Page:Popular Science Monthly Volume 2.djvu/420

404 the chest heaves, is composed of very various substances. It contains mineral salts, such as chlorures, sulphates, phosphates of potassium, soda, lime, magnesia, coloring-matters, fatty particles, neutral substances of the nature of starch, and nitrogenized products, such as albumen and fibrin. The salts undergo slight changes in the torrent of circulation; they are eliminated by the chief emunctories. The neutral matters of the nature of starch are converted into glycogene and fat. The fatty particles undergo in the blood only such oxidizations as produce certain derivatives of the same order. And, last, the nitrogenized products are made over into fibrin, musculin, ossëin, pepsin, pancreatin, compounds all differing very slightly. It is the first portion of the chemical process which is effected in the principal fluid of the body. All these materials, elaborated at different points of the circulating current, and designed to be assimilated, are destroyed in the very organs in which they had been fixed. The glycogene is transformed into sugar, which is burned, yielding water and carbonic acid; the fatty acids are partly eliminated by the skin, and partly burned. As to the plastic matters which form the web of the tissues, we know little about the chemical relation which connects these with their products of destruction—urea, creatine, cholesterine, uric acid, and xanthine. Such is a rapid sketch of the principal chemical phenomena which, taking place throughout the entire system, kindle everywhere an evolution of more or less intense heat. There is no central organ, then, for feeding the vital fire—every anatomical element performs its share; and, if a nearly uniform temperature exists throughout the body, it is because the blood diffuses heat regularly into the various parts it bathes.

Now, how can the amount of heat to which these reactions may give rise be ascertained? Lavoisier arrived at it in a very simple manner. After comparing the oxygen absorbed by the animal with the carbonic acid and watery vapor thrown off, he deduced the weight of the carbon and hydrogen burned, by assuming that the formation of carbonic acid and of water produces in the system the same amount of heat that it would produce if taking place by means of free carbon and hydrogen. This is very nearly the result he obtained: A man weighing 132 pounds burns in 24 hours, at the average temperature of Paris, very nearly 11 ounces of carbon, and $11/14$ of an ounce of hydrogen, and thus develops 3,297 heat units. During the same period he loses through his lungs and skin 2 $3/4$ pounds of watery vapor, which take from him 697 heat-units. There remain, then, nearly 2,600 heat-units to account for. Other analogous estimates have been made, and physiologists have deduced from them the conclusion that a man of average weight produces in our climate 3,250 heat-units every day; that is to say, a sufficient amount of heat to raise seven gallons of water to the boiling-point. These figures, though approximations, give a sufficiently clear notion of the power of the animal economy to generate heat.