Page:Popular Science Monthly Volume 3.djvu/458

444 the body may waste for a lengthened period and yet live, it rapidly dies when the source of heat is removed or even greatly lessened.

The production of heat in the body, so wonderful in the process and amount, results only from the chemical combination of the elements of food, whether on the minute scale of the atoms of the several tissues, or on the larger one connected with respiration, and is thence called the combustion of food. As familiar illustrations of the production of heat from chemical change, we may mention that, when cold oil of vitriol and cold water are added together, the mixture becomes so hot that the hand cannot bear it, and the heating of hay-stacks, and also of barley in the process of malting, is well known. This action in the body is not restricted to changes in one element alone, but proceeds with all; yet it is chiefly due to a combination of three elements, viz., oxygen, hydrogen, and carbon, and requires for its support fat, starch, or sugar, or other digestible food composed of those substances, precisely as coal and wood supply fuel for lire without the body.

This effect is made extremely striking, by Prof. Frankland, in the following table, which shows the amount of heat generated from so small a quantity as ten grains of certain foods during their complete combustion within the body, and the force which scientific calculations have shown to be equivalent to that amount of heat. The original quantity used by Prof. Frankland has been reduced by Dr. Letheby to ten grains, for the convenience of English readers:

Thus we prove that an ounce of fresh lean meat, if entirely burned in the body, would produce heat sufficient to raise about 70 lbs. of water 1° Fahr., or a gallon of water about 7° Fahr. In like manner, one ounce of fresh butter would produce ten times that amount of heat; but it must be added that, as the combustion which is effected within the body is not always complete, the actual effect is less than that now indicated.

It may thus be shown that the division of foods into the two great classes of flesh-formers and heat-generators is not to be taken too incisively, for while a food is renewing flesh it also produces heat, and while the heat-generating food is acting it may also produce a part of flesh in the form of fat; but, although they are so closely associated