505. The heat of the bird is increased somewhat when it is prepared for incubation. Some eggs were taken from under a sitting hen whose temperature was 104°, at the time when the chick was about three-parts formed. A hole was broken in the shell and the bulb of a thermometer introduced; the quicksilver rose to 99½°; but in some eggs that were addled it was proved that their heat was not so high by two degrees, so that the life of the living egg assisted to support its own temperature. 506. These facts sufficiently show the dependence of the faculty of generating heat and of producing cold on the powers of life. But the processes by which, under the agency and control of the vital powers, these different results are effected, are various, and even opposite. 507. The power of generating heat is connected in the closest manner with the function of respiration, and is directly dependent upon it. The evidence of this is indubitable. For— 508. i. Respiration is combustion, and, like ordinary combustion, is attended with the production of heat. In ordinary combustion oxygen disappears, and a new compound is formed, consisting of oxygen combined with the combustible matter; that is, an oxidized body is generated. On burning a piece of iron wire in oxygen, the oxygen disappears, and the iron increases in weight. The oxygen combines with the iron, forming a new product, oxide of iron, and the weight of this new substance is found on examination to be exactly equal to the weight of the wire originally employed, added to the quantity of oxygen which has disappeared. 509. It is precisely the same in respiration. In this process oxygen combines with combustible matter, carbon: the oxygen disappears, and a new body, carbonic acid, is generated. 510. ii. One phenomenon which invariably accompanies the combination of oxygen with combustible matter is the extrication of heat. Whenever a substance passes from a rarer into a denser state; when, for example, a gas is converted into a liquid or solid, or when a liquid solidifies, heat is evolved; because, according to the ordinary theory of combustion, the denser substance has a less capacity for caloric than the rarer, and consequently in passing from a rare into a dense state, a quantity of caloric previously combined or latent within it is set free. The combined or latent caloric contained in a body is termed its specific caloric; the caloric which is evolved on its change of state is named free or sensible caloric.