The human body uses the mechanical power chiefly in muscular work; the heat is used in warming the body and causing the evaporation of moisture from its surface. The animal organism is much superior to the mechanical engine. It is more economic in the use of fuel; it has a nervous organization rendering it sensible to impressions and capable of directing its energies. The human machine is capable of adapting itself to many circumstances and changes in the demands upon it. But to enable the body to continue to perform these functions indefinitely it must be properly fed, and a proper feeding of the body requires a knowledge of its composition and the exchanges which are constantly going on. This knowledge is to be derived from a study of its metabolism, the analysis of foods, and a determination of their heat values. =The Classification of Foods.=—Foods are divided into proteins, carbohydrates, fats, mineral salts, and water. The body is composed of materials belonging to the same groups. Proteins form the principal part of muscles, bones, and many other tissues of animal bodies; they also constitute some of the most important vegetable structures. _Proteins_ are probably the most complex compounds in nature; all contain carbon, hydrogen, oxygen, and nitrogen, generally sulphur, and sometimes phosphorus also. They are, with rare exceptions, colloids, that is, glue-like, non-crystalline bodies, which even in solution cannot pass through animal membranes. The building stones of the body are the amino-acids. All proteins contain them, but the kinds and proportions are not always the same. It has been shown by biologic experiments that life and growth cannot be maintained when certain amino-acids are deficient. Proteins are of use to the human body as tissue formers, and, secondly, as producers of energy, but they also have a food value as flavoring agents, rendering the food appetizing, and so are to a certain extent stimulants. The palatability of meats and soups are due to their presence. The amino-acids have decided heat value. The necessity for proteins in the diet has been abundantly demonstrated. Many investigations have shown that when the food contains no protein the waste of nitrogen continues, no matter how abundant the supply of carbohydrates and fats. In other words, a continuous protein cleavage is demanded by the animal organism, and no other nutriments can serve as a substitute for protein to meet this demand. If the food contains no protein, the body tissues will be depleted. It cannot be said that carbohydrates or fats are an essential part of the diet in the sense that protein is, because it is possible to substitute one for the other to produce energy, or to substitute proteins for both.