But now the reader may ask another question: If, as you say, the body in many ways behaves like a furnace, with food serving as fuel, from which heat is produced, why cannot we find out the amount of food or fuel the body uses by measuring the amount of heat it evolves? Why not do to man what we did to a piece of coal or to a portion of food?

But you will at once raise the objection that such an experiment would involve the sacrifice of a human being, for you would have to "burn" him up; a phase of the experiment which would concern the subject of it even more than the experimenter. On second reflection, however, you will notice that your objection does not really hold. For in man - in all "things that have the breath of life" - the "furnace" is inside of him. In reality, there are millions and millions of these "furnaces" represented by the millions and millions of cells. The food after careful preparation by the digestive system reaches these cells, is there joined by oxygen from the lungs, and then "oxidized." No light need be applied either to the cells themselves or to the body as a whole.

Just how the cells do their "oxidation" work is a mystery which has not yet been solved, though physiologists have a workable hypothesis to explain it. What the cells do we have not been able to repeat in a test tube; which is another way of saying that the mystery of man is man.

Let us return. We take our man and put him into a chamber so constructed as to enable us to measure the heat he produces in the course of a day.* We do not have to do to man what we did to coal - apply a light; his cells have their own way of "burning" material without any help from us. We find that this man weighing 160 pounds evolves 2200 calories in twenty-four hours. He receives "square meals" during the day. What deduction may we draw from such an experiment? Obviously enough that the man must have "burnt up" food to the extent of 2200 calories, and that therefore food yielding 2200 calories must be supplied to him every day so that he may have the necessary energy; so that he may continue to live.

* Those interested in the actual details may consult Sherman: Chemistry of Food and Nutrition, p. 158 (Macmillan Co., 1918).

By repeating such an experiment with hundreds of different men, of all sizes and all ages, we can arrive at an "average" figure.