Davy's "Elements of Chemical Philosophy" examined.—His Memoir
on some combinations of Phosphorus and Sulphur, &c.—He
discovers Hydro-phosphoric gas.—Important Illustrations of
the Theory of Definite Proportionals—Bodies precipitated from
water are Hydrats.—His letter to Sir Joseph Banks on a new
detonating compound.—He is injured in the eye by its explosion.—His
second letter on the subject.—His paper on the Substances
produced in different chemical processes on Fluor Spar.—His
work on Agricultural Chemistry.
The "Elements of Chemical Philosophy," a work
to which he has alluded in several of the preceding
letters, was published in June 1812. It is dedicated
to Lady Davy, to whom he offers it "as a pledge
that he shall continue to pursue Science with unabated
ardour."
This work, although only a small part of the
great labour he proposed to accomplish, must be
considered as one of high importance to the cause
of science. It has not perhaps announced any discoveries
which had not been previously communicated
to the Royal Society, but it has brought
together his original results, and arranged them in
one simple and digested plan—it has given coherence
to disjointed facts, and has exhibited their
mutual bearings upon each other, and their general
relations to previously established truths.
Very shortly after the publication of this first
part, it was asserted by a scientific critic that the
work could never be completed upon the plan on
which it had commenced, which was little less than
a system of chemistry, in which all the facts were
to be verified by the author: an undertaking far
too gigantic for the most intrepid and laborious experimentalist
to accomplish. There was too much
truth in the remark:—the life of the Author has
closed—the work remains unfinished.
Although it bears the title of "Elements," its
plan and execution are rather adapted for the adept
than the Tyro in science; it has, however, enabled
the discoverer to expand several of his opinions with
a freedom which is not consistent with the studied
compression and elaborate brevity that necessarily
characterise the style of a Philosophical Memoir,—and
thus far it may have served the more humble
labourer.
The first impression which this volume must produce,
is that of admiration at the rapid and triumphant
progress of Chemistry, during the period
of a very few years; while a comparison of this
work with others, even of very recent date, will
show how much we are indebted for this progress
to the unrivalled labours of Davy.
The first part of his projected system, which constitutes
the volume under review, extends only to
the general laws of chemical changes, and to the
primary combinations of undecompounded bodies.
It is resolved into seven divisions, upon each of
which I propose to offer some remarks.
The First Division embraces the consideration
of the three different forms of matter, viz. Solidity,
Liquidity, and elastic Fluidity; and that of
the active powers on which they depend, and by
which they are changed, such as Gravitation, Cohesion,
Calorific repulsion, or Heat, and Attractions
chemical and electrical;—the laws of which
he has expounded in a lucid and masterly manner;
although it will be only necessary to quote the
following passage, to show that the greatest philosopher
may occasionally slide into error. "In
solids, the attractive force predominates over the
repulsive; in fluids, and in elastic fluids, they may
be regarded as in different states of equilibrium;
and in ethereal substances, the repulsive must be
considered as predominating over and destroying
the attractive force." A reviewer has very justly
observed, that it is difficult to conceive how so much
error and confusion could have been collected, by
such an author, into so short a sentence. It is a
solecism to say that two forces may exist in different
states of equilibrium; besides, it is generally
admitted that the repulsive force alone exists in
elastic fluids, and that it is only compensated by
external pressure, or gravitation.
In treating the subject of Heat, he maintains the
same opinion, though in a manner somewhat more
subdued, as that which he had formed at the very
commencement of his scientific career,[101]—that it is
nothing else than motion, and that the laws of Heat
are the same as the laws of Motion.
In taking a general view of the subject of Chemical
Attraction, there is a remarkable clearness
in his enunciation of its several propositions, and
a great felicity in the selection of its illustrations.
He combats the theory of Berthollet, respecting
the influence of mass, with singular success, and
confirms the general law, that all bodies combine
chemically, in certain definite proportions to be
expressed by numbers; so that, if one number be
employed to denote the smallest quantity in which
a body combines, all other quantities of the same
body will be as multiples of this number; and the
smallest proportions in which the undecompounded
substances enter into union being known, the constitution
of the compound they form may be learnt;
and the element which unites chemically in the
smallest quantity being expressed by unity, all the
other elements may be represented by the relations
of their quantities to unity. Unfortunately, however,
there has existed amongst philosophers a want
of agreement as to the unit to which the relative
values of the other numbers shall be referred. Mr.
Dalton selected Hydrogen as the unit; Davy followed
his example, but doubled the weight of
oxygen; while Wollaston, Thompson, and Berzelius,
have proposed oxygen as the most convenient
unit, since that element enters into the greatest
number of combinations.
To Dalton is now universally conceded the glory
of having established the laws of definite proportions;
but in unfolding them, he has employed
expressions which involve speculations as to their
physical cause, and has thus given to that, which
is nothing more than a copious collection of facts,
the appearance of a refined theory. It may be
perfectly true, as Mr. Dalton supposes, that all
bodies are composed of ultimate atoms; but in the
present state of our knowledge, we can neither form
any idea of the nature of such atoms, nor of the
manner in which they may be grouped together.
We are therefore indebted to Davy for having,
by his early and powerful example, taught the chemist
how to disentangle fact from hypothesis, and
to investigate the doctrine of proportionals, without
any reference to the atomic theory which has been
proposed for its explanation.
The Second Division treats of Radiant or
Ethereal Matter, and of its effects in producing
vision, heat, and chemical changes. It contains
some refined speculations respecting the possible
conversion of terrestrial bodies into light and heat,
and vice versâ.
The Third Division presents us with an account
of "Empyreal undecompounded Substances,"
or those which support combustion; together with
that of the compounds which they form with each
other. Upon this occasion, Davy has completely
rescued us from the trammels of the Anti-phlogistic
theory, and has shown that, so far from the process
of combustion depending upon the position or transfer
of oxygen, it is a general result of the actions of
any substances possessed of strong chemical attractions,
or different electrical relations, and that it
takes place in all cases in which an intense and
violent motion can be conceived to be communicated
to the corpuscules of bodies, without any regard
to the peculiar nature of the substances engaged.
The announcement of the general law is
followed by a history of the only two undecompounded
bodies included under this arrangement,
viz. Oxygen, and Chlorine.[102] In naming a class of
bodies by their relations to combustion, he distinctly
states that he merely intends to signify that the
production of heat and light is more characteristic
of their actions, than of those of any other substances;
and that they are, at the same time, opposed
to all other undecompounded substances by
their electrical relations, being always in Voltaic
combinations attracted to, or elicited from the positive
surface; whereas all other known undecompounded
substances are separated at the negative
surface.
The Fourth Division comprises the history
of Undecompounded Inflammables, or Acidiferous
Substances, not Metallic, and that of their binary
combinations with oxygen and chlorine, or with
each other.
The bodies considered under this division, are
the following:—Hydrogen, Azote, Sulphur, Phosphorus,
and Boracium, or Boron. Under the history
of Sulphur, he gives us the true theory of the process
by which sulphuric acid is produced by the
combustion of that body in mixture with nitre, and
which had never before been explained in any
chemical work.
The Fifth Division contains the Metals; their
primary combinations with other undecompounded
bodies, and with each other.
In the order of classification adopted on this occasion,
the newly discovered inflammable metals,
producing by combustion alkalies, alkaline earths,
and earths, commence the series; next come those
which produce oxides; and lastly, those which produce
acids. Thus are we presented with a chain
of gradations of resemblance which may be traced
throughout the whole series of metallic bodies.
The Sixth Division comprehends certain bodies
(the Fluoric Principle, and the Ammoniacal Amalgam)
which present some extraordinary and anomalous
results. It is worthy of remark, that, at the
period at which this work was written, Davy considered
the peculiar acid developed from fluor spar,
by the action of sulphuric acid, as a compound of an
acid unknown in a separate state, and water; whence
he proposed to call it Hydro-fluoric acid,—a term
extremely objectionable from its ambiguity, since it
would indicate either hydrogen or water as one of
its constituents. At the conclusion, however, of
this chapter, in consequence of having observed certain
phenomena displayed by this gas, when in combination
with silica and boracic acid, he for a moment
seems to have caught the truth, but it as
quickly eluded his grasp, and he dismisses the conjecture
which it was his good fortune some years
afterwards to verify, viz. that the fluoric acid is a
compound of an unknown principle, analogous to
chlorine, with hydrogen and water, and that fluor
spar is a compound of the same principle with
calcium, or the base of lime.
The Seventh Division offers to the chemical
enquirer various speculations, as to the probable nature
of certain bodies hitherto undecompounded.
He observes, that "we know nothing of the true
elements belonging to nature; but as far as we can
reason from the relations of the properties of matter,
that hydrogen is the substance which approaches
nearest to what the elements may be supposed to be.
It has energetic powers of combination, its parts are
highly repulsive of each other, and attractive of the
particles of other matter; it enters into combination
in a quantity very much smaller than any other
substance, and in this respect it is approached by no
known body. After hydrogen, oxygen perhaps
partakes most of the elementary character: it has a
greater energy of attraction, and, with the exception
just stated, enters into combination in the smallest
proportion."
In conclusion, he hints at the possibility of the
same ponderable matter in different electrical states,
or in different arrangements, constituting substances
chemically different, and he thinks that there are
parallel cases in the different states in which bodies
are found connected with their different relations to
temperature: thus, steam, ice, and water, are the
same ponderable matter; and certain quantities of
steam and ice mixed together produce ice-cold water.
"That the forms of natural bodies may depend
upon different arrangements of the same particles of
matter, has been a favourite hypothesis, advanced in
the earliest era of physical research, and often supported
by the reasonings of the ablest philosophers.
This sublime chemical speculation, sanctioned by the
authority of Hooke, Newton, and Boscovich, must
not be confounded with the ideas advanced by the
alchemists, concerning the convertibility of the elements
into each other. The possible transmutation
of metals has generally been reasoned upon, not as a
philosophical research, but as an empirical process.
Those who have asserted the actual production of
the precious metals, or their decomposition, or who
have defended the chimera of the philosopher's stone,
have been either impostors, or men deluded by impostors.
In this age of rational enquiry, it will be
useless to decry the practices of the adepts, or to
caution the public against confounding the hypothetical
views respecting the elements founded upon
distinct analogies, with the dreams of alchemical
visionaries, most of whom, as an author of the last
century justly observed, professed an art without
principles, the beginning of which was deceit, and
the end poverty."
On the 18th of June 1812, Davy presented to the
Royal Society a paper entitled "On some Combinations
of Phosphorus and Sulphur; and on some
other subjects of Chemical Inquiry."
By the researches detailed in this Memoir, he
accomplished three important objects: he established
the existence of some new compounds—furnished
additional evidence in support of the doctrine of
definite proportions—and ascertained that most of
the substances obtained from aqueous solutions by
precipitation, are compounds of water, or Hydrats.
In the first place, he recognised the formation of
two distinct compounds of phosphorus and chlorine:
one, solid, white, and crystalline in its appearance;
the other, fluid, limpid as water, and volatile. The
latter body he found to contain just double as much
chlorine as the former.
On experimenting upon this latter body with
water, he obtained a crystallized substance which he
proposed to call Hydro-phosphorous acid, since it
consists of pure phosphorous acid and water. By
decomposition in close vessels, it is resolved into
phosphoric acid, and a peculiar gas, consisting of one
proportional of phosphorus and four of hydrogen,
and for which he proposed the term Hydro-phosphorous
gas. The reader, no doubt, will be immediately
struck with the impropriety of a nomenclature
in which the prefix Hydro is made to
express water in the former, and hydrogen in the
latter instance.
In examining the results of the mutual decomposition
of water and the phosphoric compounds of
chlorine, Davy remarks, that it is scarcely possible
to imagine more perfect demonstrations of the laws
of definite combination: no products are formed
except the new combinations, (phosphoric acid from
the solid, phosphorous acid, from the liquid compound,
and in both muriatic acid;) neither oxygen,
hydrogen, chlorine, nor phosphorus, is disengaged;
and therefore the ratio in which any two of them
combine being known, the ratio in which the rest
combine, in these cases, may be determined by calculation.
Lastly, he ascertained that most of the substances
obtained by precipitation from aqueous solutions are
compounds of water: thus zircona, magnesia, and
silica, when precipitated and dried at 212°, still contain
definite proportions of water; and many of the
substances which had been considered as metallic
oxides, he found, when obtained from solutions, to
agree in this respect; and that their colours and
other properties are materially influenced by this
combined water.
On the 5th of November 1812, was read before
the Royal Society a letter addressed by Davy to
Sir Joseph Banks, on the subject of the detonating
compound already alluded to in his communications
to Mr. Children. He expresses his anxiety to have
the circumstances made public as speedily as possible,
since experiments upon the substance may be
connected with very dangerous results.
He had some time before received information
from Paris of a combination having been effected
between chlorine and azote, and that it was distinguished
by detonating properties; but he was wholly
ignorant of the mode by which it had been prepared,
and he could not obtain any information
upon this point from any of the French journals.
So curious and important a result could not fail
to interest him, as he had himself been long engaged
in experiments on the action of azote and chlorine,
without gaining any decided proofs of their power
of combining with each other. It was evident from
the notice, that this new body could not be formed
in any operations in which heat is concerned; he
therefore attempted to combine the elements by presenting
them to each other artificially cooled, the
azote being in a nascent state. For this purpose he
introduced chlorine into a solution of ammonia; a
violent action ensued, and minute films of a yellow
colour were observed on the surface of the liquor,
but they immediately resolved themselves into gas.
As he was about to repeat the experiment with
some other ammoniacal compounds, Mr. Children
reminded him of the circumstance which he had
previously communicated to him in a letter, that
Mr. James Burton, junr, on exposing chlorine to a
solution of nitrate of ammonia, had observed the
formation of a yellow oil, but which he had not been
able to collect. Davy availed himself of the hint,
and obtained the substance in question: on examining
its properties by the application of heat, the
tube in which it was contained was shivered to
atoms by its explosion, and he received a severe
wound in the transparent cornea, which was followed
by inflammation, and disabled him from
pursuing his enquiry.
In the following July, however, he communicated
in a second letter to Sir Joseph Banks, the continuation
of this enquiry, and furnished a full and satisfactory
history of the body in question. Having
procured it in sufficient quantity, he attempted to
effect its analysis by the action of mercury, but a
violent detonation occurred, and he was again
wounded in the head and hands; fortunately, however,
the injury was slight, in consequence of his
having taken the precaution to defend his face by a
plate of glass attached to a proper cap.
In a subsequent experiment, by using smaller
quantities, and recently distilled mercury, he succeeded
in obtaining results without any violence of
action: the mercury united with the chlorine, and
the azote was disengaged; from which he was enabled
to conclude that it was composed of four
volumes of chlorine and one volume of azote. For
this new body Davy suggested the name of Azotane;
but I have already observed, that his nomenclature
of the compounds of chlorine has never been adopted;
the detonating substance is now very properly
denominated Chloride of Nitrogen.
Shortly after the publication of this paper, M.
Berzelius, in a letter to Professor Gilbert, asserted
that "Azotane" is nothing more than dry nitro-muriatic
acid, since it dissolves slowly in water, and
forms a weak aqua regia. "These few observations,"
says he, "show clearly that Davy's analysis of this
substance is inaccurate, and that he corrected his
results in consequence of theoretical views."
This was an imputation upon the philosophical
character of Davy, which excited in him no small
degree of indignation. In reply he says, "It is
difficult to discover what meaning M. Berzelius
attaches to the term dry nitro-muriatic acid; and it
is wholly unnecessary to refute so unfounded and
vague an assertion."
On July 8, 1813, a paper was read by Davy before
the Royal Society, entitled "Some Experiments
and Observations on the Substances produced in different
chemical processes on Fluor Spar."
The views which he formerly entertained with respect
to the fluoric acid have been already noticed:[103]
in the present paper he renounces his previous opinions,
and establishes, by experiments of the most
satisfactory character, that the base of fluoric acid is
a highly energetic body not hitherto obtained in an
insulated form, and the properties peculiar to which
are as yet unknown. It appears, however, to belong
to the class of negative electrics, and, like oxygen
and chlorine, to have a powerful affinity for hydrogen
and metallic substances. With hydrogen, it
constitutes the peculiar and very powerful acid long
known by the name of fluoric acid,—with boron,
the fluoboric, and with silicium, the silicated-fluoric,
acids. Although this theory had originally suggested
itself to the mind of Davy, yet the chemical
world is unquestionably indebted to M. Ampère for
establishing it; and the English chemist has very
justly acknowledged the obligation. "During the
period that I was engaged in these investigations,"
says he, "I received two letters from M. Ampère, of
Paris, containing many ingenious and original arguments
in favour of the analogy between the muriatic
and fluoric compounds. M. Ampère communicated
his views to me in the most liberal manner:
they were formed in consequence of my ideas
on chlorine, and supported by reasonings drawn from
the experiments of MM. Gay Lussac and Thénard."
It has been stated that Davy gave his last public
lecture on the 9th of April 1812; he however afterwards
delivered an occasional lecture to the Managers,
on his own discoveries, and did not formally
resign his professorship until the next year.
The following record has been extracted from the
Journal of the Institution.
"Minutes of the Proceedings of a general Monthly
Meeting of the Members of the Royal Institution,
held on Monday, April 5, 1813.
"Earl of Winchelsea, President, in the Chair.
"This being the meeting appointed by Article 2.
chap. xix. of the bye-laws, for putting in nomination
from the chair the professors for the year ensuing,
Sir Humphry Davy rose, and begged leave to resign
his situation of Professor of Chemistry; but he by
no means wished to give up his connection with the
Royal Institution, as he should ever be happy to
communicate his researches, in the first instance, to
the Institution, in the manner he did in the presence
of the members last Wednesday, and to do all
in his power to promote the interest and success of
this Institution.
"Sir H. Davy having retired, Earl Spencer moved,
That the thanks of this Meeting be returned to Sir
H. Davy, for the inestimable services rendered by
him to the Royal Institution. This motion was seconded
by the Earl of Darnley, and on being put,
was carried unanimously.
"Earl Spencer further moved, That in order more
strongly to mark the high sense entertained by this
Meeting of the merits of Sir H. Davy, he be elected
Honorary Professor of Chemistry; which, on being
seconded by the Earl of Darnley, met with unanimous
approbation.
"The Chairman having declared the Professorship
of Chemistry vacant, put in nomination William
Thomas Brande, Esq. F.R.S. as a candidate for
that office, with a salary of 200l. per annum.
"On Monday, June 7, 1813, William Thomas
Brande, Esq. was unanimously elected."
In March 1813, Davy published his "Elements
of Agricultural Chemistry," being the substance of
a course of lectures which he had, for ten successive
seasons, delivered before the members of the Board
of Agriculture, to whom the work is inscribed, as a
mark of the author's respect.
This work, which may be considered as the only
system of philosophical agriculture ever published
in this country, has not only contributed to the advancement
of science, but to that for which he has
an equal claim upon our gratitude,—the diffusion
of a taste amongst the higher classes for its cultivation;
for it has been wisely remarked, that not he
alone is to be esteemed a benefactor to mankind
who makes an useful discovery, but he, also, who
can point out an innocent pleasure.
It has been already stated, that Davy became
early impressed with the importance of the subject:—that
in future life its investigation should have
been to him so fertile a source of pleasure, may be
readily imagined, when it is remembered with what
passionate delight he contemplated the ever varying
forms of creation. "I am," said he, "a lover of
Nature, with an ungratified imagination, and I shall
continue to search for untasted charms—for hidden
beauties." In unfolding, then, the secrets of vegetable
life, he did but remove the veil from his mistress.
From the same poetical feeling sprang his
love of angling: it was a pursuit which carried
him into the wild and beautiful scenery of Nature,
amongst the mountain lakes, and the clear and
lovely streams that gush from elevated hills, or
make their way through the cavities of calcareous
strata.[104] In the early spring, it led him forth upon
the fresh turf in the vernal sunshine, to scent the
odour of the bank perfumed by the violet, and
enamelled with the primrose, while his heart participated
in the renovated gladness of Nature.
I had hoped that, amidst the voluminous correspondence
of my late friend Mr. Arthur Young,
some important letters might have been found from
Davy on agricultural subjects; but the communications
which took place between them were generally
in conversation, and I have therefore only been able
to procure two letters, which I shall here insert:
the first will show that, during his tours, his attention
was alive to the practices of husbandry; and
the second will prove that he had once seriously
contemplated the labour of writing the agricultural
history of his native county.
TO ARTHUR YOUNG, ESQ.
Killarney, June 1806.
DEAR SIR,
You have been of great and durable service to
Ireland. I have met with a number of persons who
have been enlightened by your labours, and who
now follow an enlightened system of Agriculture.
One very intelligent gentleman you will recollect,—Mr.
Bolton of Waterford: he is zealously pursuing
improvements, and is instructing his neighbours by
precept and example. I am, &c.
H. Davy.
The above letter contains also some observations
on a chemical mixture, but which is unintelligible
from our being ignorant of the conversation to
which it refers.
TO THE SAME.
April, 1807.
DEAR SIR,
I called this morning with the hope of seeing
you, and of gaining some explanation on the subject
of your note. I shall not be able to leave London
until the middle of July, and I must return
early in October.
I do not think there would be sufficient time
between these periods for accomplishing the objects
you mention; nor do I think myself qualified to
write upon the agriculture of a county. I wished
likewise to devote the leisure of this summer to
the preparation of my lectures on the Chemistry of
Agriculture for publication. I have a great deal of
information concerning the mineralogy and geology
of Cornwall, but none concerning the farming.
If the business admits of being postponed, I
might perhaps be able to accomplish it next summer;
that is, by devoting a part of this summer,
and the whole of my next: but I would rather confine
myself to my own province, the mineralogy
and geology of the county, and leave the agriculture
to abler hands.
Be pleased to receive my thanks, and to communicate
them to the President for the honour of the
proposal. I remain, &c.
H. Davy.
The majority of my readers will probably concur
in the wisdom of this decision: they will consider
that to have doomed Davy to a drudgery of this
nature, would have been wasting talents upon an
object which might be accomplished by smaller
means. From my acquaintance, however, with Cornwall,
I am induced to form a different opinion.
Davy never approached even those subjects which
had already received from others the most thorough
investigation, without extracting from them new
and important truths. What, then, might not have
been expected from his genius, when applied to a
department upon which the light of science had
scarcely dawned?
It is only in a primitive country like Cornwall,
that the natural relations between the varieties of
soil and the subjacent rocks can be studied with
success: as we advance to alluvial districts, such
relations become gradually less distinct and apparent,
and are ultimately lost in the confused complication
of the soil itself, and in that general obscurity
which envelopes every object in the ulterior stages
of decomposition. We can, therefore, only hope to
succeed in such an investigation by a patient and
laborious examination of a primitive country, after
which we may be enabled to extend our enquiries
with greater advantage through those regions which
are more completely covered with soil, and obscured
by luxuriant vegetation; as the eye, acquainted
with the human figure, on gazing upon a beautiful
statue, traces the outline of the limbs, and the swelling
contour of its form, through the flowing draperies
which invest it. The importance of the subject,
as well as the general interest it has excited,
induce me to offer an analysis of his "Elements of
Agricultural Chemistry."
The work is divided into eight lectures; and in
his introductory chapter, after adverting to the difficulties
which the enquiry presents to the lecturer,
he offers a general view of the objects of the course,
and of the order in which he proposes to discuss
them.
"Agricultural Chemistry has not yet received a
regular and systematic form. It has been pursued
by competent experimenters for a short time only;
the doctrines have not as yet been collected into an
elementary treatise; and on an occasion when I am
obliged to trust so much to my own arrangements,
and to my own limited information, I cannot but
feel diffident as to the interest that may be excited,
and doubtful of the success of the undertaking. I
know, however, that your candour will induce you
not to expect any thing like a finished work upon a
science as yet in its infancy; and I am sure you
will receive with indulgence the first attempt made
to illustrate it, in a distinct course of lectures.
"Agricultural Chemistry has for its objects all
those changes in the arrangements of matter connected
with the growth and nourishment of plants;
the comparative values of their produce as food;
the constitution of soils; and the manner in which
lands are enriched by manure, or rendered fertile by
the different processes of cultivation." That such
objects are intimately connected with the doctrines
of chemistry, he proceeds to show by several appropriate
and striking illustrations.
"If land be unproductive, and a system of ameliorating
it is to be attempted, the sure method of
obtaining the object is, by determining the cause of
its sterility, which must necessarily depend upon
some defect in the constitution of the soil, which
may be easily discovered by chemical analysis. Are
any of the salts of iron present? they may be decomposed
by lime. Is there an excess of siliceous
sand? the system of improvement must depend on
the application of clay and calcareous matter. Is
there a defect of calcareous matter? the remedy is
obvious. Is an excess of vegetable matter indicated?
it may be removed by liming, paring, and
burning. Is there a deficiency of vegetable matter?
it is to be supplied by manure."
"In the selection also of the remedy, after the discovery
of the evil, chemical knowledge is of the
highest importance. Limestone varies in its composition,
and by its indiscriminate application we
may aggravate the sterility we seek to obviate.
Peat earth is an excellent manure, but it may contain
such an excess of iron as to be absolutely poisonous
to plants. How are such difficulties to be
met but by the resources of chemistry? It is also
evident that the scientific agriculturist should possess
a general knowledge of the nature and composition
of material bodies, and the laws of their
changes; for the surface of the earth, the atmosphere,
and the water deposited from it, must, either
together or separately, afford all the principles concerned
in vegetation; and it is only by examining
the chemical nature of these principles, that we are
capable of discovering what is the food of plants,
and the manner in which this food is supplied and
prepared for their nourishment."
Davy likewise advocates the necessity of studying
"the phenomena of vegetation, as an important
branch of the science of organized nature; for, although
exalted above inorganic matter, vegetables
are yet in a great measure dependent for their existence
upon its laws. They receive their nourishment
from the external elements; they assimilate it
by means of peculiar organs; and it is by examining
their physical and chemical constitution, and the
substances and powers which act upon them, and
the modifications which they undergo, that the
scientific principles of Agricultural Chemistry are
obtained."
With respect, however, to the practical utility of
this latter branch, different opinions have been entertained.
I confess, I am inclined to agree with
an able reviewer[105] when he says, "It is the proper
business of the chemist to examine and ascertain the
nature and properties of dead and inorganized matter,
and the various combinations which, according
to chemical laws, it is capable of forming. The
chemical composition of organized bodies, and of
the products which they form, fall likewise under
his cognizance; but when he proceeds to consider
the physical constitution of these bodies, and the
manner in which they act in forming their products,
he no longer works with the instruments of the
laboratory, or conducts processes which can be properly
imitated there."
In concluding his introductory observations, he
remarks upon the prejudice which persons, who
argue in favour of practice and experience, very
commonly entertain against all attempts to improve
agriculture by philosophical enquiries and chemical
methods. "That much vague speculation may be
found in the works of those who have lightly taken
up agricultural chemistry, it is impossible to deny.
It is not uncommon to find a number of changes
rung upon a string of technical terms, such as oxygen,
hydrogen, carbon, and azote, as if the science
depended upon words, rather than upon things.
But this is, in fact, an argument for the necessity of
the establishment of just principles of chemistry on
the subject.—If a person journeying in the night
wishes to avoid being led astray by the ignis fatuus,
the most secure method is to carry a lamp in his
own hand."
"There is no idea more unfounded than that a
great devotion of time, and a minute knowledge of
general chemistry, are necessary for pursuing experiments
on the nature of soils, or the properties of
manures. The expense connected with chemical
enquiries is extremely trifling: a small closet is
sufficient for containing all the materials required."
In the Second Lecture, he enters upon the
consideration of the general powers of matter, such
as gravitation, cohesion, chemical attraction, heat,
light, and electricity; and then proceeds to examine
the elements of matter, and the laws of their combinations
and arrangements.
To an audience constituted of persons who were
not familiar with the elementary principles of the
science, it might have been very necessary for the
lecturer to enter upon such preliminary details; but
there cannot be any good reason for his having published
them in his system. As they are to be found
in every work on chemistry, it will not be necessary
to bestow upon them any further notice.
In the Third Lecture, he enters into a description
of the organization and living system of plants;
in which he connects together into a general view,
the observations of the most enlightened philosophers
who have studied the physiology of vegetation—those
of Grew, Malpighi, Sennebier, Hales, Decandolle,
Saussure, Bonnet, Darwin, Smith, and
above all, of Mr. Knight, whose enquiries upon
these subjects are not only the latest, but by far the
most satisfactory and conclusive.
As there is little in these descriptions that may
not be found in the original authors, I shall not unnecessarily
trespass upon the time of the reader by
relating them. In the latter part of this lecture, he
describes the properties and ultimate composition of
the proximate principles of which vegetable matter
consists, and into which it may be resolved by different
processes of art; such are gum, starch, sugar,
albumen, gluten, extract, tannin, resin, oils, &c. &c.
But since the publication of this work, vegetable
analysis has advanced to a degree of refinement
which could scarcely have been anticipated in so
short a period, and consequently many of his statements
appear deficient; but his general directions
for conducting an analysis of any vegetable substance,
with a degree of accuracy sufficient for the
views of the agriculturist, remain unimpeached.
The most valuable, and more strictly original part
of this lecture, is his statement of the quantity of
soluble or nutritive matters contained in varieties of
the different substances that are used as articles of
food, either for man or cattle, and which he has
displayed in a tabular form.
The analyses were his own, and were conducted
with a view to a knowledge of the general nature
and quantity of the products, rather than to that of
their intimate chemical composition. He proceeded
upon the assumption, that the excellence of the different
articles, as food, will be in a great measure proportional
to the quantities of soluble matter they afford;
although he admits that these quantities cannot be regarded
as absolutely denoting their value. Albuminous
or glutinous matters have the characters of animal
substances; sugar is more, and extractive matter
less nourishing than any other principles composed
of carbon, hydrogen, and oxygen. Certain combinations
likewise of these substances may be more nutritive
than others. There are some principles also,
which, although soluble in the vessels of the chemist,
pass through the alimentary canal of animals without
change; such is tannin: on the other hand,
there are bodies which, although sparingly soluble
in water, are readily acted upon by the gastric juice;
gluten is a principle of this description.
Shortly after Dr. Wollaston published his scale
of chemical equivalents, it occurred to me that by
applying the sliding rule to a series of nutritive substances,
arranged according to the analyses of Davy,
some curious and important problems[106] might be
solved; or at least, that the accuracy of the conclusions
might be thus conveniently submitted to the
test of practice. I accordingly superintended the
construction of such an instrument, and submitted
it to Davy, who expressed his approbation of the
principle, but doubted how far the accuracy of his
analyses would justify the experiment.
To such a scheme, however, I soon found that
there existed a much more serious objection. The
operation of the insoluble matter had been wholly
neglected; and whatever views the chemist may
entertain, the experience of the physiologist has
established, beyond doubt, the influence of such
matter in the process of digestion. The capacity
of the alimentary organs of graminivorous animals
sufficiently proves that they were designed for the
reception of a large bulk of food, and not for provender
in which the nutritive matter is concentrated;
and since the gramineous and leguminous
vegetables do not present this matter in a separate
state, and the animal is not furnished with
an apparatus by which he can remove it, the obvious
inference is, that he was designed to feed indiscriminately
upon the whole; and that, unless
bulk be taken into the account, no fair inference
can be deduced as to the nutritive value of different
vegetables.
Notwithstanding the difficulties which prevent
our arriving at any thing like an accurate conclusion
upon so complicated a subject, the results may
be received as affording some general views with
regard to the comparative value of different nutritive
vegetables. It would thus appear that at
least a fourth part of the weight of the potatoe
consists of nutritive matter, which is principally
starch;—that wheat consists of as much as ninety-five,
barley of ninety-two, oats of seventy-five, rye
of eighty, and peas and beans of about fifty-seven
per cent. of nutritive matter.
The Fourth Lecture comprises subjects of the
utmost importance, and must be considered as constituting
by far the most original and valuable
division of the work. It treats of soils,—their
constituent parts, their chemical analysis, their uses,
their improvement, and of the rocks and strata
found beneath their surface.
In the execution of this part of his labours, he
has not only improved on the processes of Fordyce
and Kirwan, but he has enriched the subject with
much interesting and novel research.
"Soils, although extremely diversified in appearance
and quality, consist of comparatively few elements,
which are in various states of chemical combination,
or of mechanical mixture.
"These substances are silica, lime, alumina, magnesia,
the oxides of iron, and of manganese; animal
and vegetable matters in a state of decomposition;
together with certain saline bodies, such as common
salt, sulphate of magnesia, sometimes sulphate of
iron, nitrates of lime and magnesia, sulphate of potash,
and the carbonates of potash and soda.
"The silica in soils is usually combined with
alumina and oxide of iron; or with alumina, lime,
magnesia, and oxide of iron, forming gravel and
sand of different degrees of fineness. The carbonate
of lime is usually in an impalpable form; but sometimes
in the state of calcareous sand. The magnesia,
if not combined in the gravel and sand of the
soil, is in a fine powder united to carbonic acid.
The impalpable part of the soil, which is commonly
called clay or loam, consists of silica, alumina, lime,
and magnesia; and is, in fact, visually of the same
composition as the hard sand, but more finely divided.
The vegetable, or animal matters (and the
first is by far the most common in soils,) exist in
different states of decomposition. They are sometimes
fibrous, sometimes entirely broken down and
mixed with the soil.
"To form a just idea of soils, it is necessary to
conceive different rocks decomposed, or ground into
parts and powder of different degrees of fineness;
some of their soluble parts dissolved by water, and
that water adhering to the mass, and the whole
mixed with larger or smaller quantities of the remains
of vegetables and animals, in different stages
of decay."
Soils, then, would appear to have been originally
produced from the disintegration of rocks and strata;
and hence there must be at least as many varieties
of them, as there are species of rocks exposed at the
surface of the earth; and they may be distinguished
by names derived from the rocks from which they
were formed. Thus, if a fine red earth be found immediately
above decomposing basalt, it may be denominated
basaltic soil. If fragments of quartz and
mica be found abundant, it may be denominated
granitic soil; and the same principles may be extended
to other analogous cases.
A general knowledge then of geology becomes
essential to the scientific agriculturist, not only to
enable him to form a correct judgment with respect
to the connection between the varieties of soil and
the subjacent rocks, but to direct him to the different
mineral substances which may be associated
together in their vicinity, and which may contain
principles capable of extending their fertility, or of
correcting the circumstances upon which their poverty
or barrenness may depend.
With this conviction, Davy proceeds to offer a
general view of the nature and position of rocks and
strata in nature; but which, I confess, appears to
me to be wholly useless to those who have any
acquaintance with the subject, and far too meagre
to convey any instruction to those who have not
made this branch of science an object of study.
Upon this view, however, he has grounded a
number of valuable remarks; although his observations
appear to have been too limited to enable
him to do justice to a subject of such extent and
importance. Had he fulfilled his intention of making
a survey of the county of Cornwall, the science
must have been greatly advanced by his labours, for
there is no district in Great Britain so rich in fact,
and so capable of elucidating the history of soil, and
the advantages of cultivation, when conducted on
the principles of chemical philosophy. The soils
superincumbent upon the different rocks are distinct
and characteristic; and even in the same species
varieties may be observed, in consequence of geological
peculiarities. I have, for instance, found
that the fertility of a granitic soil is increased by
the abundance of felspar in the parent rock;—that
of a slaty soil by the degree of inclination or dip of
the strata: but the most extraordinary circumstance
perhaps connected with this subject, is the very
remarkable fertility of the land which lies over the
junction of these rocks,—so obvious indeed is it,
that the eye alone is sufficient to trace it.
We are indebted to the author, in this lecture,
for some very ingenious and important remarks on
the relations of different soils to heat and moisture,
and for a series of experiments by which his views
are supported.
Some soils, he observes, are more easily heated
and more easily cooled than others: for example,
those that consist principally of a stiff white clay
are heated with difficulty; and being usually very
moist, they retain their heat only for a short time.
Chalks also are difficultly heated; but being dryer,
they retain their heat longer, less being consumed
in the process of evaporation.
A black soil, and those that contain much carbonaceous
or ferruginous matter, acquire a higher temperature
by exposure to the sun, than pale-coloured
soils.
When soils are perfectly dry, those that most
readily become heated, most rapidly cool; but the
darkest-coloured dry soil, abounding in animal and
vegetable matters, cools more slowly than a wet pale
soil, composed entirely of earthy matter.
These results Davy gained by experiments made
on different kinds of soils, exposed for a given time
to the sun, and in the shade; the degrees of heating
and cooling having been accurately ascertained by
the thermometer.
Nothing can be more evident, than that the
genial heat of the soil, particularly in spring, must
be of the highest importance to the rising plant.
And when the leaves are fully developed, the
ground is shaded, and any injurious influence,
which in the summer might be expected from too
great a heat, entirely prevented; so that the
temperature of the surface, when bare and exposed to
the rays of the sun, affords at least one indication
of the degree of its fertility; and the thermometer
may therefore be sometimes a useful instrument to
the purchaser or improver of lands.
Water is said to exist in soils, either in a state of
chemical combination, or of cohesive attraction. It
is in the latter state only that it can be absorbed by
the roots of plants, unless in the case of the decomposition
of animal and vegetable substances. The
more divided the parts of the soil are, the greater is
its attractive power for water; and the addition of
vegetable and animal matters still farther increases
this power.
The quality of soils to absorb water from air, is
much connected with fertility. Davy informs us
that he has compared this absorbent power in numerous
instances, and that he always found it greatest
in the most productive lands: he states, however,
the important fact, that those soils, such for instance
as stiff clays, which take up the greatest quantity
of water, when it is poured upon them in a fluid
form, are not such as absorb most moisture from
the atmosphere in dry weather. They cake, and
present only a small surface to the air, and the vegetation
on them is generally burnt up almost as
readily as on sands.
There is probably no district in which the importance
of moisture in relation to fertility is more
apparent than in Cornwall; and there is a provincial
saying, that the land will bear a shower every weekday,
and two upon a Sunday: indeed, of such importance
is moisture, that it is by no means an uncommon
practice to encourage the growth of weeds,
in order to diminish the evaporation; a necessity
which arises from the excess of siliceous matter in
the soil.
To those who are disposed to prosecute this enquiry,
I should recommend a perusal of Mr. Leslie's
treatise on the "Relations of Air to Heat and
Moisture."
I must not quit the consideration of this lecture,
without adverting to the directions with which its
author has furnished the philosophical farmer for
analysing the different varieties of soil; and which
are so clear, so perfect, and above all so simple, that
they are now introduced into all elementary works
on chemistry, as the only guide to such researches.
His method for ascertaining the quantity of carbonate
of lime in any specimen, consists in determining
the loss of weight which takes place on its admixture
with muriatic acid; for since carbonate of lime,
in all its states, contains a determinate proportion of
carbonic acid, it is evident that, by estimating the
quantity of elastic matter given out, the proportion
of carbonate of lime will be known. For conducting
this experiment, he contrived a very simple and
ingenious piece of pneumatic apparatus, in which
the bulk of the carbonic acid is at once measured
by the quantity of water it displaces.
In his Fifth Lecture he enters upon the nature
of the atmosphere, and its influence on vegetables:
he also examines the process of the germination of
seeds, and the functions of plants in their different
stages of growth; and concludes with a general
view of the progress of vegetation.
I shall merely mention a few of the more interesting
points in this enquiry.
In illustrating the importance of water to the
vegetable creation, he observes that the atmosphere
always contains water in its elastic and invisible
form, the quantity of which will vary with the temperature.
In proportion as the weather is hotter,
the quantity is greater; and it is its condensation
by diminution of temperature, which gives rise to
the phenomena of dew and mist. The leaves of
living plants appear to act upon this vapour, and to
absorb it. Some vegetables increase in weight from
this cause, when suspended in the atmosphere, and
unconnected with the soil; such are the house-leek,
and different species of the aloe. In very intense
heats, and when the soil is dry, the life of plants
seems to be preserved by the absorbent powers of
their leaves; and it is a beautiful circumstance in
the economy of Nature, that aqueous vapour is most
abundant in the atmosphere when it is most needed
for the purposes of life; and that when other sources
of its supply are cut off, this is most copious.[107]
If water in its elastic and fluid states be essentially
necessary to the economy of vegetation, so
even in its solid form, it is not without its uses.
Snow and ice are bad conductors of heat; and at a
period when the severity of the winter threatens
the extinction of vegetable life, Nature kindly
throws her snowy mantle over the surface; while in
early spring the solution of the snow becomes the
first nourishment of the plant; at the same time, the
expansion of water in the act of congelation, and
the subsequent contraction of its bulk during a
thaw, tend to pulverise the soil, to separate its parts
from each other, and, by making it more permeable
to the influence of the air, to prepare it for the offices
it is destined to perform.
He next proceeds to consider the action of the
atmosphere on plants, and to connect it with a general
view of the progress of vegetation. He commences
with examining its relations to germination.
"If a healthy seed be moistened and exposed to
air at a temperature not below 45°, it soon germinates;
it shoots forth a plume which rises upwards,
and a radicle which descends.
"If the air be confined, it is found that, in the
process of germination, the oxygen, or a part of it,
is absorbed. The azote remains unaltered; no carbonic
acid is taken away from the air; on the contrary,
some is added." Upon this point, critics have
been disposed to break a lance with Sir Humphry.
The doctrine, let it be observed, is at variance with
the numerous experiments made on this subject by
Scheele, Cruickshank, and De Saussure; the results
of which agree in proving, that if seeds be confined
and made to germinate in a given portion of air, not
a part only, but the whole of the oxygen is consumed;
and that its place is supplied, not merely by
some, but by an equal bulk of carbonic acid.
Objections have been also started to his theory of
the chemical changes which the seed undergoes
during the process of germination: but were I to
enter upon these discussions, time and space would
alike fail me, to say nothing of the patience of the
reader, which would be exhausted long before we
could arrive at any satisfactory conclusion. I shall
for the same reasons pass over his observations upon
the influence exerted upon growing plants on the
air: the subject is involved in much difficulty,
which can be only removed by fresh experiments;
nor, after all, is the great question, whether the
purity of the atmosphere is maintained by vegetation,
of any practical moment,—it is one which partakes
more of curiosity than of use, and might
therefore have been well dispensed with in a system
of agriculture.
He agrees with many other philosophers in considering
"the process of malting as merely one in
which germination is artificially produced, and in
which the starch is changed into sugar, which sugar
is afterwards, by fermentation, converted into spirit.
"It is," he continues, "very evident from the
chemical principles of germination, that the process
should be carried on no farther than to produce the
sprouting of the radicle, and should be checked as
soon as this has made its distinct appearance. If it
is pushed to such a degree as to occasion the perfect
developement of the radicle and the plume, a considerable
quantity of saccharine matter will have
been consumed in producing their expansion, and
there will be less spirit formed in fermentation, or
produced in distillation.
"As this circumstance is of some importance, I
made, in October 1806, an experiment relating to it.
I ascertained by the action of alcohol, the relative
proportions of saccharine matter in two equal quantities
of the same barley; in one of which the germination
had proceeded so far as to occasion protrusion
of the radicle to nearly a quarter of an inch
beyond the grain in most of the specimens, and in
the other of which it had been checked before the
radicle was a line in length; the quantity of sugar
afforded by the last was to that in the first nearly as
six to five."
The whole of this subject appears to be debateable
ground between the physiologists and chemists: the
one considering the change of starch into sugar as
the result of the vital action of the seed; the other
affirming that the growth of the germ is in no way necessary
to the result, and is to be considered as a mere
indication of the due degree of change being effected
in the organic matter, or, in other words, that when
the organized parts exhibit a certain degree of developement,
then the inorganic matter is most completely
changed. All growth beyond this is injurious,
as leading to a consumption of the inorganic
matter. All less than this is not otherwise disadvantageous,
than as an indication that the inorganic
matter is not duly changed. This change, it is
farther affirmed, so far from depending upon vegetable
life, can be wrought on the matter of the seed
after it is even reduced to powder, or is separated in
the form of starch. At all events, it must be admitted
as a beautiful arrangement in nature, that
the same agents which urge on the developement
of the organized parts, should, at the same time,
assist in preparing food for their support.
From this subject Davy is very naturally led to
the consideration of the ravages inflicted upon the
infant plant by insects; the saccharine matter in the
cotyledons at the time of their change into seed-leaves,
rendering them exceedingly liable to such
attacks. He appears to have bestowed much attention
on the turnip-fly, a colyopterous insect, which
fixes itself upon the seed-leaves of the turnip at
the time that they are beginning to perform their
functions. He relates the several remedies which
have been proposed for this evil; and from letters
which have been put in my possession, addressed to
Dr. Cartwright as early as the year 1804, he appears
to have been engaged with that gentleman in experiments
made by sprinkling the young plants with
lime and urine.
After alluding to the parasitical plants of different
species, which attach themselves to trees and shrubs,
feed on their juices, destroy their health, and finally
their life, for which, at present, there does not exist
any remedy, he thus concludes his lecture:
"To enumerate all the animal destroyers, and
tyrants of the vegetable kingdom, would be to give
a catalogue of the greater number of the classes in
Zoology. Every species of plant almost is the
peculiar resting-place, or dominion, of some insect
tribe; and from the locust, the caterpillar, and snail,
to the minute aphis, a wonderful variety of the inferior
insects are nourished, and live by their ravages
upon the vegetable world.
"The Hessian fly, still more destructive to wheat
than the one which ravages the turnip plant, has in
some seasons threatened the United States with a
famine. And the French government is at this
time[108] issuing decrees with a view to occasion the
destruction of the larvæ of the grasshopper.
"In general, wet weather is most favourable to
the propagation of mildew, funguses, rust, and the
small parasitical vegetables; dry weather, to the increase
of the insect tribes. Nature, amidst all her
changes, is continually directing her resources towards
the production and multiplication of life;
and in the wise and grand economy of the whole
system, even the agents that appear injurious to
the hopes, and destructive to the comforts of man,
are in fact ultimately connected with a more exalted
state of his powers and his condition. His industry
is awakened, his activity kept alive, even by the
defects of climates and season. By the accidents
which interfere with his efforts, he is made to exert
his talents, to look farther into futurity, and to consider
the vegetable kingdom, not as a secure and
unalterable inheritance spontaneously providing for
his wants, but as a doubtful and insecure possession,
to be preserved only by labour, and extended
and perfected by ingenuity."
His Sixth Lecture treats of manures of animal
and vegetable origin, and of the general principles
with respect to their uses and modes of application.
It is evident that plants, by their growth, must
gradually exhaust the soil of its richer and more nutrient
parts; and these can be alone restored by the
application of manures. It is equally obvious, that
if a soil be sterile from any defect in its constitution,
such a defect can be only remedied by artificial additions.
Hence the introduction of foreign matter
into the earth, for the purpose of accelerating vegetation,
and of increasing the produce of its crops, is
a practice which has been pursued since the earliest
period of agriculture. Unfortunately, however, the
greatest ignorance has prevailed in all ages with regard
to the best modes of rendering such a resource
available; and the farmer, instead of enriching the
soil, has too frequently given his treasures to the
winds. "It is quite lamentable," says an intelligent
writer,[109] "to survey a farm-yard in many parts of
the kingdom; to see the abundance of vegetable
matter that is trodden for months under-foot, over a
surface of perhaps half an acre of land, exposed to
all the rains that fall, by which its more soluble and
richer parts are washed away, or perhaps carried
down to poison the water of some stagnant pool,
which the unfortunate cattle are afterwards compelled
to drink. From the yard, the manure is
often carted to the field, at the time when the land
is rendered impenetrable by frost; or, if this operation
be delayed to a less unseasonable period, it is
then frequently laid down in small heaps, or sometimes
spread over the surface, exposed for many
days to the sun, the winds, and the rain, as if with
the direct design of dissipating those more volatile
parts which it ought to be the farmer's first endeavour
to preserve.
"Nothing can be so likely to remove ignorance
so deplorable, and prejudices so inveterate, as the
diffusion of real knowledge concerning the nature of
manures, and their mode of action on soils, and on
the plants which grow in them."
Davy, fully sensible of the practical importance
of the subject, and impressed with the conviction
that it was capable of being materially elucidated by
the recent discoveries of chemistry, determined to
put forth his strength, in order to bring this department
of agriculture under the dominion of science;
and upon this occasion our philosopher presents
himself in the only character in which he ever ought
to appear—in that of an original experimentalist.
His first step in the enquiry was to ascertain whether
solid substances can pass from the soil through
the minute pores in the fibres of the root. He tried
an experiment by introducing a growing plant of
peppermint into water which held in suspension a
quantity of impalpably powdered charcoal: but
after a fortnight, upon cutting through different
parts of the roots, no carbonaceous matter could be
discovered in them, nor were the smallest fibres
even blackened,—though this must have happened,
had the charcoal been absorbed in a solid form. If
a substance so essential to plants as carbonaceous
matter cannot be introduced except in a state of
solution into their organs, he very justly concludes
that other less essential bodies must be in the same
case.
He also proved by experiment that solutions of
sugar, mucilage, jelly, and other principles, unless
considerably diluted, clogged up the vegetable organs
with solid matter, and prevented the transpiration
by the leaves: when, however, this precaution
was taken, the plants grew most luxuriantly
in such liquids.
He next proceeded to determine whether soluble
vegetable substances passed in an unchanged state
into the roots of plants, by comparing the products
of the analysis of the roots of plants of mint which
had grown, some in common water, some in a solution
of sugar: the results favoured the opinion
that they were so absorbed. It appeared, moreover,
that substances even poisonous to vegetables did
not offer an objection to this law. He introduced
the roots of a primrose into a weak solution of
oxide of iron in vinegar, and suffered them to remain
in it till the leaves became yellow; the roots
were then carefully washed in distilled water,
bruised, and boiled in a small quantity of the same
fluid: the decoction of them passed through a filtre
was examined, and found to contain iron; so that
this metal must have been taken up by the vessels
or pores in the root.
If to these facts are added those connected with
the changes which animal and vegetable substances
undergo by the process of putrefaction, we have all
the data necessary for forming a rational theory,
to guide us in the management and application of
manures.
Davy has very satisfactorily shown the cases in
which putrefaction or fermentation should be encouraged,
and avoided. As a general rule, it may
be stated, that when manure consists principally
of matter soluble in water, its fermentation or putrefaction
should be prevented as much as possible;
but on the contrary, when it contains a large proportion
of vegetable or animal fibre, such processes
become necessary.
To prevent manures from decomposing, he recommends
that they should be preserved dry, defended
from the contact of the air, and kept as
cool as possible. Salt and alcohol, he observes, appear
to owe their powers of preserving animal and
vegetable substances to their attraction for water,
by which they prevent its decomposing action, and
likewise to their excluding air. The importance
of this latter circumstance he illustrates by the success
of M. Appart's method of preserving meat.
By allowing the fermentation of manure to proceed
beneath the soil, rather than in the farm-yard,
we not only preserve elements which would otherwise
be dissipated, but we obtain several incidental
advantages; for example, the production of heat,
which is useful in promoting the germination of
the seed. This must be particularly favourable
to the wheat crop, in preserving a genial temperature
beneath the surface late in autumn, and
during winter.
Again:—it is a general principle in chemistry,
that in all cases of decomposition, substances combine
much more readily at the moment of their
disengagement, than after they have been perfectly
formed. And in fermentation beneath the soil, the
fluid matter produced is applied instantly, even
whilst it is warm, to the organs of the plant, and
consequently is more likely to be efficient than in
manure that has gone through the process, and of
which all the principles have already entered into
new combinations.
He examines with much attention the various
animal and vegetable matters which have been used
as manure, and furnishes the farmer with a number
of practical remarks on their nature and mode
of operation. For these, the reader must refer to
the work itself; for my limits will not allow me
to enter into the consideration of rape-cake—malt-dust—linseed-cake—sea-weeds—peat—wood-ashes—fish—bones—hair,
woollen rags, and feathers—blood,
&c. &c.; to each of which he assigns peculiar
qualities and virtues.
As he regards the due regulation of the fermentative
process of the utmost importance, he has
furnished some valuable hints for the conduct of
the farmer upon this occasion. He considers that
a compact marle, or a tenacious clay, offers the best
protection against the air; and before the dung is
covered over, or, as it were, sealed up, he recommends
that it should be dried as much as possible.
If at any time it should heat strongly, he advises
the farmer to turn it over, and thus cool it by
exposure to the air; for the practice sometimes
adopted of watering dunghills is inconsistent with
just chemical views. It may cool the dung for a
short time; but moisture, it will be remembered, is
a principal agent in all processes of decomposition.
In cases of the fermentation of dung, there are
simple tests by which the rapidity of the process, and
consequently the injury done, may be discovered.
If, for instance, a thermometer plunged into the mass
does not rise above 100°, it may be concluded that
there is not much danger of the escape of aëriform
matter; but should it exceed this, the dung ought
to be immediately spread abroad.
When a piece of paper moistened in muriatic acid,
held over the steams arising from a dunghill, gives
dense fumes, it is a certain test that the decomposition
is going too far; for this indicates that volatile
alkali is disengaged.
It may be truly said that, under the hand of
Davy, the coldest realities blossomed into poetry:
the concluding passage of this lecture certainly sanctions
such an opinion, and is highly characteristic of
that peculiar genius to which I have before alluded.[110]
A subject less calculated than a heap of manure to
call forth a glowing sentiment, can scarcely be imagined.
"The doctrine," says he, "of the proper application
of manures from organized substances, offers
an illustration of an important part of the economy
of nature, and of the happy order in which it is
arranged. The death and decay of animal substances
tend to resolve organized forms into chemical
constituents; and the pernicious effluvia disengaged
in the process seem to point out the propriety
of burying them in the soil, where they are
fitted to become the food of vegetables. The fermentation
and putrefaction of organized substances
in the free atmosphere are noxious processes; beneath
the surface of the ground they are salutary
operations. In this case the food of plants is prepared
where it can be used; and that which would
offend the senses, and injure the health, if exposed,
is converted by gradual processes into forms of
beauty and of usefulness; the fetid gas is rendered
a constituent of the aroma of the flower, and
what might be poison, becomes nourishment to
man and animals."
The Seventh Lecture is devoted to the investigation
of manures of a mineral origin. He
commences the subject by refuting the opinion of
Schrader and Braconnot, that the different earthy
and saline substances found in plants arise from new
arrangements of the elements of air and water, by
the agencies of their living organs.
In 1801, he made an experiment on the growth
of oats, supplied with a limited quantity of distilled
water, in a soil composed of pure carbonate of lime.
The soil and the water were placed in a vessel of
iron, which was included in a large jar, connected
with the free atmosphere by a tube, so curved as to
prevent the possibility of any dust, or fluid, or solid
matter, from entering into the jar. His object was
to ascertain whether any siliceous earth would be
formed in the process of vegetation; but the oats
grew very feebly, and began to be yellow before
any flowers formed. The entire plants were burnt,
and their ashes compared with those from an equal
number of grains of oat. Less siliceous earth was
given by the plants than by the grains; but their
ashes yielded much more carbonate of lime.
Numerous other authorities might be quoted to the
same effect. Jacquin states that the ashes of Glasswort
(Salsola-Soda) when it grows in inland situations,
afford the vegetable alkali; but when on the
sea-shore, the fossile or mineral alkali. Du Hamel
also found, that plants which usually grow on the sea-shore,
made small progress when planted in soils
containing little common salt. The Sunflower,
when growing on lands not containing nitre, does
not afford that substance; though when watered by
its solution, it yields nitre abundantly. De Saussure
made plants grow in solutions of different salts;
and he ascertained that, in all cases, certain portions
of the salts were absorbed by the plant, and found
unaltered in their organs.
It may be admitted then as established, that the
mineral principles found in plants are derived from
the soils in which they vegetate. This fact becomes
the foundation of the theory respecting the operation
of mineral manure.
Davy observes, that "the only substances which
can with propriety be called fossile manures, and
which are found unmixed with the remains of any
organized beings, are certain alkaline earths, or alkalies,
and their combinations." If he intends to limit
the term to those bodies only which find their way
into the structure of plants, his definition may be
correct; but I am inclined to take a much wider
view of the subject, and to include all those mineral
substances which promote vegetation by modifying
the texture of the soil:—but of this hereafter.
Lime, not only from its importance, but from the
controversies which it has occasioned, ranks first in
the list of mineral manures.
That disputes concerning the uses of lime and its
carbonate, should have long existed, and be still continued
amongst a class of persons who, whatever may
be their practical knowledge, are not acquainted
with the composition of the substances about which
they differ, is certainly by no means extraordinary.
Davy, therefore, very properly introduces the subject,
by a description of the nature and qualities
of these bodies, and by marking the distinctions
between quicklime and its carbonate.
The substance commonly known by the name of
Limestone is a compound of lime and carbonic acid,
associated generally with other earthy bodies, the
nature and proportions of which vary in different
species. "When a limestone does not copiously
effervesce in acids, and is sufficiently hard to scratch
glass, it contains siliceous, and probably aluminous
earth. When it is deep brown or red, or strongly
coloured of any of the shades of brown or yellow, it
contains oxide of iron; when it is not sufficiently
hard to scratch glass, but effervesces slowly, and
makes the dilute nitric acid in which it effervesces
milky, it contains magnesia; and when it is black,
and emits a fetid smell if rubbed, it contains coally
or bituminous matter."
As the agricultural value of limestone is materially
modified by the substances with which it may
be associated, their analysis becomes an object of
much importance, and the author has accordingly
proposed a simple method of effecting it.
Before any opinion can be formed of the manner
in which these different ingredients operate, it is
necessary that the action of the pure calcareous element
as a manure should be thoroughly understood.
In its caustic state, whether used in powder, or
dissolved in water, lime is injurious to plants.
Davy informs us that he has, in several instances,
killed grass by watering it with lime water; but in
its combination with carbonic acid, it is an useful
ingredient in soils.
When newly-burnt lime is exposed to the atmosphere,
it soon falls into powder, from uniting with
the moisture of the air; and the same effect is immediately
produced by throwing water upon it,
when it heats violently, and the water disappears:
in this state it is commonly called slacked lime:
chemists have named it the hydrat of lime; and
when this hydrat becomes a carbonate, by long exposure
to the air, its water is in part expelled, and
the carbonic acid takes its place.
Lime, whether freshly burnt, or slacked, acts
powerfully on moist fibrous vegetable matters, and
forms with them a compost, of which a part is
usually soluble in water. By this operation, it renders
inert vegetable matter active; and as charcoal
and oxygen (the elements of carbonic acid) abound
in vegetables, it is itself, at the same time, converted
into a carbonate. But limestone simply powdered,
marls, or chalks, do not thus act on vegetable matter;
and hence the operation of quicklime and
mild lime depends on principles altogether different.
Quicklime acts on any hard vegetable matter,
so as to render it more readily soluble; the
mild limes, or carbonates, act only by improving the
texture of the soil, or by supplying a due proportion
of calcareous matter: thus almost all soils which do
not effervesce with acids, are improved by mild lime
and sand, more than clays. I apprehend that it is
upon this principle the application of shelly sand
proves beneficial in Cornwall, although I have ascertained
that, on some occasions, its value depends
upon its chemical action upon mineral bodies in
the soil.
Soils abounding in soluble vegetable manures are
injured by quicklime, as it tends to decompose their
soluble matters, or to form with them compounds
less soluble than the pure vegetable substance.
With animal manures, it is equally exceptionable,
unless indeed they be too rich, or it becomes necessary
to prevent noxious effluvia: for since it decomposes
them, it destroys their efficacy, and tends to
render the extractive matter insoluble.
The limestones containing alumina and silex are
less fitted for the purposes of manure than pure
limestones; but the lime formed from them has no
noxious quality. Such stones are less efficacious,
merely because they furnish a smaller quantity of
quicklime. Those, however, that contain magnesia,
if indiscreetly used, may be very detrimental.
It had been long known to farmers in the neighbourhood
of Doncaster, that lime made from a certain
limestone, when applied to the land, often
injured the crops considerably. Mr. Tennant discovered
that this limestone contained magnesia;
and on mixing some calcined magnesia with soil,
in which he sowed different seeds, he found that
they either died, or very imperfectly vegetated;
and with great justice and ingenuity, he referred
the bad effects of the peculiar limestone to the
magnesian earth it contained. In prosecuting the
enquiry, Davy however ascertained that there were
cases in which this magnesian lime was used with
good effect,—in small quantities, for example, on
rich land: and during his chemical consideration of
the question, he was led to the following satisfactory
solution.
"Magnesia has a much weaker attraction for carbonic
acid than lime, and will remain in the state
of caustic or calcined magnesia for many months,
though exposed to the air; and as long as any
caustic lime remains, the magnesia cannot be combined
with carbonic acid, for lime instantly attracts
carbonic acid from magnesia. When therefore a
magnesian limestone is burnt, the magnesia is deprived
of its carbonic acid much sooner than the
lime, and in this state it is a poison to plants. That
more magnesian lime may be used upon rich soils,[111]
seems to be owing to the circumstance, that the
decomposition of the manure in them supplies carbonic
acid, and thus converts it into a mild carbonate.
Besides being used in the forms of lime
and carbonate of lime, calcareous matter is applied
for the purposes of agriculture in other combinations.
The principal body of this kind is gypsum,
or sulphate of lime; respecting the uses and operation
of which very discordant opinions have been
formed.
Its beneficial operation has been referred to two
causes, viz. to its power of attracting moisture from
the air, or to its assisting the putrefaction of animal
substances; but Davy has shown by experiments
that neither of these theories can be supported by
facts.
The most extraordinary circumstance perhaps connected
with the history of this mineral manure, is
the very opposite opinions which have been formed
respecting its value. In this country, although
there are various testimonies in its favour, it has
never been employed with the signal success which
marked its adoption in America, and which was
so palpable and extraordinary as at once to have
ensured its universal introduction.
I was some years since assured by Mr. Maclure
of Philadelphia, that whenever any doubt or hesitation
betrayed itself with respect to its fertilizing
agency, it was only necessary to sprinkle a small
quantity in a meadow, to satisfy the most sceptical;
and that this was usually done in the form of letters
or characters, which in a short time became so much
more luxuriant than the surrounding grass, as to be
visible at a considerable distance. It is, I understand,
chiefly applied to grass lands as a top-dressing;
and the American farmers[112] explain its operation
upon its solubility in water, and its consequent
absorption by the roots of the grass. Davy, in
examining the ashes of sainfoin, clover, and rye-grass,
which had grown in soils manured by gypsum,
found considerable quantities of that substance;
and he thinks it probable that it was intimately
connected with their woody fibre. He attempts to
explain the reason why the application of gypsum
is not generally efficacious, by supposing that most
of the cultivated soils may already contain it in
sufficient quantities for the use of the grasses. I
strongly suspect, however, that it will be hereafter
discovered to depend upon the nature of the soil in
its hygrometric relations. From the facts already
recorded, it would appear that it never answers near
the sea, nor in wet lands. In consequence of its
solubility, it is enabled to penetrate and pervade the
whole vegetable structure; and the experiments of
Davy have proved its presence in the ashes of plants
exposed to its operation, and have rendered it probable
that it enters into union with their woody
fibre, by which the density of their textures will be
increased, and consequently the evaporation from
their leaves diminished; I am from such considerations
induced to think that gypsum does not
act by effecting any chemical change in the soil, but
solely by diminishing the plants evaporation. This
idea seems to be borne out by the evidence furnished
by the different circumstances attending the
operation of this manure: we find, for example,
that succulent vegetables, planted on dry soils, are
those which are principally benefited by its application,
and that the various grasses so manured
retain their verdure, even in the dryest season and
on the most arid lands; at the same time, we find
that these crops, especially clover, acquire a proportionate
increase in the density of their fibres,
that is to say, that they become much more rank
and stubborn, and often to such a degree does this
take place, that in America, where its effects are
best understood, sheep not uncommonly refuse to
feed upon them. Upon the same principle we find
that, under circumstances or in situations where the
evaporation of a plant is provided for by a constant
supply of moisture, the effects of gypsum cease to
be apparent.
Davy hints at a process by which gypsum may
be formed in a soil containing sulphate of iron, by
the action of calcareous manure,[113] and which was
first pointed out by Dr. Pearson. I can confirm
this statement by the results of experiments I formerly
made in Cornwall, where soil containing this
salt of iron had been manured by shelly sand.
In pursuing his enquiry into the efficacy of mineral
manure, Davy proceeds to investigate the
efficacy of the fixed alkalies, and observes that their
general tendency is to give solubility to vegetable
matters, and in this way to render carbonaceous and
other substances capable of being taken up by the
tubes in the radical fibres of plants. The vegetable
alkali has likewise a strong attraction for water, and
even in small quantities may tend to give a due
degree of moisture to the soil, or to other manures.
He considers that pure salt may act, like gypsum,
phosphate of lime, and the alkalies, by entering into
the composition of the plant. Upon the subject of
salt, however, his remarks are very meagre and unsatisfactory:
at the time he composed his lecture,
the subject had not excited that public attention
which the writings of Mr. Parkes, Sir Thomas
Bernard, and others, have since awakened.
Had our philosopher undertaken the agricultural
survey of Cornwall, his lecture on mineral manure
must have been very considerably extended. He
would have learnt that various rocks reduced to
small fragments, are commonly applied as dressing;
he would have explained the cause of the fertility
so generally associated with hornblende rocks;—he
would have speculated upon the influence of iron
in giving fruitfulness; and above all, he would have
taught the agriculturist the scientific use of calcareous
sand, by pointing out the description of
lands which are most likely to be benefited by its
application.
The Eighth Lecture concludes the subject of
the chemistry of Agriculture, by establishing the
theory of the operation of burning lands. He considers
the process to be useful in rendering the soil
less compact, and less tenacious and retentive of
moisture; and that, when properly applied, it is
capable of converting a matter that was stiff, damp,
and cold, into one powdery, dry, and warm, and
much more proper as a bed for vegetable life. He
states the great objection made by speculative chemists
to paring and burning, to be the unavoidable
destruction of vegetable and animal matter, or the
manure of the soil; but he considers that, in those
cases in which the texture of its earthy ingredients
is permanently improved, there is more than a compensation
for so temporary a disadvantage; and that
in some soils, where there is an excess of inert vegetable
matter, the destruction of it must be beneficial,
and that the carbonaceous matter remaining in the
ashes may be more useful to the crop than the vegetable
fibre from which it was produced.
In this view of the subject it is evident, that all
poor siliceous sands must be injured by the operation;
"and here," says Davy, "practice is found to
accord with theory. Mr. Arthur Young, in his
Essay on Manures, states, 'that he found burning
injure sand;' and the operation is never performed
by good agriculturists upon siliceous sandy soils,
after they have been once brought into cultivation.
An intelligent farmer in Mount's Bay told me, that
he had pared and burned a small field several years
ago, which he had not been able to bring again into
good condition. I examined the spot,—the grass
was very poor and scanty, and the soil an arid siliceous
sand." Irrigation, or watering land, is a practice,
he observes, which at first view appears the
reverse of torrefaction; and, in general, the operation
of water in nature is to bring earthy substances
into an extreme state of division. But in the artificial
watering of meadows, the beneficial effects
may depend upon many different causes, some chemical,
some mechanical. It may act as a simple
supply of moisture to the roots, or it may carry into
the soil foreign matter, or diffuse that which exists
in it more equally through its substance.
He concludes with some valuable scientific observations
upon the process of fallowing, by which he
attempts to correct the prejudices which have existed
with regard to its benefits. He points out, on
the other hand, the great advantages of the convertible
system of husbandry, by which the whole of
the manure is employed; and those parts of it which
are not fitted for one crop, remain as nourishment
for another. These views he illustrates by a reference
to the course of crops adopted by Mr. Coke,
in which "the turnip is the first in the order of
succession; and this crop is manured with recent
dung, which immediately affords sufficient soluble
matter for its nourishment; and the heat produced
in fermentation assists the germination of the seed
and the growth of the plant. After turnips, barley
with grass seeds is sown; and the land having been
little exhausted by the turnip crop, affords the soluble
parts of the decomposing manure to the grain.
The grasses, rye-grass, and clover remain, which
derive a small part only of their organized matter
from the soil, and probably consume the gypsum in
the manure which would be useless to other crops;
these plants likewise, by their large system of leaves,
absorb a considerable quantity of nourishment from
the atmosphere; and when ploughed in at the end of
two years, the decay of their roots and leaves affords
manure for the wheat crop; and at this period of
the course, the woody fibre of the farm-yard manure,
which contains the phosphate of lime and the
other difficultly soluble parts, is broken down; and
as soon as the most exhausting crop is taken, recent
manure is again supplied."
At the end of his system is added an Appendix,
containing "An Account of the results of Experiments
on the produce and nutritive qualities of the
Grasses and other plants used as the food of animals;
instituted by John Duke of Bedford." But as these
experiments do not admit either of abridgement or
analysis, the reader must refer to the original source
for information.
I shall conclude this long, and, I fear, somewhat
tedious review, with the animated appeal so earnestly
addressed by the illustrious author to the philosophical
readers of his work.
"I trust that the enquiry will be pursued by
others; and that in proportion as chemical philosophy
advances towards perfection, it will afford new
aids to agriculture: there are sufficient motives connected
both with pleasure and profit, to encourage
ingenious men to pursue this new path of investigation.
Science cannot long be despised by any persons
as the mere speculation of theorists, but must
soon be considered by all ranks of men in its true
point of view, as the refinement of common sense
guided by experience, gradually substituting sound
and rational principles for vague popular prejudices.
"The soil offers inexhaustible resources, which,
when properly appreciated and employed, must increase
our wealth, our population, and our physical
strength.
"We possess advantages in the use of machinery,
and the division of labour, belonging to no other
nation. And the same energy of character, the
same extent of resources, which have always distinguished
the people of the British Islands, and made
them excel in arms, commerce, letters, and philosophy,
apply with the happiest effects to the improvement
of the cultivation of the earth. Nothing is
impossible to labour, aided by ingenuity. The true
objects of the agriculturist are likewise those of the
patriot. Men value most what they have gained
with effort; a just confidence in their own powers
results from success; they love their country better,
because they have seen it improved by their own
talents and industry; and they identify with their
interests the existence of those institutions which
have afforded them security, independence, and the
multiplied enjoyments of civilized life."
END OF THE FIRST VOLUME.
LONDON:
PRINTED BY SAMUEL BENTLEY,
Dorset Street, Fleet Street.
