pendent on the life and developement of plants. Plants not only afford the means of nutrition for the growth and continuance of animal organization, but they likewise furnish that which is essential to the support of the important vital process of respiration; for besides separating all noxious matters from the atmosphere, they are an inexhaustible source of pure oxygen, thus supplying the loss which the air is continually sustaining. Animals, on the other hand, expire carbon (as carbonic acid) which plants inspire; and thus the composition of the medium in which both exist, namely, the atmosphere, is preserved constantly unchanged. It may be asked, is the quantity of carbonic acid in the atmosphere, which scarcely amounts to one-thousandth part, sufficient for the wants of the whole vegetation on the surface of the earth? Is it possible that the carbon of plants has its origin from the air alone? This question is very easily answered. It is known that a column of air of 2,216·66 lbs. Hessian rests upon every square foot Hessian of the surface of the earth; the diameter of the earth and its superficies are likewise known, so that the whole weight of the atmosphere can be calculated with the utmost exactness. The thousandth part of this is carbonic acid, which contains upwards of twenty-seven per cent. of carbon. By this calculation it can be shown that the atmosphere contains 3000 billion lbs. Hessian of carbon; a quantity which amounts to more than the weight of all the plants, and of all the strata of coal and brown coal, which exist upon the earth. This carbon is therefore more than adequate to all the purposes for which it is required. The quantity of carbon contained in sea-water is proportionally still greater.'-p. 21.

Again:

The proper, constant, and inexhaustible sources of oxygen gas are the tropics and warm climates, where a sky seldom clouded permits the glowing rays of the sun to shine upon an immeasurably luxuriant vegetation. The temperate and frigid zones, where artificial warmth must replace the deficient heat of the sun, produce, on the contrary, carbonic acid in superabundance, which is expended in the nutrition of the tropical plants. The same stream of air which moves by the revolution of the earth from the equator to the poles, brings to us in its passage from the equator the oxygen generated there, and carries away the carbonic acid formed during our winter.

'Plants thus improve the air by the removal of carbonic acid, and by the restoration of oxygen, which is immediately applied to the use of man and animals. Vegetable culture heightens the salubrity of a country; and a previously healthy country would be rendered quite uninhabitable by the cessation of all cultivation.'-p. 23.

Although the above extracts are much compressed, we trust they will convey to our readers some idea of the cogency and beauty of the arguments by which Professor Liebig has established his propositions. They leave no doubt as to the sublime and perfect arrangements by which much of the economy of nature is maintained; they point directly, in the words of our author, to an infinite wisdom, for the unfathomable profundity

of

of which language has no expression. The importance of the conclusions thus established to a scientific system of agriculture is too obvious to require comment.

How does it happen,' asks Professor Liebig, that the absorption of carbon from the atmosphere by plants is doubted by all botanists and vegetable physiologists, and that by the greater number the purification of the air by means of them is wholly denied? These doubts have arisen from the action of plants on the air in the absence of light, that is, during the night.'-p. 26.

These doubts and difficulties are discussed and dissipated by our author in a most masterly chapter, which, however, we cannot quote at present. He candidly acknowledges that

"The opinion is not new that the carbonic acid of the air serves for the nutriment of plants, and that its carbon is assimilated by them; it has been admitted, defended, and argued for, by the soundest and most intelligent natural philosophers, namely, by Priestley, Sennebier, De Saussure, and even by Ingenhousz himself. There scarcely exists a theory in natural science in favour of which there are more clear and decisive arguments. How, then, are we to account for its not being received in its full extent by most other physiologists-for its being even disputed by many-and considered by a few as quite refuted?'-p. 34.

This Professor Liebig attributes to two causes. First, that most botanists and physiologists have not availed themselves of the assistance of chemistry in their researches, owing to their slender knowledge of that science; secondly, that those who have experimented, in all good faith, on this very point, have made their researches in a manner totally opposed to all the principles of experimental philosophy. They were utterly unacquainted with the art of experimenting, which, as he justly says, can only be learned in the laboratory. Both accusations are true to a certain extent; it is certain that if physiologists had availed themelves of chemistry they would have advanced farther: as also that if certain experimenters had practically learned the art of research, they would never have thought of attaching any importance to the results of such experiments as Professor Liebig describes : but we venture to offer a third explanation, namely, that the arguments for the doctrines established by this writer were never till now laid down in a clear and logical manner; and the having done this entitles him to the same honour as if these doctrines had originated with him. In fact, when the illustrious philosophers whose names are mentioned above made their researches, chemistry was not sufficiently advanced to afford the same means of deciding the question as it does now. In the opinion held by our author, which indeed it is the chief object of his work to inculcate, that it is to chemistry we must look for the

future

future improvement of physiology and of agriculture, we cordially concur. The next generation, both of physiologists and of eminent agriculturists, we confidently predict, will be men accomplished in the art of chemical research; and for this we shall be mainly indebted to Professor Liebig.

Passing over an interesting section on the assimilation of hydrogen by plants, we must briefly allude to that on the source of the nitrogen in the vegetable kingdom. This element is highly important, as being an essential part of those vegetable products which serve as food for man and animals. Indeed Boussingault had proved that the nutritive power of different species of vegetable food is in proportion to the nitrogen they contain.

Without entering into minute details, we may state that Professor Liebig has shown that all the nitrogen of plants and animals is derived from ammonia; and that this ammonia is furnished by the atmosphere, from which it is brought to the earth in every shower of rain. Its quantity in the atmosphere is relatively very small, but amply sufficient for all the demands of the animal and vegetable kingdom. Indeed, as all the nitrogen of past generations of plants and animals must, in the process of putrefaction, have been sent into the atmosphere in the form of ammonia, its presence in the air might have been anticipated. It is to Professor Liebig, however, that we owe the experimental proof of the fact. He has shown that "the ammonia contained in rain and snow-water always possessed an offensive smell of perspiration and putrid matters a fact which leaves no doubt respecting its origin' (p. 76). From the rain-water it is absorbed into the plants; and our author has shown that, previous to its undergoing those chemical metamorphoses which cause its assimilation, it may be detected in the juices of almost all plants.

Although, in the case of land not manured, all the ammonia is derived from the atmosphere, it is otherwise in those cases where animal manure is employed. One chief use of animal manure is to yield more ammonia than the air can furnish; and for this purpose, those kinds of manure are obviously the best which contain the largest proportion of ammonia or of nitrogen. Hence the high value of liquid manure compared with solid, the former being far richer in nitrogen than the latter :

'Agriculture differs essentially from the cultivation of forests, inasmuch as its principal object consists in the production of nitrogen in some form capable of assimilation by animals; while the object of forestculture is confined principally to the production of carbon.'—p. 85. Wheat, for example, is composed of two principles, starch and gluten; of which the latter alone contains nitrogen. Now an increased supply of nitrogen in the form of ammonia not only

increases

increases the number of seeds obtained from one plant, but also the proportion of gluten to starch, in other words the nutritive power, of those seeds. Thus 100 parts of wheat grown on land manured with cow-dung, a manure containing the smallest proportion of nitrogen, afforded only 11.97 parts of gluten; while the same quantity grown on a soil manured with human urine, which is very rich in nitrogen, yielded the largest proportion of gluten yet found, namely, 35.1 per cent.

Professor Liebig, after bringing forward numerous proofs that it is ammonia which yields to plants all their nitrogen, then proceeds to explain the principle on which gypsum, burnt clay, and ferruginous earths act in promoting fertility. All these substances possess the property of absorbing and fixing the ammonia, whether derived from the air or from manure. Many other substances have the same effect; such as powdered charcoal, diluted acids, &c., and some of these will no doubt be employed hereafter. It is easy to see why gypsum, for example, does not equally improve all soils. In some there is already a sufficient quantity either of gypsum, or of some analogous substance, to fix all the ammonia that they receive. If sterile, their sterility must depend on some other cause for no conclusion,' says the author, can have a better foundation than this, that it is the ammonia of the atmosphere (where manure is not used) that furnishes nitrogen to plants.'

6

We have already seen that the carbon is furnished by carbonic acid, while water yields the oxygen and hydrogen:

⚫ Carbonic acid, water, and ammonia contain the (organic) elements necessary for the support of animals and vegetables. The same substances are the ultimate products of the chemical processes of decay and putrefaction. All the innumerable products, therefore, of vitality resume, after death, the original form from which they sprung. And thus death-the complete dissolution of an existing generation-becomes the source of life for a new one.'-pp. 91, 92.

We earnestly recommend this section to our readers as being equally interesting with that on carbon, and argued with at least equal talent. To do it justice, we ought to have copied it entire. But we have shown, we trust, its importance; and we confidently anticipate from it practical applications of immense value to the agriculturist.

'But another question,' says our author, arises. Are the conditions already considered the only ones necessary to the life of vegetables? It will now be shown that they are not.'-p. 92.

This leads him to the consideration of the inorganic or mineral constituents of plants. And here we have another admirable specimen of the manner in which he handles an obscure and diffi

cult

cult subject. He first points out that all plants contain, although in small quantity, certain mineral substances, often different in different plants, but generally the same in the same species. Thus, for example, the stems and leaves of all the gramineæ invariably contain silicate of potash, while phosphate of magnesia and ammonia are found in their seeds. He then shows that those alkaline or earthy bases which are found in the ashes of plants in the form of carbonates, existed originally in the plants in the form of salts, that is, combined with vegetable acids which have been destroyed by the combustion. As certain of these vegetable acids are peculiar to certain species, and constantly occur in them, he concludes that they are essential to the developement of the species in which they occur; and as they occur in combination with alkaline bases, it is obvious that these bases also are essential to the plants.

In many cases-for example, in wheat-the acids as well as the bases are of mineral origin; and in others, such as opium and cinchona bark, the bases are organic, while the acids are partly mineral and partly vegetable. Further, it appears that one base or acid may, within certain limits, supply the place of another, without injury to the plant; while, in most cases, the absence of the proper mineral base or acid arrests entirely the developement of the plant. Thus, opium contains variable proportions of sulphuric and meconic acids; and when there is much of the latter there is always a deficiency of the former. In cinchona bark, quinine and lime are found; and the more lime is present, the less quinine does the bark contain. Again, pine-wood in one soil has been found to contain much lime, little potash, no magnesia; in a different soil, less lime, more potash, and a certain quantity of magnesia; but in both, the power of the bases taken together to neutralize acids was almost exactly equal. Nay, a third specimen, containing potash, soda, lime, and magnesia, was still found to have the same neutralizing power. These curious facts, all taken from the researches of the most accurate observers, but observed without special reference to this point, and consequently beyond all suspicion, lead to the conclusion that each vegetable requires a definite amount of mineral bases to combine with its proper acid or acids; and consequently that these bases have an important function to perform in the economy of the plant. In many cases this function can only be performed by one base and one acid. Thus, in wheat-straw silica is the acid and potash the base; and without these materials, happily present in most soils, wheat cannot thrive. It may be, indeed, that the silica and potash are not combined; the potash might be, and probably is, in part combined in wheat with an organic acid; but

the