that agency is introduces us to the second branch of Priestley's discovery.
He had put some mice in a glass containing atmospheric air, closely stopped, and found, as usual, that they died of suffocation as soon as the air became sufficiently impure by their breathing; an absolutely poisonous quality being gradually assumed. But, if a few vegetable leaves, or a small plant, were placed in the glass, and exposed to the sun, in a very short time the poisonous quality disappeared, and the power of supporting animal life was regained. Here, then, was an unexpected result—a discovery that gave a solution to all the difficulty, and which has been verified in its minutest details by more modern experiments. It has revealed the great and interesting fact that plants and animals stand in a relation of antagonism to one another; that whatever changes the one tends to impress on the air, the other undoes; and that, while animals discharge their duty in consequence of their being living and moving things, plants perform theirs under the influence of the light of the sun; for these changes do not go on in the dark.
Let us look at these facts by the aid of modern chemistry, premising that oxygen is an invisible substance, existing in the air, and that carbonic acid arises from its union with carbon. When carbon burns, it is merely uniting with atmospheric oxygen, and the resulting carbonic acid escapes away under an invisible form. So, too, when a man breathes, he draws in oxygen from the air; it is distributed to all parts of his system, and, combining therein with carbon, turns into carbonic acid, which is expelled when he throws out his breath. Every animal, therefore, to use the language of chemistry, is an oxidizing machine, the physical end of its existence being to rob the air of oxygen, and put back, in its stead, carbonic-acid gas.
With plants it is just the reverse. As long as the sun is shining upon them, they take carbonic acid from the air, and, decomposing it by their leaves, they set free its oxygen, which escapes away; its carbon they appropriate. With it they form their various parts, their stems, roots, flowers, seeds; but they do this only so long as the sun shines, and when night or winter comes the process stops.
The animal, therefore, takes from the air oxygen, and turns it into carbonic acid; the plant takes that carbonic acid, and turns it back into oxygen, which has thus discharged the great office of carrying carbon from the bodies of animals, and transferring it to the systems of plants. In what an interesting relation do the two kingdoms, the animal and the vegetable, thus stand to one another, not alone as respects the air in maintaining its constitution uniform by a mutual antagonization, but also as respects their own structures! The elements of which plants are formed have all been derived from the preëxisting parts of animals; and the elements of which animals consist, from the preëxisting parts of plants. To the classical scholar, what a
