_Heat Value and Digestibility of Foods._—One of the chief functions of food is to supply the body with heat and energy; the food must be capable of digestion and absorption. Herein lies the exact value of any food to the consumer. In science the figures that are given for the digestibility of the various foods refer to the completeness or extent to which the food is dissolved and transferred to the circulation, and an indigestible one is that of which a considerable portion passes out of the system into the feces without being disintegrated and absorbed. _Animal food_ is more completely digested than vegetable food, as shown by the difference of nitrogen in the feces. In meats 97 per cent. of the protein and 98 per cent. of the fat are absorbed. Lean meat is more rapidly digested than fat, and the flesh of young animals than that of older ones. The breast of chicken, fresh beef, and mutton are among the most digestible of the solid foods. Raw and rare meats are more easily digested than well-done meats; in other words, cooking lessens the digestibility of meats. Steak should be broiled and never fried; all fried foods are difficult to digest. Veal and pork are both difficult to digest. _Eggs_ are almost as nutritious as meat; their digestibility is unsurpassed and only equalled by a few foods, such as milk and oysters. They are most easily digested when soft boiled or poached. Dry toast finely broken up and mixed with a soft-boiled egg aids in its digestion. Soft-boiled eggs are more easily digested than raw eggs, but the latter are less irritating to the stomach, probably because they are digested in the intestines. It has been found that two poached or soft-boiled eggs leave the stomach in from two to three hours; that is, in the same time as milk, oysters, white bread, and light fish. _Milk._—Although one of the most completely digested of foods in a mixed diet, milk is not quite so completely digested as meat and eggs. When milk is the sole food (milk diet) the proportion digested depends partly on the amount consumed. With the consumption of 3½ pints of milk daily the loss of milk solids varies from 10 to 11.16 per cent. Young children digest milk more completely than adults. The addition of aërated waters or crackers broken up in the milk prevent the formation of tough clots, and hence render it more digestible. Hot (not boiled) milk is more digestible than cold. Boiling increases the toughness of the curd, but it destroys the bacteria. Buttermilk and koumiss are more easily digested than cows’ milk.