The assimilation of carbon first claims our attention. The composition of the atmos pheric air, from whatever part of the earth's surface it is taken, is, as is well known, constant, in so far as the relative volumes of oxygen and nitrogen are concerned; while the variations in the carbonic acid, except when there are obvious causes for an excess (as, for example, an over-crowded room), are very slight, and, as a general rule, deviate scarcely at all from 4 volumes in 10,000 of air. Yet causes disturbing this uniformity are perpetually at work, Prof. Mulder, adopting Lavoisier's and Davy's experiments, according to which a man consumes about 20 cubic ft. of oxygen in 24 hours (and later observers place the daily quantity at 45 cubic ft.), calculated his yearly consumption at more than 9,500 feet. Considering the enormous numbers of men and animals on the surface of the globe, and the lamps, fires, furnaces, etc., ever burning, the atmosphere would apparently soon cease to be fit for the support of life (1) in consequence of the great diminution of oxygen, a gas essential to life, and (2) in consequence of the great excess of carbonic acid, a gas deleterious to life. The cause of this marvelous uniform ity of atmospheric air under these circumstances is that function of plants by which they absorb their carbon. It is to the experiments and observations of Priestley in 1771, Ingenhousz in 1776, Senebier in 1807, and many later observers, that we are indebted for the knowledge of the great general fact, that plants take up the carbonic acid from the air, reduce it in their organism, and retain the carbon for the composition of their own organisms, while they restore the oxygen gas to the atmosphere. It is chiefly by the leaves, which may be regarded as the respiratory organs, that this process is carried on. It is needless here to notice the questions as to whether it is only during light, or constantly, that these changes go on; whether different rays of the spectrum act with more or less power in liberating the oxygen, etc. Independently of the proof afforded, for example, by placing green plants in a mixture of 70 parts of common air and 30 parts of carbonic acid, and finding that, in the course of four hours, the carbonic acid has been almost entirely replaced by oxygen, we have obvious evidence in the case of lichens growing on a naked rock, that the carbon which they contain must be obtained from the atmosphere. In the case of aquatic plants, the process is identical, the atmos pheric air being dissolved in the water. Carbon, in some form of combination or other, probably forms about two-thirds of the weight of a dried plant. the assimilation of hydrogen from the decomposition of water in and by the plant, is not capable of the .same direct proof as that of carbon; but there are strong grounds for believing in its occurrence. This view is supported by the composition of wax, the resins, volatile oils, etc., and indeed it is difficult to see from what other source the hydrogen could be de rived. In that case, the water, like the carbonic acid, contributes its oxygen to the air_ The nitrogen, which enters into certain constituents of plants, is derived from ammonia, and not, as might have been supposed, directly from the air, of the volume• of which it forms about four-fifths. It has been shown by the direct observations of Boussingault, that plants cannot assimilate nitrogen, that those which have been made to absorb it by placing their roots in nitrogenous water, throw it off unchanged, and that vegetation cannot exist in a soil which contains no substances readily convertible into ammonia. The indifference of nitrogen to other elements, and the extreme readiness with which ammonia becomes decomposed, and enters into different combinations (the amides, imides, amido-acids, compound ammonias, and probably also the albmninates, be ing derivatives of it), together with the conclusion we draw from the action of liquids containing ammonia, or matter convertible into it, as gas•liquid, fluid sew• age, etc., confirm this view. The ammonia taken up by plants is obtained partly from the air, and partly from the soil. In the air it is formed after thunder storms, and it Is further supplied to the atmosphere by putrefactive processes, ani mal excretions, and volcanic action. -It is indeed found in snow and in all rain-water, and is thus conveyed to the soil. Although direct experiments show that the air con tained in the pores of the soil is richer in ammonia than ordinary atmospheric air, it is easy to show that a plant can derive its ammonia from the latter alone, by a reference to the vegetation on naked rocks, or by growing plants in powdered charcoal duly moist ened with rain-water. Hence both air and soil contribute the ammonia from which the
nitrogen is fixed in the plant. The oxygen which occurs in the various constituents of the plant is derived from the decomposed carbonic acid and water, and corresponds to the difference between the amount contained in those absorbed compounds and the amount liberated to the atmosphere. The sulphur that occurs in the albnminates and certain ethereal oils must be derived from the soil, since it does not occur either free or in combination in the air; and as the only form in which it is found in common soil is as sulphates, plants must have the property of decomposing these salts, and appropriating their sulphur after reduction. Extensive experience has proved that certain inorgunte constttuents are as indispensable to the life and development of the plant as the organic elements we have been considering; and further, that special plants require special mor genic constituents, as is shown in works on agricultural chemistry. The two following facts seem well established: (1) that the roots of plants exert a special selective power, and absorb sonic salts, and reject others that are also in solution in the water of the soil; and (2) that the top or vegetable soil has the pqwer of absorbing and retaining the most necessary mineral ingredients, and does not allow them to be carried deep into the ground by the rain; but for the discussion of this subject we can only refer to the experimental researches of Liebig, Mulder, Huxtable, 'Way, etc. It would be altogether out of place to enter into the consideration of the prodigious synthetic and analytic power of the vegetable cells; we shall merely indicate bow some of the most important vegetable compounds are probably formed; beginning with the vegetable acids "Even," says prof. Gregory, "when carbonic acid and water are brought together in the cell, this is not enough. There must be present, first, albuminous matter, without which no active cell can exist; secondly, mineral matter, especially alkalies, phosphates, and salts. All these conditions being fulfilled, and light being admitted, we may suppose the first organic acid formed to be oxalic acid, the least complex of all.' —Organic Chemistry, 4th ed. p. 541. Putting it in the briefest terms, the cell separates 2 equiv alents of oxygen from 2 equivalents of carbonic acid, and yields 1 equivalent of anby &row oxalic acid, or 0408 — 02 = Two equivalents of dry oxalic acid, C8114018, by taking up 2 equivalents of water, and losing 8 equivalents of oxygen, yield hydrated malic acid, Tartaric and citric acids are capable of a similar production. Thus, 2 equivalents of dry.oxalic acid, combining with 2 of water, and losing 6 of oxygen, yield hydrated tartaric acid or Ca140,0,21:10); and similarly, 3 equivalents of oxalic acid, combining with 2 of water, and losing 12 of oxygen, yield hydrated citric acid (012118014, or 012118011,8110). In like manner every vegetable acid, and every one of the neutral compounds of carbon, hydrogen, and oxygen, may be derived from some less complex compound, containing more hydrogen than itself; or it may be supposed to be derived directly from carbonic acid and water, oxygen being, on every supposition, given off. "As the proportion of oxygen to carbon diminishes, the acids become weaker, till the oxygen exactly suffices to form water with the hydrogen, when we have either very feeble acids, or neutral bodies, such as sugar, gum, and starch. As the oxygen is still further diminished, we have neutral, bitter, and acrid compounds, or colored bodies, or such as yield coloring matters, with ammonia and oxygen; further on still we have aromatic oils, and volatile, quasi-resinous, crystallizable acids; then resins; and lastly, when all the oxygen is expelled, certain oils, which are carbo•hydrogens."— Gregory, op. cit., p. 543. AlthOugh each individual substance is doubtless in reality derived from some substance only a little less complicated than itself, the final result, in so far as showing their mode of construction is concerned, is the same as if they were all obtained directly from carbonic acid and water. The following tabular view (compiled by Gregory) will serve to show how all the leading groups of vegetable compounds are produced from carbonic acid and water by deoxidation: