Of the many persons who, for economical or hygienic reasons, have tried to adjust their diet better, some have undertaken the task without a fundamental knowledge of the physiological and caloric value of foods, their composition, or the nutritional needs of the body, and have done themselves more harm than good. It is possible for us to measure the value of our foods, and to express it in terms of heat units; and with a knowledge of the bodily needs, we may supply ourselves with foods in approximately the amounts needed, and in the best combinations. Food oxidized in the body produces the same amount of heat as that burned outside the body, and the instrument by which the heat value of any substance is determined is called a calorimeter. The unit of measure of heat is called the calorie or heat unit. The calorimeter consists of a double chamber, the outer one containing a given quantity of water. The inner chamber is thus surrounded by a water jacket. In it is placed a definite amount of pure, water-free food to be tested; for example, an ounce of sugar. By means of an electric connection, the sugar is ignited and burned, and the heat produced thereby is imparted to the water in the outer chamber. When the process is complete, the difference in the temperature of the water is noted, and the amount of heat generated is computed. The calorie is the amount of heat necessary to raise the temperature of one pound of water four degrees F., or one kilogram one degree C. In this way, the heat values of pure protein, fat, starch, and sugar have been determined. In the laboratories of the United States government, the composition and caloric value of practically every food substance known has been worked out. Any person can have access to these tables of food values by applying to the government, or by purchasing from almost any bookstore any one of the several books on food values, that are on the market. (See pages 23-27 of this book.)