This gas is formed by the mixture of oxygen with carbon (charcoal). It is absorbed very readily by water, and is perhaps best known in the form of soda water; the aërated liquid sold under that name being nothing more than water strongly impregnated with carbonic acid gas. It is formed by a variety of processes--by breathing, by combustion, by fermentation, and otherwise. In every case, however, its formation is attended with heat. And now, having thus briefly introduced the subject, we may mention, that on this fact is founded the theory which attempts to explain the means by which the animal temperature is produced and maintained. It is founded on the fact, that whenever oxygen enters into combination with carbon, and forms carbonic acid gas, heat is always produced. The most usual manner in which this is effected is by combustion; the substance which burns, such as wood, or tallow, or coal gas, for example, consists principally of carbon, and on being ignited, the oxygen of the atmosphere is made to combine with it, and carbonic acid is the result. Every body knows that heat is produced by this process; but there are many instances in which the same effect may take place without being so readily understood. Heat and light are so constantly found united, that we can hardly conceive how so large a substance as the human body can be kept constantly warm without the aid of fire. It is, however, effected by a chemical process identically the same as combustion, except that light is not produced. The lungs may be regarded as the furnace of the body, from which it derives its supply of heat; the fuel is the carbon in the blood; and the wind-pipe is a chimney serving a double purpose: first, to allow of the passage of fresh air for the process, and then to convey away the vapour which is produced by it: for the breath which issues from our lungs is just as much deteriorated in quality as that which escapes from the chimney of a large furnace after passing through the fire. This, then, is the process by which the animal heat is maintained. The blood comes to the lungs loaded with carbon; the air descends the wind-pipe, consisting of one-third oxygen; the carbon of the blood and the oxygen of the air unite; the blood is purified, and carbonic acid gas is produced. This is attended with heat; the purified blood is capable of absorbing all this heat, and does so. In its progress through the body, as the blood again becomes impure, it gradually parts with the heat so acquired, and on again being purified, it receives a fresh supply. Nothing can be more simple and beautiful than this process; it is in accordance with every great operation in nature, which is always effected in the most direct and simple manner; and the proofs that this is the manner in which nature effects her object in this instance, are numerous and unanswerable.