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⁾
 

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. 
 

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. 
 

2) See Annales de Chimie, of 1792. 

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⁾
 

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. 
 

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. 
 

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. 
 

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. 
 

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. 
 

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. 
 

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. 
 

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. 
 

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. 

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. 
 

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]