ducers of organic compounds for the support of animal life. Taking another view, animals may be considered as performing a part as essential to vegetable life, that of disorganizers; what is excrementious from them being so reduced as to have the character rather of inorganic than of organic compounds, whether it be carbonic acid with which they contaminate the air in respiration, their gaseous excrement, or their liquid and consistent, derived from the other excreting organs and passages of the body. These matters which are destructive to animals, and not only to the animals that void them, but to animals generally, may be held to be the highest kind and most appropriate food of plants. And the more we reflect on this, the more we are convinced of its truth-the more we must admire the connection and mutual dependThe animal enriching the air for the use of the plant, the plant purifying the air for the use of the animal, and the same in regard to the soil, afford a lesson to man of a very instructive kind, most beneficial when carried practically into effect, most injurious when neglected; in the one instance insuring fertility, and I may add salubrity; in the other, the production of sterility and disease.

ence.

Let us now, for a moment, take a glance at the composition of plants and animals. Both may be considered as composed of nearly the same elements,-few in number, but variously united, so as to give rise to very many different compounds. The principal constituent elements of both are carbon, hydrogen, and azote, oxygen, lime, potash, silica, and phosphorus. Of these, carbon and silica preponderate in plants (silica, indeed, strictly is confined to plants); azote and phosphorus preponderate in animals. In plants, a large proportion of carbon and silica are expended in forming the woody fibre, the framework of the vegetable structure, and the epidermis, the resisting outer covering; whilst in animals, the azote and phosphorus are as largely expended in producing the organs of locomotion,-the muscles and bones. And in each instance we witness the usual happy economy of nature, and fitness of means to an end. Plants being fixed to the soil, take from it that which is almost always abundant in a fertile soil, silica, a substance, even in a thin

and delicate layer, imparting great power of resistance, and far less soluble when acted on by rain, than the less common, or at least less abundant, phosphate of lime. Animals, on the contrary, being able to range abroad in quest of food, select such as contain phosphate of lime and azote, and in such compounds as admit of digestion and assimilation, and conversion into bone and muscle, following, in so doing, their natural tastes, undoubtedly instinctively directed.

Leaving these general views, it may be well to consider the subject we have entered upon somewhat in its details.

Physiologists who have directed their attention especially to the food of animals, have arrived at the conclusion, that, amidst the extraordinary variety of articles capable of supporting animal life, there are three which may be considered as of most importance, and, as it were, elementary alimentary substances-substances which are found in milk, viz., an albuminous matter, the curd; an oily matter, the cream; a saccharine matter, the sugar-of-milk. It seems to be proved by a wide induction of facts, that articles containing these substances, or their analogues, such as starch for sugar, muscle for curd, any kind of fat for cream, are fit for the food of animals generally, and that no articles are fit that do not contain more or less of these. These substances, taken into the stomach, are converted into a pultaceous semifluid chyme, from whence a milk-like chyle is formed, and from whence blood, by which every part of the body is nourished, in its constant circulation.

The results of the inquiries of physiologists, as regards the food of vegetables, have not been so well defined and satisfactory. As the sap of plants is a fluid, and transparent, we are sure that complete solution is essential as a preliminary, and that nothing enters the spongioles of the roots, organic in its structure—a state of perfect solution being incompatible with such structure. The principal part of the sap is water; in it are dissolved carbonic acid, phosphate of lime, carbonate of lime, carbonate of potash, and in very many instances silica. And these inorganic substances, I apprehend, are to the plant for its food what the organic substances before mentioned are to the animal for the same purpose; and these are not less elementary than those, as

nutritive principles. The sap so impregnated passes from the rootlets by ascending vessels to the leaves, undergoing some change in its passage, but a greater change in the leaves, where carbonic acid gas is decomposed under the influence of light, oxygen evolved, and woody fibre either formed completely, or a substance formed about to become woody fibre, and to be deposited by the sap in its descent through another order of vessels. And as in the animal frame, very different compounds are secreted by different glands, so too, in the vegetable, a vast variety of compounds are produced by an analogous function of secretion; tubes and cells in the latter corresponding to glands in the former, the ultimate structure of which is also similar, the glands being congeries of tubes or cells.

Returning to the sap, it may be asked—and it is an important question-how are certain of the substances, which I have mentioned as essential to this nutritive fluid, dissolved in the water of the sap, such as phosphate of lime, carbonate of lime, silica,-themselves insoluble in water? My belief is, and it is founded upon experiments which I have made, that their solution is effected by the carbonic acid in the sap. It is well known how soluble carbonate of lime, and I may add carbonate of magnesia, is in water containing carbonic acid; it is quite certain that phosphate of lime is also soluble in the same, and that not in an inconsiderable degree; and the experiments which I have made on silica, to me are convincing that it likewise is soluble in water impregnated with carbonic acid, though in a degree very much less than phosphate of lime.

Taking this for granted, a certain simplicity is imparted to the nutritive process of plants. A fluid medium, water, holding a gaseous acid, carbonic acid, is the menstruum of the inorganic substances derived from the soil, which the plant requires for its healthy growth. This compound solution becomes exposed, in the leaves, to the action of light, and to the evaporating agency of the winds; the carbonic acid undergoes decomposition, as already mentioned, carbon being detained for the use of the plant, oxygen being exhaled; a portion of the water is removed by evaporation,

and, in consequence, the solvent power of the menstruum is diminished, and depositions of silica, and carbonate of lime, and other ingredients, take place. This view, it appears to me, is not only recommended by its simplicity, but also by a certain beauty and exactness of adjustment, and economy of means. Is it not very admirable that a gaseous acid, which, with water, is to yield to the plant, by decomposition, its organic elements, should be the solvent and vehicle of its inorganic parts?

Limited as I am in the delivery of this discourse by time, my main object has been to give general views, precise in themselves, and I believe correct, and involving principles capable of being carried out into practice, the test and confirmation of scientific truths.

On the practical part of the subject of Manures, it is not my intention to enter at any length. I have neither time for it, nor the experience to do it justice, or to treat of it in the manner I could wish. In the farther observations which I propose to make, I shall restrict myself to such remarks as I hope may be suggestive,―may excite curiosity and inquiry, and so have a chance of being useful; intelligent inquiry being, as I think, the one thing perhaps the most needful,without which agriculture can never speedily advance, and under which it cannot fail to advance, and from an art, which it is at present, and obscure in many of its parts, become a science, as certain as to results as is compatible with the uncertainty of the weather, and of other circumstances not under the controlling power of man.

What are the principal sources of manures, or of fertilizing means? I shall briefly speak of them under a few heads. 1st, Atmospheric, chiefly in the form of rain; 2dly, Of animal matter; 3dly, Of vegetable matter; and, lastly, Of mineral.

1st, Of the fertilizing means derived from the atmosphere. The atmosphere, as you are aware, is a mixture of two gases, azote and oxygen, in certain, almost constant proportions; and of carbonic acid and aqueous vapour, the former in small quantity, little variable,—the latter in a variable quantity, and extremely variable according to circumstances of

temperature and other influences on which its increase and diminution depend. And, besides these ingredients, there is reason to infer, that various other substances are either suspended, floating as minute particles or dissolved in the air, such as carbonate of ammonia, common salt, and some other salts derived from the sea, and dust of several kinds. The most important of all these atmospheric ingredients are, undoubtedly, the oxygen, azote, carbonic acid, aqueous vapour, and ammoniacal salt. I should exceed my limits as to time, were I to enter into particulars on the fertilizing agency of either one of these substances. Aqueous vapour condensed and precipitated, you know, occasions rain,-is, in fact, rain. When it falls, it brings down with it carbonic acid, some oxygen and azote, and, there is reason to believe, a minute portion of carbonate of ammonia-all dissolved in it. Moistening dead vegetable and animal matter at the surface of the earth, it favours their decomposition, and the evolution of carbonio acid and of carbonate of ammonia; penetrating beneath the surface, descending into the soil, it has a like effect there; and there becoming impregnated by its solvent power with what is found in a fertile soil, the substances already alluded to, as phosphate of lime, carbonate of lime and magnesia, carbonate of potash, and silica, it passes into the growing plant, absorbed by its roots, and becomes its nutritive sap. Thus complicated is rain in its agency; so various are the circumstances which concur to this agency, and so happily are they connected, one favouring the action of the other, and all promoting the process of vegetation. Reflecting on these circumstances, we cease to wonder at the growth of forests in a state of nature, in which for a long series of years, vegetable matter, living or dead, is constantly accumulating, deriving its elements solely from the atmosphere and the soil, and by what it abstracts from the former greatly enriching the latter.

2dly. Of the fertilizing means derived from animal matter I have already alluded to the composition of animal matter, and how, as regards its ultimate elements, excepting that it contains no silica, it differs, compared with those of vegetables, rather in proportion than kind. Readily putrefying