CHAPTER IX.

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.

[101] See Page 44.

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.

[102] _Iodine_, _Fluorine_, &c. had not been discovered at this

period.

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, phosphor_ous_

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."

[103] See page 364.

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 200_l._ 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.

[104] See his Salmonia, Edit. 2. p. 9.

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."

[105] Edinburgh Review, vol. 22, page 253.

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.

[106] For example:--What weight of wheat is equivalent to a given

weight of oats, barley, rye, &c.? Suppose three hundred pounds of

potatoes feed twenty head of cattle for any given time, how many

will the same weight of oats feed?

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]

[107] The history of his native county would have furnished

him with a parallel instance of the intelligence and design

which Nature displays in connecting the wants and necessities

of the different parts of creation, with the power and means

of supplying them. In a primitive country like Cornwall, the

siliceous soil necessarily requires much moisture, and we may

perceive that the cause which occasions, at the same time

supplies this want; for the rocks elevated above the surface,

solicit a tribute from every passing cloud; while in alluvial

and flat districts, where the soil is rich, deep, and retentive

of moisture, the clouds float undisturbed over the plains, and

the country frequently enjoys that uninterrupted series of dry

weather which is so necessary to its fertility. Linnæus observes,

that the plants which chiefly grow upon the summit of mountains,

are rarely found in any other situation, except in marshes,

because the clouds arrested in their progress by such elevations,

keep the air in a state of perpetual moisture.

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.

[108] January 1813.

"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.

[109] Edinburgh Review, vol. xxii. p. 270.

"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.

[110] Page 191.

"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."

[111] These facts have been confirmed by agriculturists, who

could not possibly have had any favourite theory to support. Dr.

Fenwick tells us, (Essays on Calcareous Manures, p. 11. 1798,)

that in the county of Durham, the farmers always distinguish

between _hot_ and _mild_ limes. They never apply the former

to exhausted lands, or to any soil that has been long under a

course of tillage, unless it be very deep and rich. In peaty

soils, and in new, sour, and wild lands, the _hot_ limes, on

the contrary, are preferred to the _mild_ ones. Dr. Fenwick

made some experiments to ascertain the cause of the differences

between these varieties of lime; and though he failed to discover

that by analysis which Mr. Tennant subsequently ascertained, he

nevertheless arrived at a just conclusion by simple observation;

and was led to believe, that "what farmers term _hot_ limes, are

such as re-absorb their fixed air more slowly, and therefore

continue longer to exert the peculiar action of quicklime."

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.

[112] When this substance was first introduced into America,

which is nearly forty years since, it was imported from the

quarries of Montmartre, and in such request was it, that a bushel

of wheat was usually given for the same measure of gypsum:

it is now, I believe, obtained from Nova Scotia; I have not

heard that it has been found within the States. It may perhaps

serve to convey some idea of the extent to which it has been

applied, when I state, that Mr. Maclure assured me that not

less than three hundred vessels are constantly employed in the

traffic, and that in Philadelphia twenty merchants, at least,

are engaged in supplying the demand for it. Its efficacy appears

to be considerably increased by applying it in a minute state

of division; and a want of attention to this circumstance may

possibly have been one of the causes which have rendered its

advantages less conspicuous in England. In America, three or four

hundred mills, of a peculiar construction, have been erected in

different parts for the purpose of grinding it.

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.

[113] Gypsum is readily produced by the admixture of decomposing

pyrites and calcareous matter: in proof of which the Mineralogist

can produce specimens of oyster shells studded with crystals of

selenite from Shotover; and alum from the _aluminous shale_ at

the Hurlet Mine near Glasgow.

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.

Transcriber's note:

Minor spelling and punctuation inconsistencies, and hyphenated

words, have been harmonized. The formatting of the letters

has been regularized. Every effort to decipher the hand written

letter by Sir Humphry Davy has been made, however, there are no

guarantees that it is correct.