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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!










End of Project Gutenberg's Familiar Letters of Chemistry, by Justus Liebig