History of science index


By John Ingen-Housz

Body Physician to their Imperial and Royal Majesties, F. R. S. 
Foreign Honorary Member of the Board of Agriculture, 
Etc., Etc. 

The surest way to find out the real nourishment of organized bodies seems to be, to inquire what is the substance, without which they inevitably perish, and which alone is sufficient to continue their life. All animals require two ingredients for the continuation of their life; viz. atmospheric air and moist food, derived either from animal or vegetable substances, which food being received in the stomach, or some reservoir destined for that purpose, and being gradually digested and changed into different substances in the different organs, is applied to the whole economy of the animal body. Vegetables being deprived of progressive motion, by which means the most part of animals go in search of food, must find, in the narrow compass of space they occupy, every thing necessary for their subsistence. As they are in contact with two substances only, the earth and the atmospheric air, their nourishment must exist in either of them, or in both. The earth is necessary to the plants, as the only means to fix them stedfastly to the spot, by spreading through it their roots; but as earth contains generally moisture, salts, air, &c. nature has taken advantage from this circumstance, so that the filaments of the roots pump from the soil all that is offered to their suckers, and can be absorbed by them; but as some plants may live and thrive without being in contact with any earth, we ought to take it for granted, that the soil, or what exists in the soil, is not the only food of plants. Water is necessary to all organized beings, as without it no circulation of juices could be carried on; but from this necessity it can only be deduced, that water is a vehicle of the food, and by no means that it is the true nourishment of animals or vegetables - the less so, as it is an incontrovertible fact, that several plants can live without being in contact with water. Thus the agave, cactus, aloe, cacalia, &c. live in the most dry rocks in the hottest climates, where it does not rain sometimes in the space of several months, and where the burning sun pierces all other plants, and even deprives the trees of all their leaves, and, what is extraordinary, the most part of such plants are full of juices. The nocturnal dew cannot give sufficient nourishment to such plants, as all other plants would also maintain themselves with it. But to be certain that those plants do not subsist by dew, we ought to consider only that some plants of that species may be kept alive in the hot-houses, either in pots, without being watered, or by hanging them up from the ceiling. 

Now, as by what I have said here, it seems to be probable, that neither water nor soil is, or contains all the true nourishment of vegetables, it must be concluded, that plants must find it in the atmospheric air; for this is the only ingredient without which all vegetables perish. A plant shut up in vacuo soon dies; and it dies in all sorts of aerial fluids which are incapable of supporting animal life - such as fixed air, inflammable air, phlogisticated air, or azote, &c. It is true, Dr. Priestley and Mr Scheele have propagated a doctrine diametrically opposite to what I have here advanced, by saying that plants thrive wonderfully in putrid air, and perish in pure or dephlogisticated air. This doctrine, though generally adopted, and very ingeniously applied, by Sir John Pringle and others, to illustrate the mutual reference, established by the Author of nature, between the vegetable kingdom and the animal creation, is refuted by my experiments, by which I think I have proved, that plants shut up in vital air live so much the longer, as this air is superior in purity to atmospheric air. I have explained the manner of making these experiments with success; and I have indicated the reason why, of two plants, the one shut up with common air, the other with the same quantity of vital air, (both kept in the dark,) the plant placed is common air can only be kept alive during a certain very limited time, whereas the plant shut up in vital air may be kept alive much longer, even as long as there is vital air enough remaining to cover the whole plant.1)
From these, and many other considerations, I have deduced, that from the two organized kingdoms, the animal and the vegetable, the animal derives its nourishment from the vegetable; but that the vegetable creation is independent of the animal world, provides for itself, and derives its subsistence chiefly from the atmosphere.2)

When I engaged in the experiments on vegetables, which I have published, 1779 in English, and, in 1780, more fully in French, Dr. Priestley had already observed that vegetables possessed a power of correcting bad air, which, however, was denied by Mr. Scheele, in Sweden, who found that plants, instead of correcting bad air, corrupted good air. This contradiction struck Dr. Priestley so much, that he employed the summer of 1778 in repeating his former experiments; and, after the most accurate researches, he concluded, that though there seems to be such a power in plants, yet that, very often, they have quite a contrary effect, as Mr. Scheele found; but that he did not know what the reason of this uncertain effect of plants on air was.3) Dr Priestley, in 1778, found, by accident, that by exposing well water a long while to the sun, it produced a filmy, greenish sediment, which produced pure air in the sun-shine: by examining this matter with a microscope, he found it destitute of organization and pronounced it to be neither an animal nor a vegetable substance, but a substance sui generis, to which he gave the name of green matter. Mr. Berthollet found also that by exposing dephlogisticated marine acid to the sun, vital air was produced, and Mr. Scheele, in Sweden, found, that the same air was also produced from nitrous acid exposed to the light of the sun. 

I was fortunate enough to discover the true reason, why plants did sometimes correct bad air, and sometimes made it worse, which reason was never so much as even suspected either by Dr. Priestley or by Scheele; and indeed if either of them had had the least suspicion of it, their known eagerness for fame would not have allowed them to keep the discovery from the public eye, and Dr. Priestley would not have gone much farther than Mr. Scheele did; viz. to acknowledge openly, (even in his book printed 1779,) that he had been mistaken, and that he was entirely ignorant of the reason why vegetables are so inconstant in their effects on the air in contact with them.4)

1) See Experiences sur les Vegetaux, tom. II. sect. i, ii, iii, iv, & lvi. 
2) See Experiences sur les Vegetaux, tom. II. p. 190. 490. 
3) See Dr. Priestley's work on Airs, Vol- iv. published 1779, of which the quotations may be found in the preface of my book, Experiments on Vegetables, published 1779 p. xxiii-xxxii, and in the Avant-propos and Preface of my Experiences sur les Vegetaux, Vol. I. second edition, printed at Paris. 
4) See Experiments on Vegetables, p. xxviii, and following. 

I discovered, in the summer 1779, that all vegetables are incessantly occupied in decomposing the air in contact with them, changing a great portion of it into fixed air, now called carbonic acid, which, being specifically heavier than atmospheric air, tends naturally to fall downwards, and being miscible with moist, salts, and different sorts of earthly substances, is apt to combine with them. I found, that the roots, flowers, and fruits are incessantly employed in this kind of decomposition, even in the middle of the sunshine; but that the leaves and green stalks alone cease to perform this operation, during the time the sun, or an unshaded clear day-light shines upon them; during which time, they throw out a considerable quantity of the finest vital air, and moreover make the air in contact with them purer, or more approaching to the nature of vital air.1)

I did not doubt that this continual decomposition of atmospheric air must have a general utility for the subsistence of the vegetables themselves, and that they derived principally their true food from this operation, by changing this decomposed air into various juices, salts, mucilage, oils, &c. much the same as in graminivorous animals the simple grass changes, in the various organs, into the numerous and very heterogeneous fluids and solids. It would certainly be a very difficult, if not impossible task, to give a clear and satisfactory theory, by which these various changes, compositions, decompositions, new combinations, &c. performed upon one single species of food, such as grass, may be explained. The same incomprehensible transformations are going on in vegetables. If once it was satisfactorily proved, that plants can subsist on what they find in atmospheric air, without any other substance, we ought to content ourselves with the fact alone; for it would be in vain to attempt to penetrate the mystery of all the changes this air undergoes in the organs of these living beings; no more need we anxiously to investigate by what means, in a man who lives only on rice and water, all the various transmutations of this simple food are performed. This mystery is above the reach of our very limited understandings: and truly, who could pretend to understand how a part of this rice forms in some places hard bones, in others soft fat, and in one single place a liquid, of which a single drop poured into the womb of a woman excites in that organ the most wonderful of all operations - the production of an embryo, which is to bear the strongest mark of resemblance to the man whose sperm gave rise to this progeny. 

The new light which chemistry has received in our age, affords us the means of understanding many phenomena, which we were either ignorant of, or which nobody understood anything of before. The new discoveries on the nature of water, air, salts, &c. open the door to an infinite number and variety of new discoveries. The identity of the same principle of all acids called oxygen, which the French chemists have established, throws new light on the difference which exists in the various acids already known, and on the changes which these acids undergo. Thus the same acidifying principle, attaching itself to a different basis, le radical,* becomes either nitrous, vitriolic, marine, or any other acid: with carbon it becomes fixed air, or carbonic acid; with sulphur, it becomes vitriolic, or sulphuric acid; with phosphorus, it becomes phosphoric acid; with azote, it becomes nitrous acid, &c. - (This last a capital discovery of Mr. Cavendish.) It may thus be reasonably supposed, that some acids taken into our body may lose, in the various operations of our organs, their former radical, and combine with a new one, and by this combination entirely change their nature. Without such like changes taking place in our organs, how could we account for the generation of the great quantity of phosphoric acid existing almost everywhere in our bodies, (which acid has already got the name, by some eminent chemists, of animal acid,) principally in our bones? whereas we find no where the marine acid, though of all others we take in the greatest quantity of it. We find, in several liquids, its basis, the fossil alkali; as in bile, semen, urine, &c.; but we find nothing of its acid, nor even the marine salt, undecomposed, except in the serum of the blood, and the chylus, in which this salt has not yet undergone the elaboration of the vital organs. It seems, therefore, as if all the acids, the marine, vegetable, the carbonic, &c. were in our organs transformed, for the greatest part, into the animal or phosphoric acid. It seems at least probable, that, without supposing this change of acids to take place in our bodies, we could not account for the great abundance of phosphoric acid existing in our bodies; for though it really exists in some of our foods, yet the quantity of it is but small. 

1) I have published facts, which prove that vital air, produced by vigorous plants in the sunshine, is of the greatest purity in itself, and that the air thrown out by them, in the shade, or in the dark, is in itself - that is to say, being free from other air, the most active poison in destroying animal life yet known. (See my work on Vegetables, French editionw,page 182-185.) 
2) *See Fourcroy's Works.

If plants imbibe fixed air, or carbonic acid, it is not more difficult to believe that this substance may be transformed, elaborated, or modified into various other substances and salts in the organs of the plants, than it is difficult to believe that the above-mentioned changes take place in the human body. Who could believe, without demonstrative proof, that the aerial fluid, the carbonic acid, constitutes about 40/100 of limestone and that this stone having lost its hardness, by being deprived of this aerial fluid recovers its former consistency by recovering this fluid.1)

As the carbonic acid is composed of the acidifying principle, the oxygen, and the carbon or coal, plants may derive from these two principles some of the most essential substances we find in them: their acids, their oils, their mucilage, &c. these ingredients, together with the azote absorbed also with the atmospheric air, being elaborated in their organs, variously modified and combined, in a manner somewhat analogous to the wonderful, though to the human understanding incomprehensible, elaborations and combinations which we observe in the bodies of animals. 

Mr. Hassenfratz2) delivered, in the month of June, 1792, to the Royal Academy of Paris, three papers on the nourishment of plants, which met with general approbation. The principal part of the doctrine contained in these three memoirs, viz. that coal or carbon, constitutes the principal nutritive substance of plants, is much admired, and adopted by the celebrated Mr. Kirwan, in his Dissertation on Manures, published in the fifth volume of the Irish Philos. Transactions. 

In the first of these papers, Mr. Hassenfratz says, that water and air are not alone sufficient to nourish plants, but that the development, or growth, of these beings is owing to the carbon, which, being originally lodged in the seed, is expended in this business. 

In the second memoir he attempts to prove, that the carbonic acid, or fixed air, is not a nutritive ingredient of plants, and that the act of vegetation does not decompose the carbonic acid; but, he says, on the contrary, this carbonic acid is, as Dr. Ingen-housz has discovered, formed by plants in the dark, and drawn from the plants and the oxygen of the water decomposed by vegetables Mr. Kirwan differs in this respect from Mr, Hassenfratz, and thinks that the carbonic acid is decomposed by the act of vegetation. 

In the third memoir he asserts, that the carbon, the true nourishment of plants is derived by the roots from the soils, where it is ready found in a state of sufficient solution, or suspension, to be absorbed by the suckers, and carried through the whole plant. He thinks that the vigour of the plants depend chiefly upon the quantity of carbon, with which the soil is impregnated; and he gives the name of carbon to the brown sediment of the infusion of dung which remains after the water is evaporated. 

1) According to M. Lavoisier, the founder of the new chemistry, the carbonic acid is composed of 72/100 of oxygen, which is the basis of vital air, and 28/100 Of carbonic substance, called carbon; water is composed of 85/100 of oxygen, and 15/100 of hydrogen; common air is composed of 27/100 of oxygen of vital air, and 73/100 of nitrogen, or azote; oxygen air is composed of oxygen, or the acidifying principle, and the matter of fire and light. 

2) See Annales de Chimie, of 1792. 

The doctrine contained in these memoirs, as well as the important experiments to which they relate, require, I think, farther investigation, before it can be proved or clearly understood. Let me be allowed to throw out some hints and considerations which may, perhaps, shew the way towards the true mystery of the manner which nature employs to feed the plants. All seeds contain a certain quantity of food, by which the plant may be kept alive in the beginning of its growth; some have a considerable portion of mucilaginous matter, such as seed of quince; some have, besides this mucilage, a very thick cover or pulp, by which the seed is surrounded, such as the seed of grapes, apples, pears, melons, cucumbers; which substances serve also as food for animals. All those substances by which many grains are thickly covered, yield a great quantity of fixed air, or carbonic acid, when the seed lodged in them begins to vegetate; but this substance being exhausted at the close of their fermentation and putrefaction, the embryo plants must be capable of providing for themselves. A new born child may also live a few days without food, being nourished by some nutritious matter which it brings with it when born, and which it had imbibed by the mouth when in the womb, and part of which nourishment was prepared in the pectoral gland of the child; as it is well known that all children, male as well as female, come into the world with a portion of true milk elaborated in their own breasts. Thus also the yolk of the egg is drawn into the stomach of the chick when ready to break its prison, by means of which yolk it is nourished, till it has acquired strength enough to go in search of food. The mothers of animals endowed with breasts feed, during a certain time, their offspring by their milk. Many animals, such as birds of different kinds wander about in search of food, to be carried to their young. Very few animals find on the spot where they exist, everything they want. But all plants are destined by nature to remain on the same spot, and therefore must possess such faculties which prepare into food some of the substances in contact with them, as soon as they have consumed the small store of food they are provided with before they vegetate. 

As the very first decomposition of the pulp surrounding the seeds is accompanied with the production of carbonic acid, and the first operation of the embryon or beginning plant, is to decompose the air in contact with it, by changing the oxygenous part of it into carbonic acid, of which it probably absorbs, in the dark and shade, the oxygen, and, in the sunshine, the carbon, throwing out at that time the oxygen alone, and keeping the carbon to itself, as nourishment: as all these different operations, I say, have one general effect, viz. the decomposing the air in contact with plants, it seems more than probable, that vegetables derive their principal food from this decomposition, and the production of fixed air, or carbonic acid. 

This supposition will acquire a degree of greater probability by considering, that all air which cannot be easily changed or decomposed into fixed air, as possessing no oxygen at all, are true poisons to plants, such as inflammable air, putrid air, and azote, contrary to Dr. Priestley and Mr. Scheele's doctrine; and that vital air itself, or an air approaching to its nature, maintains a plant remarkably well, in its full vigour; and that carbonic acid concentrated, or without a great proportion of respirable air, kills also plants, as this air, and all other airs, poisonous to vegetable life, are also destructive of animal life; which doctrine I produced the first, in contradiction to that of the two celebrated philosophers just mentioned.1)

1) See Experiences sur les Vegetaux, tom. II. Sect. Ivi. 

When I discovered, in 1779, that all vegetables decompose the common air by night, and change a part of it into fixed air;1) and when I drew from this and some other facts, the conclusion, that the plants absorb this fixed air, and turn it into their nourishment, the new doctrine of chemistry was not yet published, and being ignorant of all the beauties of this system, I was unable to reduce these facts to a proper theory. But since we have been instructed in the analysis of water and air, it is become much easier to explain the phenomena of vegetation. As it is now admitted, that fixed air, or carbonic acid, is composed of oxygen, deprived of its caloric, or matter of heat, and of carbon, it is not difficult to understand how plants provide or prepare their own nourishment by producing carbonic acid, supposing it to be demonstrated, that carbon is the principal nourishment of plants. 

From this doctrine it would naturally be inferred, that plants must grow the most rapidly at such time, when they prepare the greatest quantity of this nourishment, which is when they are in the dark; and this is just what really happens, as all plants grow with much more rapidity in a dark place than in the light, as Mr. Du Hamel and Mr. Bonnet, of Geneva, found; they call this quick growing etiolement. Plants, in general, grow less towards the middle of the day than at any other time: many do not advance at all when the sun is near the meridian; some even grow manifestly shorter towards that time. (See my work, Exp. et Obsns Nouvelles vol. II. p. 206.) 

I discovered that the roots of plants, even when exposed to the sunshine, produce carbonic acid, but that the leaves and green stalks produce this acid only in the dark; and that flowers and fruits, with a few exceptions among the last, produce at all times, even at the roots, carbonic acid. Thus there is no time lost, some parts, or the whole plant, being constantly employed in this business of preparing carbonic acid. 

Though Mr. Hassenfratz seems to believe, that plants do not derive the carbon (in his opinion their true nourishment) from the carbonic acid, but finds it ready made in the dung; I think it more probable, that plants derive it chiefly from the carbonic acid, which is a substance very easily decomposable in its two ingredients, viz, oxygen and carbon. All manures, principally dung, produce a great quantity of carbonic acid, either by itself, or by decomposing the air in contact with it. 

But here seems to start up a difficulty, how a plant or manure can draw from the atmospheric air carbonic acid; as common air contains, according to the new system, only 1/100 of it; and, according to Mr. Lavoisier, nothing at all. Though, according to those principles, it could not be accounted for theoretically, I think we have at hand facts enough from which it seems evident, that the common air can, by itself, furnish all the ingredients for the composition of carbonic acid, as we will see by and by. Do these facts argue a defect in the new system? Let a better judge than I am decide this. 

We are as yet very far from understanding the various productions, which in this world are exposed to our senses as offsprings from the infinite combinations, decompositions, chemical affinities, or attractions, &c. every where in action on the surface and in the bowels of the earth, in the atmosphere, the waters of the sea, and all others in the organized bodies of animals and vegetables, &c. The new system of chemistry, indeed, furnishes a vast deal of new light, but is yet by no means sufficient to penetrate in the deep mysteries of organized beings; for instance, the propagation of animals and vegetables has acquired from it but very little light, if any at all. The analysis of the spermatic humour, very easily to be made, affords not the least idea of the nature of its wonderful prolific power. After this digression, I return to the subject of the production of carbonic acid. 

1) See Experiences sur les Vegetaux, tom. I. p. xc. and xcii.

Calcareous stones, and alkaline salt, deprived of all their carbonic acid by fire, regain it by the sole exposure to the open air. If this production can happen by such simple means, can we be astonished that organized bodies draw it from the same source, the atmosphere; which to me seems to be the general magazine or store of all the substances which enter into the composition of all organized bodies of the animal and vegetable kingdom, and even of many others of the mineral kingdom. 

Mr. Du Hamel found, that a branch of a vine, or any other tree, conducted within a hothouse, its root remaining out of its boundaries, will there shoot forth vigorous leaves, new buds, flowers, and produce fruits, when all the other branches remaining exposed to the open air, shew no signs of life, being, with the roots, benumbed by the cold, and probably destitute of any motion or circulation of their juices. If the growth of vegetables depended on the absorption of carbon by their roots, the branch drawn in a hothouse could not thrive at all as long as the root and stem are benumbed by cold weather.1) But this branch, being in contact with no substance but air, heat, and light, must derive from the surrounding air alone all its wants to perform all the operations necessary to its growth and propagation. By watching all its phenomena, as I did, it will be found decomposing the air in contact with it, but in very different way by day and by night: and that it performs these transmutations of air chiefly within its organs, is the more probable, because all plants absorb the air in contact with it, with all its contents,2) and throw it out in a given time, much altered from what it was at the moment it was drawn in. The period of time required by a plant to renew all the air it has absorbed, I found to be less than half an hour by day and by night. (See the II. vol. of my work on Vegetables, Sect. XXII. XXIII. XXX.) 

This last assertion, which I have demonstrated by facts, will perhaps be looked upon by critical minds as somewhat paradoxical, as it seems difficult to conceive, that the same organized body can, at the same time, inhale and evaporate from the same surface the same fluid; but this double phenomena being continually performed in all parts of living animals, the difficulty of understanding it vanishes of course. Though the most part of annual plants which afford good nourishment for men, such as wheat, rye, maize, will grow in poor soils, yet they do not become thriving in a luxuriant way, but in what is called rich or well-manured soil. Those plants having a quick growth, when assisted by heat and the sun's light, come very soon to their term, or to the act of propagation of their species, or of producing seed, which being come to perfection the vital power of the plant is exhausted, and it dies. These plants being of a tender structure, and generally not spreading their roots deep in the ground, require the nicest attention in preparing the soil, so that the roots may find the least resistance in spreading, and may find as much nourishment ready prepared for being absorbed by the roots, as the plants want to become vigorous, and no more; as it is well known that too much manure, as well as too little, will prevent the plants from thriving. By want of manure the plant may be considered as starved, and by too much as chocked with food. This may perhaps be considered as somewhat analogous to a chick which will lay no eggs, or very few, by feeding it too little or too much; giving it daily, for instance, three or four ounces of good grain, it may lay every day an egg weighing about two ounces; but by cramming it with eight ounces of grain it will lay no eggs at all, or very few (I have forgot this accurate proportion). I believe (by the way) that in feeding animals either destined for food or for labour, the quantity, quality, and preparation of food necessary to obtain the end proposed is too little attended to, and that much food may be saved if this article became the object of an attentive observer. It is very true that some animals take in much more nourishment than they want, such as horses, in whose excrements are often found oats that are so far from having been digested, that they have even not lost their power of vegetation. It is very probable that a horse may be kept in good health and vigour by giving it mashed, ground, or boiled corn in a moderate quantity; malted grain would be the best, as Lord Dundonald thinks. Brown or rye bread is often given to horses in the Low Countries, on the road; the horses like it very much, and become remarkably lively after it; the same effect is obtained by beer and milk, which they also give them on the road. They find a remarkable benefit in the Low Countries by feeding the cows in the stable with boiled turnips, potatoes, and different other vegetables. They keep, by this food, their strength, and afford a great quantity of good milk. It may be with plants, perhaps, as it is with animals, too much food being hurtful to both; a dog and a cat being fed plentifully, lose their natural liveliness, grow fat, and sleep almost day and night. 

1) N..B. As the juices of the deep seated roots can never be congealed by frost, the cold never reaching so far; and as, therefore, the frost cannot congeal the juices of such a vine, but as far as its trunk is exposed to the open air, and a little way under the ground, I think this fact may be explained in a clearer way by the other theory which the reader will find hereafter. 
2) I prove this in various places of the 2d. vol. (French ed.) of my work on Vegetables. 

If coal is really the genuine food of plants, it seems to me doubtful whether the brown mud remaining after an infusion of dung is evaporated, is real coal before it has undergone an ignition. It is, as I think, to be called rather an extract, and may again be diffused through water, or dissolved as it was before the evaporation; but when it is burned into real coal, it is become almost totally insoluble in water, as all charcoal is generally well known to be. Charcoal is not only insoluble, but almost unalterable, incorruptible, possessing only when newly ignited an antiseptic power which it recovers again bv a new ignition; and I cannot help still doubting, whether real coal reduced even to impalpable powder possesses any manifest quality by which it deserves to be arranged among manures. The justly celebrated Arthur Young having put this powder to the trial, found it had no beneficial effect at all on vegetation. 
Though there is no doubt but that vegetables draw in by their roots a good deal of food, yet I think the principal business of feeding is carried on by the leaves in the atmosphere. Besides the fact of Mr. Du Hamel described on the preceding page, there are several other considerations which seem to give strength to this assertion. Many European trees, when stripped at once of all their leaves, will die. (Trees in very hot climates suffer the loss of their leaves by the scorching sun for a time in dry weather, without perishing.) I was present at the following fact: the sulphurous smoke from burning a few pounds of antimony mixed with nitre was accidentally driven by the wind upon a very thriving large pear tree, full of pears half ripe: next day I found all the leaves and pears fallen off, and the tree irrecoverably dead. A plant placed under a bell, with its roots in a phial full of water, will die when the bell is exhausted by an air pump, it will equally perish if, instead of respirable air, it is immersed in any air unfit for breathing animals. If the roots were the chief organs of feeding plants, their life might be supported in any of those airs, principally such as possess no acrimonious ingredient, as pure azote: but pure azote will kill a living plant, and prevent seeds from vegetation. 
Respirable air moderately warm is alone sufficient to make a plant vegetate without any light. I found even that seeds are hurt by a strong light, grow slowly, and are often killed, before the two lobes are become leaves, and their plumula or root is formed; and that if they survive the action of light, they remain commonly but weak or deformed plants. This shews, that in agriculture almost all seeds not covered, will perish when the sun shines upon them at the time of their beginning to swell or vegetate. My experiments have, I think, proved sufficiently, that such seeds or embryo plants perish by the light only (which is unsupportable to all very young plants, as well as plants weakened or sickened by transplantation), and not by the heat of the sun, or by want of moist.1) When a plant is reared up in the dark, either under the ground, or in a dark or shaded open place, to a certain degree of strength, light becomes more and more beneficial to it; not, however, for its advancing in size, but for acquiring strength, getting a lively green colour, and for its becoming fit for propagating its species, which propagation will not succeed without sufficient sunshine, or at least daylight; and a due degree of heat, that is to say the heat of the air, and by no means the heat of the soil; which last, though very beneficial to some plants, is rather hurtful to several, and indeed the ground being kept moist by watering it, is always kept cool by the continual evaporation; and yet plants in general thrive very well in moist ground, though always kept cool. Trees in a forest spread their principal roots to such a depth, as the heat of the atmosphere never can reach: their roots, are winter and summer in an uniform temperature of 50 - 52 degrees of Fahrenheit's thermometer. This shews that the vegetation of trees, as is that of almost all plants, being stopped, or nearly so, during the winter, is revived by the heat of the atmosphere alone, without any regard to the heat of the soil, which is scarcely subject to any alteration but at the surface. The heat of the atmosphere alone sets the juices of trees into motion, which motion sets, by propagation also, the juices of the roots into the same motion; thus the juices drawn from the roots upwards empty the filaments and suckers, which by this motion upwards must naturally become powerfully absorbent, without any degree of heat being more necessary for this absorption or attraction than is required for the suction of an ordinary pump. Boyle or Hales (if I recollect well) applying a glass tube to the trunk of a vine cut off in the spring, collected a very large quantity of juice pumped up by the roots, which motion of the fluids in vegetables depends greatly upon their irritability, according to Mr. Van Marum; which seems to be the more probable, as an electrical explosion directed through a plant, such as an euphorbia, stops immediately all motion in its juices by extinguishing the irritability.2)
1) See Experiences sur les Vegetaux, Tom . I I. pag. 447. 
2) See Lettre 2d de Mr. Van Marum a M. Ingenhousz, dans le Journal de Phisique de M. Lamethery, pour l'anne 1792. 

The roots thus absorbing the moist presented to their suckers, take in, of course, all salts, earth, metallic substances, &c. that can be dissolved in water, or in the saline matter to be found in almost all waters. Thus solvent is found to be for the most part fixed air. Though we find some of these salts with all their characteristic qualities in some plants growing in a soil impregnated with them, as are many plants growing near the sea-shore, which are full of sea-salt; yet it is not less true, that the most part of the ingredients imbibed by the roots as well as by the leaves, trunk, and branches, undergo almost a total change in the organs of the plants, even so far as to produce in one plant a wholesome food, and in another its next neighbour, a true poison. But as I have proved before that the atmosphere alone can furnish to some plants all that is wanting for all their functions, we ought not to look too anxiously among rubbish or dung, for the true and natural food of vegetables, though in those substances a greater quantity of this food is at hand ready prepared, and partly imbibed in the form of carbonic acid, mucilage, oily and saline matter, by which the plant is enabled to provide food for throwing out and nourishing more branches, flowers and fruits. I think it difficult to conceive how a large tree finds, during centuries, nourishment on the same spot, in the supposition of Mr. Hassenfratz, that its principal food is coal; and that this coal is not derived from the decomposition of the carbonic acid (of which coal constitutes nearly one-third, according to Mr. Lavoisier 28/100). That gentleman admits my discovery as well-founded, that plants produce carbonic acid in the dark; and that roots being always deprived of day light are of course incessantly occupied with this business. There exists every where in the soil common air, and common air alone is sufficient to furnish, as I have proved before, carbonic acid, even without plants. Thus there is no difficulty in tracing the source of this coal, and of conceiving how the largest tree finds, during centuries, that immense quantity of food it requires for its maintenance, growth, and abundant production of fruit or seed, all which is certainly derived in part from the soil; but I still believe chiefly from the atmosphere, by means of the leaves absorbing and decomposing the air in contact with them. In the sequel of this paper, the manner by which the roots of trees beget carbonic acid will be further traced. 
The transmutation of common air into different solid bodies, such as plants, is a very ancient doctrine; Pythagoras and Epicurus took this for an undoubted fact, and Lucretius, who has adorned his poem de Rerum Natura with his doctrine, says that air changes continually into different other substances, and that these are again decomposed into air, which afterward returns again into the composition of the different bodies; and that if this incessant rotation did not exist, every thing in this world would have been changed into air, which of course would have been at last the only substance existing. These are his words: 
Aera nunc igitur dicam, qui corpore toto 
Innumerabiliter privas mutatur in horas. 
Semper enim quodcumque fluit de rebus, id omne 
Aeris in magnum fertur mare, qui nisi contra 
Corpora retribuat rebus recreetque fluenteis, 
Omnia jam resoluta forent et in aera versa. 
Haud igitur cessat gigni de rebus, et in res 
Recidere assidue, quoniam fluere omnia constat.
 Titi Lucret. Car. de Rerum Nat. Lib. V. v. 274. 

Anaximenes said that all bodies are made of air. 
On a preceding page of this paper I hinted at a new theory of a curious fact, viz. that plants accelerate their growth in the dark, and advance the least in the middle of the day (which is an observation of Mr. Gardini). Though this theory may perhaps be erroneous, yet as it is supported on a real fact, I may be permitted to say further, that plants changing in the dark more respirable air into carbonic acid than they can digest, they throw out a large quantity of it, and thus render the air in contact with them less respirable, and that in the day they absorb with the atmospheric air so much matter of heat and light, or caloric furnished by the sun, that they cannot all digest it, and therefore throw it out as superfluous, combined with the oxygen, which has thus acquired the nature of vital air, which vital air, though not yet obtained by plants in its greatest purity, is, however, in itself full as pure as is that which we obtain from the best manganese or any other ingredient. (See my book on Vegetables, French edition, vol. II. p. 182.) 
In a letter to Sir John Sinclair, dated Dec. 2d, 1794, I quoted as a proof of carbonic acid being the principal food of plants, the fact I discovered, that the wonderful apparatus which a plant produces when it is occupied with the propagation of its species, viz. the flower, is incessantly producing carbonic acid. By this observation we may be led to the knowledge of the true natural food of vegetables; and it may be said as a further illustration, that if we were desirous to know what is the natural food of some particular animals, one of the surest methods to find it out would be, to observe what kind of nourishment the parents bring to their young. Thus we should find that a pigeon is best fed by grains, and a swallow by insects. By a similar conclusion I may infer, that the true or principal aliment of plants is respirable air decomposed. By examining the air thus decomposed, I found it consist of two substances, viz. of fixed air or carbonic acid, and phlogisticated air or azote (see my book on Vegetables, French edition, Vol. I. p. xc, and in different other places of Vols. I. and II.); but as carbonic acid contains two distinct substances, viz. coal or carbon, and oxygen, it may be questioned what ingredient of the two is the real food we look for. Mr. Hassenfratz thinks it is principally coal; though his opinion is that the plant does not derive the coal from the carbonic acid, but from the soil or dung; I am much inclined to think that both these substances serve as food; and I am moreover inclined to believe that the azote enters also the plant, and has also some share in feeding it. One of my reasons to think so is, that plants absorb continually the whole of the atmospheric air;1) and that in separating this fluid by decomposition into its constituent parts, they throw out that part of it that they cannot all digest at the time it is produced, viz. at night the azote and the carbonic acid disengaged one from another, or only mechanically, not chemically, as formerly they were mixed, and in the sunshine the oxygen almost alone, the carbon and the azote remaining within the plant at that time. Though I think it probable that the azote enters in some way or other into the composition of plants, yet I think it is not absolutely necessary for a plant, as a plant thrives admirably well without it, viz. in pure oxygen. It is true, however, that plants also die in pure carbonic acid, but in this case the plant may be perhaps considered as if it were choked with it. 
1) See Vol. II of my work on Vegetables, French ed. p. 92-94, and following; as also page 121, 173, 186, 504

I acknowledge very readily that the just-mentioned theory has not all the clearness I should wish to give it. The facts, however, quoted to support it, though contradicted during twelve years, are now admitted publicly, even by those who have been the principal champions and the most violent (even to declare my new doctrine to be a downright calumny against nature, to be revenged by nature itself) against it. 

As all the most powerful manures have one common quality, viz. to contain or to disengage a great quantity of carbonic acid; and as dung applied too liberally injures vegetation, it seems to be probable, that the principal attention in manuring ought to be directed to the time of applying it to the soil, which is when it is in the height of its putrid fermentation (the acid and vinous fermentation preceding the putrid fermentation cannot be observed in dung, as they pass too quick to be observed, or perhaps do not happen at all in a substance so highly putrifiable); and indeed the most skilful husbandmen think so, and practise it. Thus the great force of putrefaction, which would at last have destroyed the whole ferment, being checked by spreading the dung over the land, or ploughing it in, begins anew in a more moderate way, by the help of warmth and moist, at a time when the growing plants may absorb the fixed air or carbonic acid, either as it is when just formed, or as it is being incorporated with moist or other substances, such as different salts, and earths, of which various kinds are to be found in almost all soils; which substances brought in close contact by moisture, act on one another chemically; that is to say, by various combinations, attractions, or simple and double, or compound chemical affinities. Of all those species of synthesis, analysis, and attraction, an almost constant attendant is the disengagement of carbonic acid.1) Animal and vegetable substances probably act as manures only, when in the act of decomposition by putrefaction, during which period a great quantity of carbonic acid is produced. This putrefaction is promoted by almost all salts when mixed with those substances in moderate quantities, but is checked by a large proportion of those salts, as Sir John Pringle found. It is thus with alkaline salts, with common salt, gyps, &c. which last is a vitriolic salt with an earthy basis. This notion may account for the benefit which the Germans and the Americans derive from employing gyps as a manure. The latter find it even worth their while to draw this ingredient (gyps) from Europe. I believe, however, that of all salts the alkaline salt would be found infinitely superior to any other salt, for the reason mentioned in my letter to Sir John Sinclair. 
According to these notions, we may perhaps understand, why all those manures which undergo the quickest decomposition, ought to be oftener applied than some others, which, not being susceptible, but of a very slow decomposition, such as chalk, lime, burnt and pounded bones, gyps, impart during several years the soil with prolific quality. 

1) If we cast our regards with astonishment on the vast scene of that perpetual rotation of organized beings, changing continually one into another by acquiring life, and disappearing by death: when we consider that all living animals, by their respiration, perspiration, digestion of their food, by the putrid fermentations of their bodies after death; that all vegetables as long as they live, as well as when thev are in a state of decomposition after death; that, in short, all the operations of an infinite variety, every where obvious on the surface of the earth, have one general effect that of producing carbonic acid, even in the middle of putrefaction, by which volatile alkali, now called ammonia, is chiefly produced. If we add to this consideration, that even the inorganized bodies, those of the mineral kingdom, have in many instances, as we have already hinted, a considerable share in assisting the two other kingdoms of nature in preparing this aerial fluid; if we consider, I say, that all these innumerable operations conspire as it were, in producing one general substance; is it possible to doubt, that this fluid, the carbonic acid, has an utility as extensive in this world as is its almost universal production? 
But it may be asked, why is it not to be found in the atmospheric air, if it is almost every where produced? The reason, I think to be, that as soon as it is generated that is to say, as soon as the carbon is combined with the oxygen, and the caloric dissipated, it ceases to be intimately combined with the atmospheric air; having acquired a superior specific gravity, it quits the common stock, sinks to the ground, and becomes easily miscible with moist, different salts, &c. and thus it disappears almost as soon as it is produced, and becomes, perhaps, the first step towards the transformation of common air into solid bodies. 
See my book on Vegetables, p. 133. English ed. The article is, on the nature of the air oozing out of our skin. Dr. Priestley denies absolutely all aerial evaporation from the surface of animals and plants. Some very recent writers seem to claim this aerial evaporation from our skin as a new and their own discovery. 

We may also understand why quick-lime renders the soil more prolific than limestone; chalk being deprived of its carbonic acid, attracts it readily again from the atmosphere by the help of moisture; but the carbonic acid is strongly attracted by water in which limestone is dissolved, and by this attraction the carbonic acid enters also the lime, which is by this combination precipitated, and being thus infinitely divided, is become easily penetrable by all acid, which dislodge again the carbonic acid from it; whereas limestone, whose texture is not loosened by fire, is naturally not so easily decomposed by the action of acids which it may meet in the soil. 

It has a long while struck me with wonder, that the excrements of horses and cows are almost alone preserved (and that even in general with little care) for manuring, and that human excrements, which are infinitely richer, are too much neglected, many privies being so constructed, that either the whole is lost in common shores, or the most enriching part of it, the salts contained chiefly in the urine, sunk in the ground, or running to waste. We ought to learn from the Chinese how to preserve these precious relics of our digestions, and to restore them to the soil, on purpose to be metamorphosed by it into new food. I make no doubt but that if none of a man's ejections were dissipated, but employed in due time on the soil which was allowed to him to draw exclusively his victuals from, this soil would be so much fertilized by it, as to afford by proper labour more than sufficient food to nourish him. The alvine and urinous ejections of one day, kept together and well mixed, are more than sufficient to dung abundantly for a whole season four square feet of ground. Now on a space of four square feet will grow more potatoes or other vegetables, than a man could well consume in one day; by selling some of this superfluous portion, he may purchase what is necessary to make his potatoes, or food exchanged for them, palatable. The Chinese neglect the dung of horses and cows as we do our own dung, but they provide every where covered reservoirs for storing up. what we drop uselessly in the privies, and the streets. 

Page 1. of this paper I have advanced, that some plants derive the most part of their watery substance from the surrounding air. This assertion is not difficult to prove, by considering that all organized beings possess the faculty of producing cold in a medium hotter than their own temperature, and of engendering heat in a medium colder than is their temperature. Mr. John Hunter has demonstrated that trees produce heat in cold weather. Nothing is easier to prove, than that all plants produce also cold in hot weather, and that some possess this power in a more conspicuous way than some others. The feeling alone of a leaf of a vine in the sunshine, will convince any one that it is colder than are the leaves of the most part of neighbouring plants. The continual evaporation of plants, no doubt, has its share in producing this cold; but there is little doubt that this cold is chiefly produced by the vital organs of plants, and the construction of the leaves. A bason full of water will become sensibly hotter in the sunshine than the leaves of some plants, though there is infinitely more evaporation from the surface of the water than there is from the surface f the leaves of plants; it seems the intention of nature to prevent leaves of plants receiving too much heat, has been fulfilled partly in that part of the leaves which alone is destined to receive the direct rays of the sun, it being of a smoother texture than the opposite surfaces so as to appear almost as if it was covered with a coat of varnish; from which the rays of light are rather reflected than absorbed. We know that a bright metal will receive but little heat from the sunshine, and that a piece of black cloth will be sooner heated in the sunshine than a piece of white cloth; a green piece of paper hung in a vine will receive more heat than a leaf or the vine of the same colour. 

The production of cold by vegetables is of more importance than one would at first sight imagine, as it is chiefly by this faculty that plants are enabled to draw in a part of their moisture, which enters the plant in two different ways; 1st, together with the air in which it is dissolved, continually drawn in by all plants, as I have proved in my second volume on Vegetables; 2d, it is absorbed by plants, being left or precipitated on their surface, in a way imperceptible to our senses, by the coolness of the plant, in the same manner as the moist of hot air is precipitated on the surface of a decanter full of cold water, or ice. 

Air never is nor can be without water, which exists in it in a double form. 

1. As water itself is a composition of two airs, vital and inflammable, or oxygen and hydrogen, in which two substances, Mr. Lavoisier found means to analyze water, and which analysis, as far as it regards the oxygen, I affirmed in my first volume on vegetables, to be performed by Vegetables, with the assistance of the sun, even before Mr. Lavoisier, as I think, published his Analysis. 

2. Air, however dry in appearance, has always a considerable portion of water combined with it, in a manner analogous to the solution of salt in water. The water thus adhering to air, is easily separated from it by various means, chiefly by cold; see page 157 of Vol. I[. of my work on Vegetables, French edition, I quote a simple experiment, which shews that the driest air contains always a great quantity of moist. 

That leaves of plants absorb and evaporate at the same time moist and air, is not more difficult to be understood, than that all internal surfaces of our bodies are incessantly moistened by a liquid transuding upon them, and that at the same time a part of that same humour should be continually reabsorbed, and returned to the general mass of humours. In the 3d article of Vol. II. of my work on Vegetables, and in some other places, I have, as I think, demonstrated that such a simultaneous absorption and emission of air is continually going on in all vegetables. 

I think it is not worth while disputing whether this simultaneous in and exhalation is carried on by distinct inhaling or exhaling vessels, or by the same vessels assuming both functions alternatively. It is not improbable that absorbent vessels in animals should also carry on the functions of transpiring vessels; or in other words, that absorbent vessels should have sometimes an inverted or retrograde motion. This is the opinion of Dr. Darwin, and of several other men of high reputation. 

Dr. Priestley denies absolutely all emission of air, as well from the skin of animals as from the surface of plants. I think, however (as I have already insinuated in the former note), I have proved the existence of this emission from both, and even that our skins evaporate fixed and azote air. See Vol. I. of my book on Vegetables. 

The economy of vegetables, as far as we traced it in the former part of this paper, may lead us insensibly to farther reflections, and to trace the reason, why the soil seems to be almost exhausted after it has produced certain vegetables, so as to have induced the cultivators of most all countries to leave the ground at rest for a whole year. Some lands require even two years rest to recruit their former power of nourishing the same sort of plants. Some plants exhaust the soil more than others; thus flax impoverishes the soil to such a degree, that in the countries where the finest flax is produced, such as in the country of Waas in North Flanders, and in the vicinity of Valenciennes, the value of one good crop is equivalent to the value of the land itself, and it is often sold for this value, which could not be the case if the ground was able to produce every year a tolerable good crop of the same plant. Virgil knew this difference of plants very well, saying, that flax, oats, and poppies exhaust (burn) the soil. 

Urit enim lini campum seges, urit avenae: 
Urunt lethaeo perfusa papavera somno.

The ground laying fallow is, however, very far from having lost the power of nourishing different plants; for, if it is left untouched, it will be found full of weeds, which owe, in a great measure, their origin to the dung of horses, and other cattle, which contain a vast number of indigested seeds of various plants. The superior wisdom of the Chinese (as we have already hinted) and the Japanese is very conspicuous on this head, as it is in many others, by preferring human dung (which contains none of such seeds) to that of cattle. Their fields are, for this reason, at least in a greater measure, so free from weeds, that a celebrated botanist, passing lately through Japan with the Dutch embassy, could scarce find any other plants on the corn fields, but the corn itself. But in these countries the laws prohibit the neglect or waste of all human excrements, and every house has proper reservoirs for this important ingredient, of which, perhaps, not ,10 part is preserved in the most part of Europe, and principally in this country, which is considered as the most enlightened and best governed of whole Europe, though in the middle of real distress for corn, (of which it sometimes imports a million sterling's worth in one year,) it throws away the substance the fittest of all for multiplying corn; and it suffers, moreover, one third of all the arable lands to lie without cultivation or inclosure. 

The utility of fallowing seems hitherto not to be well understood, any farther than ancient custom, founded on experience, has propagated the practice from father to son. The benefit derived from this practice, is commonly attributed to the utter destruction of weeds, by ploughing up the ground several times, and to the action of the atmosphere on the soil. It seems to me somewhat doubtful, whether the ancients considered the ploughing up of the ground so necessary in fallows as we do; as Virgil, whose superior skill in the husbandry of his time is generally acknowledged, speaks of the utility of fallowing, as consisting only in leaving the ground at rest, without stirring it by the plough; which rest he even prefers to the rotation of crops. 

Sic quoque mutatis requiescunt foetibus arva. 
Nec nulla interea est inaratae gratia terrae. 
Georg. 1. v. 82.

The first of these benefits, that of getting rid of the weeds, which multiply themselves by propagation, is evident enough; but it has not yet been determined, what operation the atmosphere performs on the land to restore its fertility. Those who have attempted to find it out by analyzing the soil before and after the fallow, have laboured in vain. I imagined a long while ago, and told it to several of my acquaintances, that an inverse way of examining this mysterious influence of the incumbent air on the soil, may possibly be the best, if not the only way to unriddle this difficulty; that is to say, to examine, not what the soil had gained, but what the incumbent air had lost. 

When, in 1779, I engaged in the research on the mutual influence of the vegetable kingdom upon the animal creation, or on the relation which exists between the two organized kingdoms in regard to the common element of both, the atmospheric air, I took not so much notice of what happened to the plants themselves, when confined with their own element, as what happened to the air confined with them in the light and in the dark. I contented myself, as to the plants, with finding that they suffered nothing by having been shut up with their own element during some hours. But on purpose to make the following reasoning more coherent, I must beg leave to make a short recapitulation of some articles already explained in the former part of this paper. After having satisfied myself about the safety of the plants, I found, by examining the air shut up with them, that it was affected by the plants quite differently in the day light from what it was in the night, or even in a dark place in the middle of the day time. I found that the plants communicated by day time to the air in contact with them, oxygen (vital air) or the general acidifying principle, of which the atmosphere contains about 27/100 and that at night, or in a dark place by day, they communicated to the air in contact with them fixed air, now called carbonic acid, which is composed of the same acidifying principle combined with coal, carbon, to which it has a great attraction or affinity. 

I inferred from these, and some other facts quoted before, that the plants, in the common course of nature, draw from the air, in a great measure, what is necessary for their subsistence; and that, being thus incessantly occupied in decomposing the common air, they render a part of it miscible with the ground, or with substances inherent in the earth, such as moist, salts, &c. that the carbonic acid, which is now admitted (according to my original idea) as a nourishing substance for plants, is prepared, without intermission, day and night, by the roots and flowers, and in the night by the leaves and the rest of the whole plant, must have been destined by nature to some important use for the plants themselves. An utility, whose importance must be equivalent to the wonderful action by which it is produced, not only by the plants, but by an almost universal operation carried on all over the surface of the earth, at the time principally when all vegetables want the most nourishment. Being full of this persuasion, I thought it very probable, that the soil itself must also have its share in contributing to this almost universal process of nature. By putting this my suspicion to the test of experiments, I found that the soil, even without the assistance of any plant, is incessantly employed in drawing this general and acidifying principle from the incumbent air, and in changing it into carbonic acid, by furnishing it with carbon, of which the ground is never deficient; that the soil performs this decomposition of the air, night and day, though more powerfully by day, and in warm weather than in the dark, and in cold weather; and that this decomposition is sometimes s powerful in good garden ground, that it counteracts even the influence of the most vigorous plants in the sunshine; so that the earth of a flower-pot communicates some times more carbonic acid to the air, confined by a glass bell (with which the plant and the flower-pot are covered) than the plant had communicated oxygen; which oxygen, being absorbed at the same time by the soil, the remaining air had lost more of its oxygen by the attraction of the soil, than it had acquired by the presence of the plant. See my Experiences sur les Vegetaux, tom. II. p. 119,188, 438. 

Eight cubic inches of good mouldy ground, without manure, exposed in a tea saucer to the contact of eighteen cubic inches of atmospheric air during three day and nights, in the summer, the weather being agreeably warm, but constantly hid from the sun-light, by covering the apparatus with a flower-pot, had contaminate the air to such a degree, that a wax taper could not well burn in it. One measure of this air mixed with an equal measure of nitrous air, in the eudiometer described in my work on Vegetables, was reduced to one measure, and six-and-thirty hundreds of a measure; whereas one measure of atmospheric air mixed with an equal measure of nitrous air, was reduced to one measure and two hundred parts of a measure. A similar quantity of the same soil, confined with the same quantity of air, the apparatus being left uncovered, so that the sun did shine upon it during the most part of the day, had injured the air still more, so that this air had lost the most part of it oxygen, At the same time I exposed two similar quantities of common air to the action of eight cubic inches of well manured garden mould, one of these apparatus being exposed to the sunshine, the other being covered all the time by a flower-pot. By examining the air of them, it proved to be still more injured than the air of the two former apparatus, principally that which had been exposed to the influence of the sun's light; so that this air having lost almost all its oxygen, was changed nearly into pure azote, mixed with some carbonic acid, of which acid I found manifest sign by lime-water becoming troubled when shook with any of those four airs. 

All this shews, in a manifest manner, that the soil draws incessantly from the incumbent air the oxygen, the general acidifying principle. There can be, I think, little or no doubt, that what happens to air shut up with earth, happens also to the air floating continually over the surface of the earth - that is to say, that the soil draws incessantly some oxygenous particles from the air sliding over it; so that, in the course of a whole year, a soil, principally when ploughed up several times, must have attracted a considerable quantity of the acidifying principle, principally mouldy ground, in which some decayed vegetable or animal substances capable of farther decomposition exists. Siliceous sand, either dry or moist, does scarce or nothing at all injure the air in contact with it; neither is air materially affected by being shut up with pure water. Now as all acids derive their acidity from the oxygen, of which the common store exists in the immense ocean which surrounds our globe, the atmosphere, is there not some probability that it would be possible to restore, in a moment, to the soil, what it can acquire in no less than a whole year from the incumbent air when left to itself? 

When the soil has acquired from the incumbent air a considerable portion of oxygen, either as it is existing in the common air, or combined with carbon, with which it constitutes carbonic acid, the rain will certainly carry a great deal of it to the deeper strata of the ground, and thus rob in part the superior stratum of the ground of the full benefit of this useful substance. But if by this the surface of the ground should even become almost entirely exhausted of it, it would soon recover it again from the same source, and that even in a more powerful way than it did before the rain; because the soil, when perfectly dry, has little action, if any at all, on the incumbent air. In the mean time that rain carries with it to the bowels of the earth a great deal of the ingredients useful to superficial plants, these ingredients are not lost to the vegetables in general, as they are absorbed by the deep-seated roots of trees, which roots, though incessantly occupied themselves with shifting from the air, always existing in the ground, the carbonic acids (See my work on Vegetables) must acquire by this conveyance a fresh and probably a necessary supply of it. Besides this power of shifting carbonic acid from the air by attracting its oxygen and furnishing it with carbon, plants possess a most wonderful faculty of changing water itself into vital air, or oxygen; which I have maintained as early as 1779. (See my work on Vegetables.) 

I have, as I think, sufficiently demonstrated, by facts, in the preceding part of this paper, that not only vegetables, but the soil itself (even as caustic alkalies and quick-lime), have a power of extracting the carbonic acid from the atmosphere; though the French chemists assert, that common air contains in itself no carbonic acid at all; which is not easily to be accounted for but by considering that there is in this world, going on an incessant rotation of beings, by the continual new combinations, attractions, mixtures, and affinities, in animated and inanimated beings. 

It seems to be more than probable, that the soil, laying fallow, attracts from the incumbent air more of the acidifying principle than it does when covered with plants; as at that time the contact with the air is partly hid from it, and as the vegetables themselves take it in, in proportion as it is formed. 

Would it not from all these considerations taken together appear probable, that the oxygenous principle may be in a moment imparted to an exhausted soil, by pouring upon it, a little before the sowing of fresh corn, one of the most concentrated acids, much diluted by water or divided among a heap of earth? I am of opinion, that the first trials, if thought worth while to be made, should be made with concentrated muriatic, or vitriolic acid, principally the latter, by mixing it with a sufficient quantity of water, or of dry sand or earth, so that it may be thrown or scattered over the ground as corn is usually sown. We know with sufficient certainty, that vitriolic acids (even as all other acids) is nothing else but the acidifying principle, drawn from the atmosphere by the burning of sulphur, and partly from the nitre added to the sulphur, to assist the inflammation of it by continually furnishing it with fresh. Oxygen, without which no flame can exist. 

This acidifying principle retains its aeriform nature as long as it is combined with a due proportion of caloric, or the matter of heat, which is considered as the general principle of fluidity of all liquid bodies; which caloric being consumed in the burning of the sulphur, the oxygen can no more retain its form of air, and allies itself with the sulphur as with its basis, and constitutes the sulphuric acid, in a similar way as this very oxygen, when combined intimately with azote as its basic, constitutes nitrous acid, and with phosphorus phosphoric acid. 

Though I readily acknowledge that this hint is only deduced from theory and analogy, I cannot help thinking, however, that by some agricultural philosophers it may meet with some attention, or be considered as deserving a trial in various ways. Being in possession of no facts which could serve as proper guides how to proceed in such trials, I can only throw out some loose hints, the product rather of imagination than founded on experience. Having no ground at my disposal, I could only make some preparatory experiments, which, as far as they go, were rather encouraging than derogatory to my scheme. I made them last year with my friend, the Hon. Baron Dimsdale, M.P. in his father's garden. I poured about one dram of oil of vitriol, diluted in a pint of water, in a ridge about two inches deep and three feet long; immediately after, I sowed in it twenty grains of wheat, and covered them with earth. I sowed also twenty grains in another similar ridge, on which I poured the same quantity of diluted oil of vitriol. These mixtures fermented very violently with the soil which contained a great portion of calcareous earth. A part of the calcareous earth was thus changed into gyps minutely divided, resembling a whitish powder. I repeated this, in other spots of the garden, taking, instead of wheat, oats, rye, and barley. This was also done in some flower-pots. Though the quantity of acid was in these little experiments much greater than what I would have employed on a field, yet the general result was, that, very far from having hurt the vegetation of the seeds, we found the plants all very thriving, and in the most part of these spots the plants came out earlier than those which were not manured with acids. The marine and the nitrous acids had a similar effect as the vitriolic. 

The plants thus treated were neither retarded, nor weaker than others, hut rather stouter or more vigorous for the most part. 

Though I can say nothing with any good reason about the quantity of acid to be employed on each acre of land, yet if I had the free disposal of some land, I would begin by scattering on one acre twelve pounds of concentrated oil of vitriol diluted with water, or rather divided among a heap of dry sand containing little or no chalk or other calcareous earth; on another acre I would employ nine pounds; and on a third six pounds, which, at the rate of 4d.1) the pound, would come to 25. per acre. If any advantage should be observed from it, some attentive farmers would soon find out the most advantageous proportion of this ingredient on different sorts of land. 

It would be useless to occupy ourselves with a particular theory to explain the effects of acids on the soil, after it is demonstrated that, in the natural course of things, the atmosphere imparts to the ground the very same acidifying principle which constitutes not only the vitriolic, or sulphuric acid, but also all other acids. Thus the only difference between leaving the land in fallow, and sprinkling it with a powerful acid, would be, that in the first case a whole year's crop is lost in waiting for the slow operation of the atmosphere; whereas, in the proposed method, all that the air can grant in a whole year may be communicated all at once to the ground. 

I would advise to destroy the weeds as soon as possible, by ploughing at shorter intervals, at the ordinary depth, and give the land, a day or two before sowing, a superficial ploughing, and break as much as possible the ground, after which the acid should be scattered on the soil, and soon after the seeds committed to the ground. 

Concentrated oil of vitriol was sold, a little while ago, in England and in France, at the rate of three-pence the pound, by bottles containing about 150 pounds. It is now sold at the rate of five-pence, at the time this paper was in the press, in Feb. 1796. Concentrated marine acid was sold for about the same price.

I think the first trials should be made with grain, to be sown in the spring, through fear that the rain would carry away the benefit in the winter. 

There is, perhaps, no fertile soil which has not a portion of calcareous earth, or magnesia, or of both. These two ingredients absorb very readily any acid, principally vitriolic or sulphuric acid. With the first (the calcareous earth) it constitutes gyps, with the second Epsom or bitter salt; in the alliance with either of them, a vast quantity of carbonic acid is produced, which is easily retained by the moist of the soil, and ready to be taken hold of by the vegetables as their food. 

Could this acid not be serviceable in increasing the fertility of land in general, and restore all at once the fertility of the soil, exhausted by some very impoverishing plants, such as flax? 

If I had land at my disposal, I would not hesitate to oxygenate (as a trial), in the way already explained, all such pieces of ground as were destined to lie fallow during the next season, and to sow corn on it. The loss, if it should not answer the expectation, would be trifling, but the advantage would be considerable if it should succeed. I hope, however, that some intelligent farmers will put the project immediately to the test of experiment for the next season. I would advise thereabout the following method: to plough up several times a piece of ground; for instance, an acre, on purpose to destroy the weeds, and to bury them under the ground, where they act as manure; then to divide this spot into five equal partitions, four of which to be manured in the usual way, and then oxygenated as directed below; on the fifth no manure nor acid should be employed, as this partition should be destined as a standard or a comparative experiment. On each of these five pieces of ground an equal portion of corn should be sown in the usual way, On one of the four partitions to be oxygenated I would sprinkle about two pounds of concentrated oil of vitriol diluted, on the second three pounds, on the third four pounds, and on the fourth five pounds. The time of the rise and progress, as well as the exact quantity of grain to be gathered, should be exactly taken notice of. By such an experiment the advantage or disadvantage of this new scheme would be known at once as to the renovation of soils destined to lie fallow, and the quantity of acid the most advantageous. 

If the project should succeed, a temporary rise in the price of the vitriolic and marine acid may possibly be the consequence; but the discovery of making oil of vitriol from one of the most abundant ingredients of the world, sulphur, would soon reduce it again. 

In the choice of vessels to be used to spread vitriolic acid on the soil, it is to be observed that iron ones would soon be destroyed, as vitriolic acid attacks iron very violently, but as it does not destroy lead, common tin watering pots could be lined with sheaths of lead. I would give the preference to wooden vessels


As I have made mention, more than once in this paper, of a letter I wrote to Sir John Sinclair, dated Dec. 2d, 1794, on different articles relating to agriculture, and among others, on the beneficial effects of alkaline salts in promoting vegetation, and respecting the effect of some other salts; and as that letter makes no part of this paper, I think it proper to inform the reader, that the particularly good effect of alkaline salt was so manifest in my own garden, that all the gardeners who saw it, thought it equal to the effect of the best dung. I repeated the application of that salt last year (1795) at Hartford, with the Hon. Baron Nath. Dimsdale, M.P. in his garden, and that gentleman was equally convinced as myself, of its manifest good effects. We tried, at the same time, the application of different neutral salts, the particulars of which experiments I may possibly publish on some other occasion. We made also many experiments with different solutions, and medicated liquids poured upon the ground, as well as steeping the seeds of different grains in them. Be it sufficient to say here, that of all the neutral salts we tried, the Glauber salt did seem to be one of the best in promoting vegetation; and that the steeping the seeds in water impregnated with oxygenated marine or muriatic acid (which is now much employed in bleaching linen in an expeditious way), had a particularly beneficial effect in producing vigorous and early plants- The beneficial effects of these different substances may be easily accounted for by an intelligent reader, according to the theory lain down in this paper. 

We were somewhat astonished that those seeds, viz. wheat, rye, barley and oats, which had been steeped in the above-mentioned oxygenated muriatic liquid, even during 48 hours, did thrive admirably well; whereas the same seeds steeped during so long a time in some of the other medicated liquids, were much hurt, or had lost their vegetative power. The same oxygenated liquids poured upon the ground had also a beneficial effect. 

[Published 1796]