comes to an end. This gas is a necessary part of the animal dietary. It supplies the tinder which kindles life's fuel into a vital blaze, and in other ways it assists not only the building-up but the physiological "breakdown" of the animal frame. Part of this "breakdown" or natural waste accompanying all work, like the inevitable shadow, consists of carbonic acid gas. This latter compound is made up of so much carbon and so much oxygen. It arises from the union of these two elements within the body, and is a result of the production of heat, representing, in this way, part of the ashes of the bodily fire. Viewed as an excretion, as a something to be got rid of, and as a deadly enough element in the animal domain, this carbonic acid is a thorough enemy of animal life. It is not only useless in, but hurtful to, the animal processes. Ventilation is intended as a practical warfare against the carbonic acid we have exhaled from lungs and skin; and "the breath, rebreathed," is known to be a source of danger and disease to the animal populations of our globe. Here, however, the system of natural economics appears to step in and to solve in an adequate fashion this question of carbonic acid and its uses. Just as the chemist elaborates his coal-tar colors from the refuse and formerly despised waste products of the gas-works, so Dame Nature contrives a use for the waste carbonic acid of the animal world. She introduces the green plants on the scene as her helpmates and allies in the economical work. Every green leaf we see is essentially a devourer of carbonic acid gas from the atmosphere. That which the animal gives out, the green plant takes in. Not so your mushrooms and other grovelers of the vegetable kingdom, which, having no green about them, refuse to accept the cast-off products of the animal series, and despise the carbonic acid as a poor but proud relation discards the gift of our old garments. The green plant is the recipient of the animal waste. The leaves drink in the carbonic acid which has been exhaled into the atmosphere by the tribes of animals. They receive it into their microscopic cells, each of which, with its living protoplasm and its chlorophyl or green granules, is really a little chemical laboratory devoted to the utilization of waste products. Therein, the carbonic-acid gas is received; therein, it is dexterously up, "decomposed," as chemists would have it, into its original elements, carbon and oxygen; and therein is the carbon retained as part of the food of the plant, while the oxygen, liberated from its carbon bonds, is allowed to escape back into the atmosphere, to become once again useful for the purposes of animal life.
There would thus appear to be a continual interchange taking place between the animal and plant worlds—a perpetual utilization by the latter of the waste products of the former. It is immaterial to this main point in natural economics that the reception of carbonic acid by green plants can only proceed in the presence of light. It is equally immaterial that by night these green plants become like ani-
