The _proteins_ contain nitrogen, sulphur, and phosphates. The predominance of nitrogen has given the proteins the name nitrogenous. The carbonaceous foods contain none of these elements, but are rich in carbon, hence the name carbonaceous. As previously stated, no food contains but one element of foodstuffs and all elements are formed into compounds of plant life from the elements in the soil, air and water by the action of the sun’s rays. The rays of heat and light seem to store something of their power in latent heat and energy within the combinations of these compounds. The end of plant life is the completion of its compounds,—it first generates the compounds, then matures them, and then dies. All _organic_ matter is thus formed by the action of the sun’s rays upon _inorganic_ matter. All meats are first in the form of plant life and are converted into other compounds by the chemical processes of the animal. This chemical action of the animal converts the energy latent in the foods into more concentrated form. The animal thus performs a part of the chemical work for man. The classification of foods, as previously stated, is based upon the principal organic foodstuffs contained. Proteins contain carbon and salts, and carbonaceous foods contain salts and nitrogen, but these are not in appreciable quantity. The preponderance of these elements determines the use of the foodstuffs in the body. It will be remembered that the chief uses are the production of heat and energy, the building of new tissue of the growing child, and the repair of the waste in the child and the adult. * * * * * [Sidenote: Water] No one element of food is more important for the needs of the body than water. It comprises about two-thirds of the body weight and is a component part of all foods. It is composed of oxygen and hydrogen. In order that the body may do efficient work in digestion and in the distribution of the nutrient elements of the foods, and that the evaporations from the body may be re-supplied, the water in the foods, together with the beverages drunk, should consist of about seventy-five per cent liquid to twenty-five per cent nutrient elements, or about three times as much in weight as proteins, fats, and carbohydrates combined. If undue evaporation or perspiration is occasioned, a larger proportion is required. Water passes directly into the circulation without chemical change. It is being constantly evaporated through the lungs and the skin, and every forty-five seconds it passes from the kidneys into the bladder. The average individual at normal exercise, requires about seventy-one and one-half ounces of water daily, which equals about nine glasses (one glass of water weighs one-half pound); a part of this is consumed in the food. By reference to the following tables it will be noted that water forms a large percentage of all food, particularly of green vegetables and fruits.