distinctly announced in the last volume of his Memoirs, is nothing more than a slow combustion of carbon and hydrogen, resembling that which takes place in a lamp; the atmosphere, as in all other cases, furnishing the oxygen, while the body and blood of the animal, which burns and consumes, supply the combustible matter; and the heat thus evolved maintains his elevated temperature. Hence, did not animals habitually replenish, by their food, the loss they constantly sustain in respiration, they would shortly die, as a lamp goes out when its oil is consumed. The torch of life is lighted when the child begins to breathe, is kept burning by the constant supply of aliment, and is finally extinguished in death. The obvious objection, that the lungs the furnace where this combustion was supposed to be going on should be warmer than the remote parts of the body, which is not the case, was long ago disposed of by the modified view of Lagrange, Vogel, Home, and Brande, and by the more recent confirmatory experiments of Bischoff and Magnus; which go to show, that the combustion takes place, not in the lungs, but in every part of the body to which the oxygen that supports it, absorbed in the lungs, is conveyed by the arterial circulation. Even the office of the red particles of the blood, as carriers of oxygen, of late so strikingly set forth by Liebig, was long since suggested by Sir Humphrey Davy.

We owe to Senebier the discovery of the opposite action of vegetables upon the air; namely, the decomposition of carbonic acid in the green parts of plants under the agency of the solar rays, and the evolution of its oxygen. But to Adolphe Brongniart belongs the credit of having, nearly twenty years ago, assigned the carbonic acid of the atmosphere as the source of all the carbon of plants, and consequently of animals. He developed this idea in the form of some very interesting geological speculations respecting the constitution of the atmosphere, and the state of organized existence in the earliest times. If all the carbon of existing plants and animals and their remains, as well as that which, gathered by vast primeval forests, is now buried in the bosom of the earth in the form of coal, — and, we may add, that in coral and in all the lime-rock to which marine animals have given rise, if this immense quantity of carbon, previous to the creation of organized beings, was diffused through the air in the form

of carbonic acid, it would follow, that our atmosphere, which now contains less than a thousandth part in volume of this gas, must then have contained a much larger proportion; a proportion so large, probably, as to inhibit the existence of warm-blooded terrestrial animals. And accordingly, at the epoch of the coal formations, when vegetation must have had an extraordinary luxuriance, not a single terrestrial airbreathing animal appears to have existed. But when this ancient vegetation had withdrawn a large amount of carbonic acid from the air, and replaced it with an equal bulk of oxygen, a variety of air-breathing reptiles of monstrous size and form made their appearance; animals which, from the nature of their respiration, are capable of living in much less pure air than is requisite for those of higher organization, of which they were the heralds and precursors. For it was

not until after these vast stores of carbon had been abstracted from the air, and the retiring ocean and other causes allowed a larger portion of the earth's surface to be clothed with verdure, that a number of mammiferous animals, analogous to those now existing in the world, seem to have appeared upon its surface. At present, it is not probable that vegetation, in its ensemble, is accumulating carbon upon the earth's surface, and consequently increasing the proportion of oxygen in the air, to any considerable extent. The organic and inorganic forces must be nearly in equilibrium. By so much as vegetation tends to increase the proportion of oxygen, it also tends to increase both the energy of respiration in animals and the rapidity of the decomposition (oxygenation) of vegetable and animal remains, and the consequent restoration of the carbonic acid to the atmosphere, whence it was drawn. The agency of vegetation, having brought the air to that degree of purity which comports with the active respiration of warm-blooded animals, while, at the same time, it affords sufficient pabulum for plants, the opposing influences of these two orders of beings conspire to maintain the air in a state of stability.

May we not carry our speculations into the former constitution of the atmosphere a step further back? May we not contemplate the physiological agency of vegetation, so early and largely introduced, as repairing the effects of an earlier physical operation, which must have greatly altered, in an opposite mode, the original constitution of the air?

As the earth's crust cooled down from the state of incandescence upon which geologists love to speculate, incalculable quantities of free oxygen must have been withdrawn from the air, and imprisoned in the oxydation of its mineral elements, with a force which no subsequent natural agency, at any rate upon a large scale, has been able to loosen. The vegetation which supervened, stimulated to high luxuriance by the fine bottom heat and reeking atmosphere of those times, began to repair this loss, by decomposing the carbonic acid of the air, and thus increasing the relative amount of its oxygen. Thus the beds of iron ore and of coal, which accompany and overlie each other upon the earth's rind, have withdrawn from the original atmosphere, the one a portion of its oxygen, the other a portion of its carbonic acid. When we would obtain the former in its primitive and useful state, we mingle the ore with the coal, and drive off the oxygen, surcharged with carbon, again into the air from which both originally came, by the aid of the heat which the vegetables that gathered the carbon had absorbed from the sun's rays. This idea, that plants absorb light and heat-a true force from the sun, as well as that they constitute a deoxydizing apparatus, reducing the oxydized products of animal life, is claimed by Dumas and Boussingault. The latter promulgated the opinion, that plants absorb light or caloric, in the year 1837. And Dumas sets forth the following programme, as exhibiting the doctrines of his discourse.

Produces heat,

electricity.

Absorbs heat.

Abstracts electricity.

Restores its elements to the air Derives its elements from the

air or the earth.

or to the earth. Transforms organized into min- Transforms mineral into or eral matters. ganized matters.

That plants decompose water as well as carbonic acid, at least in certain operations, and liberate its oxygen, has recently been demonstrated by Boussingault. That vegetable tissue has the same identical composition as starch and other ternary organic products was first clearly proved by Payen. The mode in which these products are consumed and restored to the air by animals was popularly taught from about the same period (as early as 1839) by Liebig and the French chemists. The discovery of the part which ammonia plays in vegetation, as one principal source of the plant's azotized food, (for Liebig's statement, that it is the sole source of the nitrogen of plants, was too hasty,) is claimed for M. Schattenmann. The essential identity of vegetable albumen with animal albumen, and consequently the general identity of the quaternary azotized neutral products of plants and animals, was first shown by Mulder; who deduced the important consequence, "that the great mass of animal substances is supplied by the vegetable kingdom." The discovery of fibrin, as such, the very material of the flesh of animals, in the juices and fruits of certain esculent plants, was long ago made by Vauquelin; its presence in wheatflour has recently been ascertained by Dumas.

That the fat of herbivorous animals is directly drawn, like their other constituents, ready-formed, from their vegetable food, has been strenuously taught by Dumas, Boussingault, and Payen, and is amply confirmed, as to the general fact, by a variety of experiments and direct observations, which our limits forbid us to notice. Professor Liebig maintains, on the contrary, as our readers are well aware, that the fat of animals is produced by a transformation of the starch, sugar, &c., of their food. During the last year or two, a very piquant discussion upon this subject has been carried on between the Giessen and the Parisian chemists; which has ended in the confirmation of an old observation of Hubert, that bees have the power of producing wax when fed upon sugar alone; whence it follows, that ordinary herbivorous

animals may produce fat from similar materials. But on the other hand, the French chemists have pretty clearly made out, that the fat of animals ordinarily comes directly from the oil and wax of their herbivorous food.

Another interesting question remains, which we have reserved to the last, because it is one which has particularly occupied the attention of Professor Draper, and forms the gist of his elaborate volume "on the forces which produce the organization of plants." The sunbeams contain light, and heat, and chemical, or, as Dr. Draper would say, tithonic rays. Which of these are the efficient agents in vegetable digestion? Which, in other words, enable the green foliage to reduce carbonic acid, and create organic matter? The old experiments of Senebier, the discoverer of this agency of light in vegetable digestion, as well as some recent researches of Mr. Hunt in England, seemed to show, that the violet and blue rays are most efficient; from which it was inferred, as, indeed, would a priori be supposed, that it is the chemical rays of the sunbeam which exert this wonderful power. This has ever been the prevailing opinion, and is taken for granted by Dumas. Observing that when he attempted to depict foliage by the daguerrotype, the green parts were not represented, Dumas remarks, that "it is as if the whole of the chemical rays essential to the photographic phenomenon had disappeared, had been absorbed and retained by the leaf. It would seem, therefore, that the chemical rays of light vanish entirely in the green parts of the plants; an extraordinary absorption, but easily explicable, when the enormous expenditure of chemical force necessary to decompose so stable a substance as carbonic acid is considered."

On the other hand, the experiments instituted some years ago by Professor Morren in Belgium, and Professor Daubeny in England, led them to the conclusion, that the yellow or most luminous rays, or, in other words, that light, strictly so called, was the efficient agent. Their experiments were made by exposing plants to sunlight transmitted through colored glass; a mode liable to obvious objections. To Dr. Draper belongs the credit of having devised and practised a much better mode of performing the experiment, namely, by exposing foliage in the solar spectrum itself to the influence of the different colors, and measuring the activ