessible to the atmosphere, to its oxygen, to its carbonic
acid; these must penetrate it, in order to secure the conditions necessary to a
happy and vigorous development of the roots. The elements present must be
brought into that peculiar state of combination which will enable them to

enter into plants. Plastic clay is wanting in these properties; but they are
imparted to it by a feeble calcination.

At Hardwicke Court, near Gloucester, I have seen a garden (Mr. Baker's)
consisting of a stiff clay, which was perfectly sterile, become by mere
burning extremely fertile. The operation was extended to a depth of three
feet. This was an expensive process, certainly; but it was effectual.

The great difference in the properties of burnt and unburnt clay is
illustrated by what is seen in brick houses, built in moist situations. In the
town of Flanders, for instance, where most buildings are of brick,
effloresences of salts cover the surfaces of the walls, like a white nap,
within a few days after they are erected. If this saline incrustation is washed
away by the rain, it soon re-appears; and this is even observed on walls
which, like the gateway of Lisle, have been erected for centuries. These
saline incrustations consist of carbonates and sulphates, with alkaline bases;
and it is well known these act an important part in vegetation. The influence
of lime in their production is manifested by their appearing first at the place
where the mortar and brick come into contact.

It will now be obvious to you, that in a mixture of clay with lime, all the
conditions exist for the solution of the silicated clay, and the solubility of
the alkaline silicates. The lime gradually dissolving in water charged with
carbonic acid, acts like milk of lime upon the clay. This explains also the
favourable influence which marl (by which term all those varieties of clay
rich in chalk are designated) exerts upon most kinds of soil. There are marly
soils which surpass all others in fertility for all kinds of plants; but I believe
marl in a burnt state must be far more effective, as well as other materials
possessing a similar composition; as, for instance, those species of
limestone which are adapted to the preparation of hydraulic cements,—for
these carry to the soil not only the alkaline bases useful to plants, but also
silica in a state capable of assimilation.

The ashes of coals and lignite are also excellent means of ameliorating
the soil, and they are used in many places for this purpose. The most
suitable may be readily known by their property of forming a gelatinous
mass when treated with acids, or by becoming, when mixed with cream of
lime, like hydraulic cement,—solid and hard as stone.

I have now, I trust, explained to your satisfaction, that the mechanical
operations of agriculture—the application of lime and chalk to lands, and
the burning of clay—depend upon one and the same scientific principle:
they are means of accelerating the decomposition of the alkaline clay
silicates, in order to provide plants, at the beginning of a new vegetation,
with certain inorganic matters indispensable for their nutrition.

LETTER XIV

My dear Sir,

I treated, in my last letter, of the means of improving the condition of the
soil for agricultural purposes by mechanical operations and mineral agents.
I have now to speak of the uses and effects of animal exuviae, and
vegetable matters or manures—properly so called.

In order to understand the nature of these, and the peculiarity of their
influence upon our fields, it is highly important to keep in mind the source
whence they are derived.

It is generally known, that if we deprive an animal of food, the weight of
its body diminishes during every moment of its existence. If this abstinence
is continued for some time, the diminution becomes apparent to the eye; all
the fat of the body disappears, the muscles decrease in firmness and bulk,
and, if the animal is allowed to die starved, scarcely anything but skin,
tendon, and bones, remain. This emaciation which occurs in a body
otherwise healthy, demonstrates to us, that during the life of an animal
every part of its living substance is undergoing a perpetual change; all its
component parts, assuming the form of lifeless compounds, are thrown off
by the skin, lungs, and urinary system, altered more or less by the secretory
organs. This change in the living body is intimately connected with the
process of respiration; it is, in truth, occasioned by the oxygen of the
atmosphere in breathing, which combines with all the various matters

within the body. At every inspiration a quantity of oxygen passes into the
blood in the lungs, and unites with its elements; but although the weight of
the oxygen thus daily entering into the body amounts to 32 or more ounces,
yet the weight of the body is not thereby increased. Exactly as much oxygen
as is imbibed in inspiration passes off in expiration, in the form of carbonic
acid and water; so that with every breath the amount of carbon and
hydrogen in the body is diminished. But the emaciation—the loss of weight
by starvation—does not simply depend upon the separation of the carbon
and hydrogen; but all the other substances which are in combination with
these elements in the living tissues pass off in the secretions. The nitrogen
undergoes a change, and is thrown out of the system by the kidneys. Their
secretion, the urine, contains not only a compound rich in nitrogen, namely
urea, but the sulphur of the tissues in the form of a sulphate, all the soluble
salts of the blood and animal fluids, common salt, the phosphates, soda and
potash. The carbon and hydrogen of the blood, of the muscular fibre, and of
all the animal tissues which can undergo change, return into the
atmosphere. The nitrogen, and all the soluble inorganic elements are carried
to the earth in the urine.

These changes take place in the healthy animal body during every
moment of life; a waste and loss of substance proceeds continually; and if
this loss is to be restored, and the original weight and substance repaired, an
adequate supply of materials must be furnished, from whence the blood and
wasted tissues may be regenerated. This supply is obtained from the food.

In an adult person in a normal or healthy condition, no sensible increase
or decrease of weight occurs from day to day. In youth the weight of the
body increases, whilst in old age it decreases. There can be no doubt that in
the adult, the food has exactly replaced the loss of substance: it has supplied
just so much carbon, hydrogen, nitrogen, and other elements, as have
passed through the skin, lungs, and urinary organs. In youth the supply is
greater than the waste. Part of the elements of the food remain to augment
the bulk of the body. In old age the waste is greater than the supply, and the
body diminishes. It is unquestionable, that, with the exception of a certain
quantity of carbon and hydrogen, which are secreted through the skin and
lungs, we obtain, in the solid and fluid excrements of man and animals, all
the elements of their food.

We obtain daily, in the form of urea, all the nitrogen taken in the food
both of the young and the adult; and further, in the urine, the whole amount
of the alkalies, soluble phosphates and sulphates, contained in all the
various aliments. In the solid excrements are found all those substances
taken in the food which have undergone no alteration in the digestive
organs, all indigestible matters, such as woody fibre, the green colouring
matter of leaves ( chlorophyle), wax, &c.

Physiology teaches us, that the process of nutrition in animals, that is,
their increase of bulk, or the restoration of wasted parts, proceeds from the
blood. The purpose of digestion and assimilation is to convert the food into
blood. In the stomach and intestines, therefore, all those substances in the
food capable of conversion into blood are separated from its other
constituents; in other words, during the passage of the food through the
intestinal canal there is a constant absorption of its nitrogen, since only
azotised substances are capable of conversion into blood; and therefore the
solid excrements are destitute of that element, except only a small portion,
in the constitution of that secretion which is formed to facilitate their
passage. With the solid excrements, the phosphates of lime and magnesia,
which were contained in the food and not assimilated, are carried off, these
salts being insoluble in water, and therefore not entering the urine.

We may obtain a clear insight into the chemical constitution of the solid
excrements without further investigation, by comparing the faeces of a dog
with his food. We give that animal flesh and bones—substances rich in
azotised matter—and we obtain, as the last product of its digestion, a
perfectly white excrement, solid while moist, but becoming in dry air a
powder. This is the phosphate of lime of the bones, with scarcely one per
cent. of foreign organic matter.

Thus we see that in the solid and fluid excrements of man and animals,
all the nitrogen—in short, all the constituent ingredients of the consumed
food, soluble and insoluble, are returned; and as food is primarily derived
from the fields, we possess in those excrements all the ingredients which we
have taken from it in the form of seeds, roots, or herbs.

One part of the crops employed for fattening sheep and cattle is
consumed by man as animal food; another part is taken directly—as flour,

potatoes, green vegetables, &c.; a third portion consists of vegetable refuse,
and straw employed as litter. None of the materials of the soil need be lost.
We can, it is obvious, get back all its constituent parts which have been
withdrawn therefrom, as fruits, grain and animals, in the fluid and solid
excrements of man, and the bones, blood and skins of the slaughtered
animals. It depends upon ourselves to collect carefully all these scattered
elements, and to restore the disturbed equilibrium of composition in the
soil. We can calculate exactly how much and which of the component parts
of the soil we export in a sheep or an ox, in a quarter of barley, wheat or
potatoes, and we can discover, from the known composition of the
excrements of man and animals, how much we have to supply to restore
what is lost to our fields.

If, however, we could procure from other sources the substances which
give to the exuviae of man and animals their value in agriculture, we should
not need the latter. It is quite indifferent for our purpose whether we supply
the ammonia (the source of nitrogen) in the form of urine, or in that of a salt
derived from coal-tar; whether we derive the phosphate of lime from bones,
apatite, or fossil excrements (the coprolithes).

The principal problem for agriculture is, how to replace those substances
which have been taken from the soil, and which cannot be furnished by the
atmosphere. If the manure supplies an imperfect compensation for this loss,
the fertility of a field or of a country decreases; if, on the contrary, more are
given to the fields, their fertility increases.

An importation of urine, or of solid excrements, from a foreign country,
is equivalent to an importation of grain and cattle. In a certain time, the
elements of those substances assume the form of grain, or of fodder, then
become flesh and bones, enter into the human body, and return again day by
day to the form they originally possessed.

The only real loss of elements we are unable to prevent is of the
phosphates, and these, in accordance with the customs of all modern
nations, are deposited in the grave. For the rest, every part of that enormous
quantity of food which a man consumes during his lifetime ( say in sixty or
seventy years), which was derived from the fields, can be obtained and
returned to them. We know with absolute certainty, that in the blood of a

young or growing animal there remains a certain quantity of phosphate of
lime and of the alkaline phosphates, to be stored up and to minister to the
growth of the bones and general bulk of the body, and that, with the
exception of this very small quantity, we receive back, in the solid and fluid
excrements, all the salts and alkaline bases, all the phosphate of lime and
magnesia, and consequently all the inorganic elements which the animal
consumes in its food.

We can thus ascertain precisely the quantity, quality, and composition of
animal excrements, without the trouble of analysing them. If we give a
horse daily 4 1/2 pounds' weight of oats, and 15 pounds of hay, and
knowing that oats give 4 per cent. and hay 9 per cent. of ashes, we can
calculate that the daily excrements of the horse will contain 21 ounces of
inorganic matter which was drawn from the fields. By analysis we can
determine the exact relative amount of silica, of phosphates, and of alkalies,
contained in the ashes of the oats and of the hay.

You will now understand that the constituents of the solid parts of animal
excrements, and therefore their qualities as manure, must vary with the
nature of the creature's food. If we feed a cow upon beetroot, or potatoes,
without hay, straw or grain, there will be no silica in her solid excrements,
but there will be phosphate of lime and magnesia. Her fluid excrements will
contain carbonate of potash and soda, together with compounds of the same
bases with inorganic acids. In one word, we have, in the fluid excrements,
all the soluble parts of the ashes of the consumed food; and in the solid
excrements, all those parts of the ashes which are insoluble in water.

If the food, after burning, leaves behind ashes containing soluble
alkaline phosphates, as is the case with bread, seeds of all kinds, and flesh,
we obtain from the animal by which they are consumed a urine holding in
solution these phosphates. If, however, the ashes of food contain no alkaline
phosphates, but abound in insoluble earthy phosphates, as hay, carrots, and
potatoes, the urine will be free from alkaline phosphates, but the earthy
phosphates will be found in the faeces. The urine of man, of carnivorous
and graminivorous animals, contains alkaline phosphates; that of
herbivorous animals is free from these salts.

The analysis of the excrements of man, of the piscivorous birds (as the
guano), of the horse, and of cattle, furnishes us with the precise knowledge
of the salts they contain, and demonstrates, that in those excrements, we
return to the fields the ashes of the plants which have served as food,—the
soluble and insoluble salts and earths indispensable to the development of
cultivated plants, and which must be furnished to them by a fertile soil.

There can be no doubt that, in supplying these excrements to the soil, we
return to it those constituents which the crops have removed from it, and we
renew its capability of nourishing new crops: in one word, we restore the
disturbed equilibrium; and consequently, knowing that the elements of the
food derived from the soil enter into the urine and solid excrements of the
animals it nourishes, we can with the greatest facility determine the exact
value of the different kinds of manure. Thus the excrements of pigs which
we have fed with peas and potatoes are principally suited for manuring
crops of potatoes and peas. In feeding a cow upon hay and turnips, we
obtain a manure containing the inorganic elements of grasses and turnips,
and which is therefore preferable for manuring turnips. The excrement of
pigeons contains the mineral elements of grain; that of rabbits, the elements
of herbs and kitchen vegetables. The fluid and solid excrements of man,
however, contain the mineral elements of grain and seeds in the greatest
quantity.

LETTER XV

My dear Sir,

You are now acquainted with my opinions respecting the effects of the
application of mineral agents to our cultivated fields, and also the rationale
of the influence of the various kinds of manures; you will, therefore, now
readily understand what I have to say of the sources whence the carbon and
nitrogen, indispensable to the growth of plants, are derived.

The growth of forests, and the produce of meadows, demonstrate that an
inexhaustible quantity of carbon is furnished for vegetation by the carbonic
acid of the atmosphere.

We obtain from an equal surface of forest, or meadow-land, where the
necessary mineral elements of the soil are present in a suitable state, and to
which no carbonaceous matter whatever is furnished in manures, an amount
of carbon, in the shape of wood and hay, quite equal, and oftimes more than
is produced by our fields, in grain, roots, and straw, upon which abundance
of manure has been heaped.

It is perfectly obvious that the atmosphere must furnish to our cultivated
fields as much carbonic acid, as it does to an equal surface of forest or
meadow, and that the carbon of this carbonic acid is assimilated, or may be
assimilated by the plants growing there, provided the conditions essential to
its assimilation, and becoming a constituent element of vegetables, exist in
the soil of these fields.

In many tropical countries the produce of the land in grain or roots,
during the whole year, depends upon one rain in the spring. If this rain is
deficient in quantity, or altogether wanting, the expectation of an abundant
harvest is diminished or destroyed.

Now it cannot be the water merely which produces this enlivening and
fertilising effect observed, and which lasts for weeks and months. The plant
receives, by means of this water, at the time of its first development, the
alkalies, alkaline earths, and phosphates, necessary to its organization. If
these elements, which are necessary previous to its assimilation of
atmospheric nourishment, be absent, its growth is retarded. In fact, the
development of a plant is in a direct ratio to the amount of the matters it
takes up from the soil. If, therefore, a soil is deficient in these mineral
constituents required by plants, they will not flourish even with an abundant
supply of water.

The produce of carbon on a meadow, or an equal surface of forest land,
is independent of a supply of carbonaceous manure, but it depends upon the
presence of certain elements of the soil which in themselves contain no
carbon, together with the existence of conditions under which their

assimilation by plants can be effected. We increase the produce of our
cultivated fields, in carbon, by a supply of lime, ashes, and marl, substances
which cannot furnish carbon to the plants, and yet it is indisputable,—being
founded upon abundant experience,—that in these substances we furnish to
the fields elements which greatly increase the bulk of their produce, and
consequently the amount of carbon.

If we admit these facts to be established, we can no longer doubt that a
deficient produce of carbon, or in other words, the barrenness of a field
does not depend upon carbonic acid, because we are able to increase the
produce, to a certain degree, by a supply of substances which do not contain
any carbon. The same source whence the meadow and the forest are
furnished with carbon, is also open to our cultivated plants. The great object
of agriculture, therefore, is to discover the means best adapted to enable
these plants to assimilate the carbon of the atmosphere which exists in it as
carbonic acid. In furnishing plants, therefore, with mineral elements, we
give them the power to appropriate carbon from a source which is
inexhaustible; whilst in the absence of these elements the most abundant
supply of carbonic acid, or of decaying vegetable matter, would not increase
the produce of a field.

With an adequate and equal supply of these essential mineral
constituents in the soil, the amount of carbonic acid absorbed by a plant
from the atmosphere in a given time is limited by the quantity which is
brought into contact with its organs of absorption.

The withdrawal of carbonic acid from the atmosphere by the vegetable
organism takes place chiefly through its leaves; this absorption requires the
contact of the carbonic acid with their surface, or with the part of the plant
by which it is absorbed.

The quantity of carbonic acid absorbed in a given time is in direct
proportion to the surface of the leaves and the amount of carbonic acid
contained in the air; that is, two plants of the same kind and the same extent
of surface of absorption, in equal times and under equal conditions, absorb
one and the same amount of carbon.

In an atmosphere containing a double proportion of carbonic acid, a
plant absorbs, under the same condition, twice the quantity of carbon.
Boussingault observed, that the leaves of the vine, inclosed in a vessel,
withdrew all the carbonic acid from a current of air which was passed
through it, however great its velocity. (Dumas Lecon, p.23.) If, therefore,
we supply double the quantity of carbonic acid to one plant, the extent of
the surface of which is only half that of another living in ordinary
atmospheric air, the former will obtain and appropriate as much carbon as
the latter. Hence results the effects of humus, and all decaying organic
substances, upon vegetation. If we suppose all the conditions for the
absorption of carbonic acid present, a young plant will increase in mass, in
a limited time, only in proportion to its absorbing surface; but if we create
in the soil a new source of carbonic acid, by decaying vegetable substances,
and the roots absorb in the same time three times as much carbonic acid
from the soil as the leaves derive from the atmosphere, the plant will
increase in weight fourfold. This fourfold increase extends to the leaves,
buds, stalks, &c., and in the increased extent of the surface, the plant
acquires an increased power of absorbing nourishment from the air, which
continues in action far beyond the time when its derivation of carbonic acid
through the roots ceases. Humus, as a source of carbonic acid in cultivated
lands, is not only useful as a means of increasing the quantity of carbon—
an effect which in most cases may be very indifferent for agricultural
purposes—but the mass of the plant having increased rapidly in a short
time, space is obtained for the assimilation of the elements of the soil
necessary for the formation of new leaves and branches.

Water evaporates incessantly from the surface of the young plant; its
quantity is in direct proportion to the temperature and the extent of the
surface. The numerous radical fibrillae replace, like so many pumps, the
evaporated water; and so long as the soil is moist, or penetrated with water,
the indispensable elements of the soil, dissolved in the water, are supplied
to the plant. The water absorbed by the plant evaporating in an aeriform
state leaves the saline and other mineral constituents within it. The relative
proportion of these elements taken up by a plant, is greater, the more
extensive the surface and more abundant the supply of water; where these
are limited, the plant soon reaches its full growth, while if their supply is
continued, a greater amount of elements necessary to enable it to

appropriate atmospheric nourishment being obtained, its development
proceeds much further. The quantity, or mass of seed produced, will
correspond to the quantity of mineral constituents present in the plant. That
plant, therefore, containing the most alkaline phosphates and earthy salts
will produce more or a greater weight of seeds than another which, in an
equal time has absorbed less of them. We consequently observe, in a hot
summer, when a further supply of mineral ingredients from the soil ceases
through want of water, that the height and strength of plants, as well as the
development of their seeds, are in direct proportion to its absorption of the
elementary parts of the soil in the preceding epochs of its growth.

The fertility of the year depends in general upon the temperature, and the
moisture or dryness of the spring, if all the conditions necessary to the
assimilation of the atmospheric nourishment be secured to our cultivated
plants. The action of humus, then, as we have explained it above, is chiefly
of value in gaining time. In agriculture, this must ever be taken into account
and in this respect humus is of importance in favouring the growth of
vegetables, cabbages, &c.

But the cerealia, and plants grown for their roots, meet on our fields, in
the remains of the preceding crop, with a quantity of decaying vegetable
substances corresponding to their contents of mineral nutriment from the
soil, and consequently with a quantity of carbonic acid adequate to their
accelerated development in the spring. A further supply of carbonic acid,
therefore, would be quite useless, without a corresponding increase of
mineral ingredients.

From a morgen of good meadow land, 2,500 pounds weight of hay,
according to the best agriculturists, are obtained on an average. This amount
is furnished without any supply of organic substances, without manure
containing carbon or nitrogen. By irrigation, and the application of ashes or
gypsum, double that amount may be grown. But assuming 2,500 pounds
weight of hay to be the maximum, we may calculate the amount of carbon
and nitrogen derived from the atmosphere by the plants of meadows.

According to elementary analysis, hay, dried at a temperature of 100 deg
Reaumur, contains 45.8 per cent. of carbon, and 1 1/2 per cent. of nitrogen.
14 per cent. of water retained by the hay, dried at common temperatures, is

driven off at 100 deg. 2,500 pounds weight of hay, therefore, corresponds to
2,150 pounds, dried at 100 deg. This shows us, that 984 pounds of carbon,
and 32.2 pounds weight of nitrogen, have been obtained in the produce of
one morgen of meadow land. Supposing that this nitrogen has been
absorbed by the plants in the form of ammonia, the atmosphere contains
39.1 pounds weight of ammonia to every 3640 pounds weight of carbonic
acid (=984 carbon, or 27 per cent.), or in other words, to every 1,000
pounds weight of carbonic acid, 10.7 pounds of ammonia, that is to about
1/100,000, the weight of the air, or 1/60,000 of its volume.

For every 100 parts of carbonic acid absorbed by the surface of the
leaves, the plant receives from the atmosphere somewhat more than one
part of ammonia.

With every 1,000 pounds of carbon, we obtain—
From a meadow . 32 7/10 pounds of nitrogen.

From cultivated fields,

In Wheat . 21 1/2 " "
Oats . 22.3 " "
Rye . 15.2 " "
Potatoes . 34.1 " "
Beetroot . 39.1 " "
Clover . 44 " "
Peas . 62 " "

Boussingault obtained from his farm at Bechelbronn, in Alsace, in five
years, in the shape of potatoes, wheat, clover, turnips, and oats, 8,383 of
carbon, and 250.7 nitrogen. In the following five years, as beetroot, wheat,
clover, turnips, oats, and rye, 8,192 of carbon, and 284.2 of nitrogen. In a
further course of six years, potatoes, wheat, clover, turnips, peas, and rye,
10,949 of carbon, 356.6 of nitrogen. In 16 years, 27,424 carbon, 858 1/2
nitrogen, which gives for every 1,000 carbon, 31.3 nitrogen.

From these interesting and unquestionable facts, we may deduce some
conclusions of the highest importance in their application to agriculture.

1. We observe that the relative proportions of carbon and nitrogen, stand
in a fixed relation to the surface of the leaves. Those plants, in which all the
nitrogen may be said to be concentrated in the seeds, as the cerealia, contain
on the whole less nitrogen than the leguminous plants, peas, and clover.

2. The produce of nitrogen on a meadow which receives no nitrogenised
manure, is greater than that of a field of wheat which has been manured.

3. The produce of nitrogen in clover and peas, which agriculturists will
acknowledge require no nitrogenised manure, is far greater than that of a
potato or turnip field, which is abundantly supplied with such manures.

Lastly. And this is the most curious deduction to be derived from the
above facts,—if we plant potatoes, wheat, turnips, peas, and clover, (plants
containing potash, lime, and silex,) upon the same land, three times
manured, we gain in 16 years, for a given quantity of carbon, the same
proportion of nitrogen which we receive from a meadow which has
received no nitrogenised manure.

On a morgen of meadow-land, we obtain in plants, containing silex,
lime, and potash, 984 carbon, 32.2 nitrogen. On a morgen of cultivated
land, in an average of 16 years, in plants containing the same mineral
elements, silex, lime, and potash, 857 carbon, 26.8 nitrogen.

If we add the carbon and nitrogen of the leaves of the beetroot, and the
stalk and leaves of the potatoes, which have not been taken into account, it
still remains evident that the cultivated fields, notwithstanding the supply of
carbonaceous and nitrogenised manures, produced no more carbon and
nitrogen than an equal surface of meadow-land supplied only with mineral
elements.

What then is the rationale of the effect of manure,—of the solid and fluid
excrements of animals?

This question can now be satisfactorily answered: that effect is the
restoration of the elementary constituents of the soil which have been
gradually drawn from it in the shape of grain and cattle. If the land I am
speaking of had not been manured during those 16 years, not more than

one-half, or perhaps than one-third part of the carbon and nitrogen would
have been produced. We owe it to the animal excrements, that it equalled in
production the meadow-land, and this, because they restored the mineral
ingredients of the soil removed by the crops. All that the supply of manure
accomplished, was to prevent the land from becoming poorer in these, than
the meadow which produces 2,500 pounds of hay. We withdraw from the
meadow in this hay as large an amount of mineral substances as we do in
one harvest of grain, and we know that the fertility of the meadow is just as
dependent upon the restoration of these ingredients to its soil, as the
cultivated land is upon manures. Two meadows of equal surface, containing
unequal quantities of inorganic elements of nourishment,—other conditions
being equal,—are very unequally fertile; that which possesses most,
furnishes most hay. If we do not restore to a meadow the withdrawn
elements, its fertility decreases. But its fertility remains unimpaired, with a
due supply of animal excrements, fluid and solid, and it not only remains
the same, but may be increased by a supply of mineral substances alone,
such as remain after the combustion of ligneous plants and other vegetables;
namely, ashes. Ashes represent the whole nourishment which vegetables
receive from the soil. By furnishing them in sufficient quantities to our
meadows, we give to the plants growing on them the power of condensing
and absorbing carbon and nitrogen by their surface. May not the effect of
the solid and fluid excrements, which are the ashes of plants and grains,
which have undergone combustion in the bodies of animals and of man, be
dependent upon the same cause? Should not the fertility, resulting from
their application, be altogether independent of the ammonia they contain?
Would not their effect be precisely the same in promoting the fertility of
cultivated plants, if we had evaporated the urine, and dried and burned the
solid excrements? Surely the cerealia and leguminous plants which we
cultivate must derive their carbon and nitrogen from the same source
whence the graminea and leguminous plants of the meadows obtain them!
No doubt can be entertained of their capability to do so.

In Virginia, upon the lowest calculation, 22 pounds weight of nitrogen
were taken on the average, yearly, from every morgen of the wheat-fields.
This would amount, in 100 years, to 2,200 pounds weight. If this were
derived from the soil, every morgen of it must have contained the
equivalent of 110,000 pounds weight of animal excrements (assuming the

latter, when dried, at the temperature of boiling water, to contain 2 per
cent.).

In Hungary, as I remarked in a former Letter, tobacco and wheat have
been grown upon the same field for centuries, without any supply of
nitrogenised manure. Is it possible that the nitrogen essential to, and
entering into, the composition of these crops, could have been drawn from
the soil?

Every year renews the foliage and fruits of our forests of beech, oak, and
chesnuts; the leaves, the acorns, the chesnuts, are rich in nitrogen; so are
cocoa-nuts, bread-fruit, and other tropical productions. This nitrogen is not
supplied by man, can it indeed be derived from any other source than the
atmosphere?

In whatever form the nitrogen supplied to plants may be contained in the
atmosphere, in whatever state it may be when absorbed, from the
atmosphere it must have been derived. Did not the fields of Virginia receive
their nitrogen from the same source as wild plants?

Is the supply of nitrogen in the excrements of animals quite a matter of
indifference, or do we receive back from our fields a quantity of the
elements of blood corresponding to this supply?

The researches of Boussingault have solved this problem in the most
satisfactory manner. If, in his grand experiments, the manure which he gave
to his fields was in the same state, i.e. dried at 110 deg in a vacuum, as it
was when analysed, these fields received, in 16 years, 1,300 pounds of
nitrogen. But we know that by drying all the nitrogen escapes which is
contained in solid animal excrements, as volatile carbonate of ammonia. In
this calculation the nitrogen of the urine, which by decomposition is
converted into carbonate of ammonia, has not been included. If we suppose
it amounted to half as much as that in the dried excrements, this would
make the quantity of nitrogen supplied to the fields 1,950 pounds.

In 16 years, however, as we have seen, only 1,517 pounds of nitrogen,
was contained in their produce of grain, straw, roots, et cetera—that is, far
less than was supplied in the manure; and in the same period the same

extent of surface of good meadow-land (one hectare = a Hessian morgen),
which received no nitrogen in manure, 2,062 pounds of nitrogen.

It is well known that in Egypt, from the deficiency of wood, the
excrement of animals is dried, and forms the principal fuel, and that the
nitrogen from the soot of this excrement was, for many centuries, imported
into Europe in the form of sal ammoniac, until a method of manufacturing
this substance was discovered at the end of the last century by Gravenhorst
of Brunswick. The fields in the delta of the Nile are supplied with no other
animal manures than the ashes of the burnt excrements, and yet they have
been proverbially fertile from a period earlier than the first dawn of history,
and that fertility continues to the present day as admirable as it was in the
earliest times. These fields receive, every year, from the inundation of the
Nile, a new soil, in its mud deposited over their surface, rich in those
mineral elements which have been withdrawn by the crops of the previous
harvest. The mud of the Nile contains as little nitrogen as the mud derived
from the Alps of Switzerland, which fertilises our fields after the
inundations of the Rhine. If this fertilising mud owed this property to
nitrogenised matters; what enormous beds of animal and vegetable exuviae
and remains ought to exist in the mountains of Africa, in heights extending
beyond the limits of perpetual snow, where no bird, no animal finds food,
from the absence of all vegetation!

Abundant evidence in support of the important truth we are discussing,
may be derived from other well known facts. Thus, the trade of Holland in
cheese may be adduced in proof and illustration thereof. We know that
cheese is derived from the plants which serve as food for cows. The
meadow-lands of Holland derive the nitrogen of cheese from the same
source as with us; i.e. the atmosphere. The milch cows of Holland remain
day and night on the grazing-grounds, and therefore, in their fluid and solid
excrements return directly to the soil all the salts and earthy elements of
their food: a very insignificant quantity only is exported in the cheese. The
fertility of these meadows can, therefore, be as little impaired as our own
fields, to which we restore all the elements of the soil, as manure, which
have been withdrawn in the crops. The only difference is, in Holland they
remain on the field, whilst we collect them at home and carry them, from
time to time, to the fields.

The nitrogen of the fluid and solid excrements of cows, is derived from
the meadow-plants, which receive it from the atmosphere; the nitrogen of
the cheese also must be drawn from the same source. The meadows of
Holland have, in the lapse of centuries, produced millions of
hundredweights of cheese. Thousands of hundredweights are annually
exported, and yet the productiveness of the meadows is in no way
diminished, although they never receive more nitrogen than they originally
contained.

Nothing then can be more certain than the fact, that an exportation of
nitrogenised products does not exhaust the fertility of a country; inasmuch
as it is not the soil, but the atmosphere, which furnishes its vegetation with
nitrogen. It follows, consequently, that we cannot increase the fertility of
our fields by a supply of nitrogenised manure, or by salts of ammonia, but
rather that their produce increases or diminishes, in a direct ratio, with the
supply of mineral elements capable of assimilation. The formation of the
constituent elements of blood, that is, of the nitrogenised principles in our
cultivated plants, depends upon the presence of inorganic matters in the
soil, without which no nitrogen can be assimilated even when there is a
most abundant supply. The ammonia contained in animal excrements
exercises a favourable effect, inasmuch as it is accompanied by the other
substances necessary to accomplish its transition into the elements of the
blood. If we supply ammonia associated with all the conditions necessary to
its assimilation, it ministers to the nourishment of the plants; but if this
artificial supply is not given they can derive all the needed nitrogen from
the atmosphere—a source, every loss from which is restored by the
decomposition of the bodies of dead animals and the decay of plants.
Ammonia certainly favours, and accelerates, the growth of plants in all
soils, wherein all the conditions of its assimilation are united; but it is
altogether without effect, as respects the production of the elements of
blood where any of these conditions are wanting. We can suppose that
asparagin, the active constituent of asparagus, the mucilaginous root of the
marsh-mallow, the nitrogenised and sulphurous ingredients of mustard-
seed, and of all cruciferous plants, may originate without the aid of the
mineral elements of the soil. But if the principles of those vegetables, which
serve as food, could be generated without the co-operation of the mineral
elements of blood, without potash, soda, phosphate of soda, phosphate of

lime, they would be useless to us and to herbivorous animals as food; they
would not fulfil the purpose for which the wisdom of the Creator has
destined them. In the absence of alkalies and the phosphates, no blood, no
milk, no muscular fibre can be formed. Without phosphate of lime our
horses, sheep and cattle, would be without bones.

In the urine and in the solid excrements of animals we carry ammonia,
and, consequently, nitrogen, to our cultivated plants, and this nitrogen is
accompanied by all the mineral elements of food exactly in the same
proportions, in which both are contained in the plants which served as food
to the animals, or what is the same, in those proportions in which both can
serve as nourishment to a new generation of plants, to which both are
essential.

The effect of an artificial supply of ammonia, as a source of nitrogen, is,
therefore, precisely analogous to that of humus as a source of carbonic acid
—it is limited to a gain of time; that is, it accelerates the development of
plants. This is of great importance, and should always be taken into account
in gardening, especially in the treatment of the kitchen-garden; and as much
as possible, in agriculture on a large scale, where the time occupied in the
growth of the plants cultivated is of importance.

When we have exactly ascertained the quantity of ashes left after the
combustion of cultivated plants which have grown upon all varieties of soil,
and have obtained correct analyses of these ashes, we shall learn with
certainty which of the constituent elements of the plants are constant and
which are changeable, and we shall arrive at an exact knowledge of the sum
of all the ingredients we withdraw from the soil in the different crops.

With this knowledge the farmer will be able to keep an exact record, of
the produce of his fields in harvest, like the account-book of a well
regulated manufactory; and then by a simple calculation he can determine
precisely the substances he must supply to each field, and the quantity of
these, in order to restore their fertility. He will be able to express, in pounds
weight, how much of this or that element he must give in order to augment
its fertility for any given kind of plants.

These researches and experiments are the great desideratum of the
present time. TO THE UNITED EFFORTS OF THE CHEMISTS OF ALL
COUNTRIES WE MAY CONFIDENTLY LOOK FOR A SOLUTION OF
THESE GREAT QUESTIONS, and by the aid of ENLIGHTENED
AGRICULTURISTS we shall arrive at a RATIONAL system of
GARDENING, HORTICULTURE, and AGRICULTURE, applicable to
every country and all kinds of soil, and which will be based upon the
immutable foundation of OBSERVED FACTS and PHILOSOPHICAL
INDUCTION.

LETTER XVI

My dear Sir,

My recent researches into the constituent ingredients of our cultivated
fields have led me to the conclusion that, of all the elements furnished to
plants by the soil and ministering to their nourishment, the phosphate of
lime—or, rather, the phosphates generally—must be regarded as the most
important.

In order to furnish you with a clear idea of the importance of the
phosphates, it may be sufficient to remind you of the fact, that the blood of
man and animals, besides common salt, always contains alkaline and earthy
phosphates. If we burn blood and examine the ashes which remain, we find
certain parts of them soluble in water, and others insoluble. The soluble
parts are, common salt and alkaline phosphates; the insoluble consist of
phosphate of lime, phosphate of magnesia, and oxide of iron.

These mineral ingredients of the blood—without the presence of which
in the food the formation of blood is impossible—both man and animals
derive either immediately, or mediately through other animals, from
vegetable substances used as food; they had been constituents of vegetables,

they had been parts of the soil upon which the vegetable substances were
developed.

If we compare the amount of the phosphates in different vegetable
substances with each other, we discover a great variety, whilst there is
scarcely any ashes of plants altogether devoid of them, and those parts of
plants which experience has taught us are the most nutritious, contain the
largest proportion. To these belong all seeds and grain, especially the
varieties of bread-corn, peas, beans, and lentils.

It is a most curious fact that if we incinerate grain or its flour, peas,
beans, and lentils, we obtain ashes, which are distinguished from the ashes
of all other parts of vegetables by the absence of alkaline carbonates. The
ashes of these seeds when recently prepared, do not effervesce with acids;
their soluble ingredients consist solely of alkaline phosphates, the insoluble
parts of phosphate of lime, phosphate of magnesia, and oxide of iron:
consequently, of the very same salts which are contained in blood, and
which are absolutely indispensable to its formation. We are thus brought to
the further indisputable conclusion that no seed suitable to become food for
man and animals can be formed in any plant without the presence and co-
operation of the phosphates. A field in which phosphate of lime, or the
alkaline phosphates, form no part of the soil, is totally incapable of
producing grain, peas, or beans.

An enormous quantity of these substances indispensable to the
nourishment of plants, is annually withdrawn from the soil and carried into
great towns, in the shape of flour, cattle, et cetera. It is certain that this
incessant removal of the phosphates must tend to exhaust the land and
diminish its capability of producing grain. The fields of Great Britain are in
a state of progressive exhaustion from this cause, as is proved by the rapid
extension of the cultivation of turnips and mangel wurzel—plants which
contain the least amount of the phosphates, and therefore require the
smallest quantity for their development. These roots contain 80 to 92 per
cent. of water. Their great bulk makes the amount of produce fallacious, as
respects their adaptation to the food of animals, inasmuch as their contents
of the ingredients of the blood, i.e. of substances which can be transformed

into flesh, stands in a direct ratio to their amount of phosphates, without
which neither blood nor flesh can be formed.

Our fields will become more and more deficient in these essential
ingredients of food, in all localities where custom and habits do not admit
the collection of the fluid and solid excrements of man, and their
application to the purposes of agriculture. In a former letter I showed you
how great a waste of phosphates is unavoidable in England, and referred to
the well-known fact that the importation of bones restored in a most
admirable manner the fertility of the fields exhausted from this cause. In the
year 1827 the importation of bones for manure amounted to 40,000 tons,
and Huskisson estimated their value to be from L 100,000 to L 200,000
sterling. The importation is still greater at present, but it is far from being
sufficient to supply the waste.

Another proof of the efficacy of the phosphates in restoring fertility to
exhausted land is afforded by the use of the guano—a manure which,
although of recent introduction into England, has found such general and
extensive application.

We believe that the importation of one hundred-weight of guano is
equivalent to the importation of eight hundred-weight of wheat—the
hundred-weight of guano assumes in a time which can be accurately
estimated the form of a quantity of food corresponding to eight hundred-
weight of wheat. The same estimate is applicable in the valuation of bones.

If it were possible to restore to the soil of England and Scotland the
phosphates which during the last fifty years have been carried to the sea by
the Thames and the Clyde, it would be equivalent to manuring with millions
of hundred-weights of bones, and the produce of the land would increase
one-third, or perhaps double itself, in five to ten years.

We cannot doubt that the same result would follow if the price of the
guano admitted the application of a quantity to the surface of the fields,
containing as much of the phosphates as have been withdrawn from them in
the same period.

If a rich and cheap source of phosphate of lime and the alkaline
phosphates were open to England, there can be no question that the
importation of foreign corn might be altogether dispensed with after a short
time. For these materials England is at present dependent upon foreign
countries, and the high price of guano and of bones prevents their general
application, and in sufficient quantity. Every year the trade in these
substances must decrease, or their price will rise as the demand for them
increases.

According to these premises, it cannot be disputed, that the annual
expense of Great Britain for the importation of bones and guano is
equivalent to a duty on corn: with this difference only, that the amount is
paid to foreigners in money.

To restore the disturbed equilibrium of constitution of the soil,—to
fertilise her fields,—England requires an enormous supply of animal
excrements, and it must, therefore, excite considerable interest to learn, that
she possesses beneath her soil beds of fossil guano, strata of animal
excrements, in a state which will probably allow of their being employed as
a manure at a very small expense. The coprolithes discovered by Dr.
Buckland, (a discovery of the highest interest to Geology,) are these
excrements; and it seems extremely probable that in these strata England
possesses the means of supplying the place of recent bones, and therefore
the principal conditions of improving agriculture—of restoring and exalting
the fertility of her fields.

In the autumn of 1842, Dr. Buckland pointed out to me a bed of
coprolithes in the neighbourhood of Clifton, from half to one foot thick,
inclosed in a limestone formation, extending as a brown stripe in the rocks,
for miles along the banks of the Severn. The limestone marl of Lyme Regis
consists, for the most part, of one-fourth part of fossil excrements and
bones. The same are abundant in the lias of Bath, Eastern and Broadway
Hill, near Evesham. Dr. Buckland mentions beds, several miles in extent,
the substance of which consists, in many places, of a fourth part of
coprolithes.

Pieces of the limestone rock in Clifton, near Bristol, which is rich in
coprolithes and organic remains, fragments of bones, teeth, &c., were

subjected to analysis, and were found to contain above 18 per cent. of
phosphate of lime. If this limestone is burned and brought in that state to the
fields, it must be a perfect substitute for bones, the efficacy of which as a
manure does not depend, as has been generally, but erroneously supposed,
upon the nitrogenised matter which they contain, but on their phosphate of
lime.

The osseous breccia found in many parts of England deserves especial
attention, as it is highly probable that in a short time it will become an
important article of commerce.

What a curious and interesting subject for contemplation! In the remains
of an extinct animal world, England is to find the means of increasing her
wealth in agricultural produce, as she has already found the great support of
her manufacturing industry in fossil fuel,—the preserved matter of primeval
forests,—the remains of a vegetable world. May this expectation be
realised! and may her excellent population be thus redeemed from poverty
and misery!

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