We have seen how Lavoisier had shown that food in the body undergoes much the same change that coal does when it is burnt; in both cases there is a union with oxygen, with the ultimate production of carbon dioxide and water, and the liberation of heat. The heat formed in the body as a result of the "oxidation" of foods, supplies our energy requirements.** If this energy is so intimately related to heat production, and if heat is measured in calories, why cannot we measure foods in terms of energy-content, by measuring the number of calories that a given quantity of food will yield when burnt? A perfectly natural question.

In actual practice the calorimeter consists of a steel bomb, often lined with copper and gold-plated, and a tightly fitting cover with screw collar attachment. A weighed sample to be tested is placed in a capsule within the bomb. The latter is now charged with oxygen under pressure, closed, and immersed in a weighed amount of water. The sample is ignited by means of an electric fuse. The water is constantly stirred and the temperature taken at short intervals by means of a carefully calibrated thermometer, usually reading to one-thousandth of one degree.

** Of course, food has other important functions, not the least of which is to build up or replace cellular tissue, but for our immediate purposes in this chapter these need not be considered.

Just as we can burn coal and determine the calories liberated, so by suitable means, we can burn any one of the many varieties of food and estimate the calories it produces. In this way we arrive at the conclusion that one food is richer than another because it liberates more calories when burnt; because, in other words, it yields more energy.

To illustrate with an example: The heat value in calories of one pound of corned beef is about 1200; that of one pound of tomatoes, 100. According to our experiment one pound of beef yields twelve times as much energy as one pound of tomatoes; or, one pound of beef yields as much energy as twelve pounds of tomatoes.