Of the fifteen to twenty substances contained in foods and comprised in the body, the principal ones are oxygen, hydrogen, carbon, nitrogen, chlorin, sodium, potassium, magnesium, iron, calcium, phosphorus, and sulphur. The differences in the forms of matter lie in the proportions in which these elements are combined. Those containing the largest proportion of nitrogen are called _Nitrogenous_ foods or _Protein_—such as meat, eggs, and some vegetables. Those containing the largest proportion of carbon are known as _Carbonaceous_—such as cereals, sugar, and fat. The four food elements, indispensable to life, either of plant or animal, are oxygen, hydrogen, carbon and nitrogen. Carbon combined with oxygen forms carbon dioxid. Oxygen, nitrogen, and carbon dioxid largely form the air. Oxygen and hydrogen form water. Calcium, iron, magnesium, sodium, and potassium are used in the formation of the various tissues and secretions of the body. The substances contained in living organisms are the same as those in inorganic matter, only in different complexities as appropriated to the needs of each organism. The difference between living and non-living matter is in the relative proportion and arrangement of the same elements. Before it is fit to supply the needs of the body, the raw material must undergo a chemical change. It has been demonstrated by scientific investigation that no unorganized elements, such as pure nitrogen, pure iron or magnesium, are assimilated by the system and converted into its various structures. While the body needs carbon, it cannot use coal; it needs nitrogen, yet it cannot appropriate it to rebuilding bone and muscle, until, by chemical action with other elements, it has been converted into complex substances called proteins. The muscles, ligaments, and labor-performing structures contain the largest amount of nitrogen. The fat contains the largest amount of carbon. The brain, the nerves, and the bones contain the largest proportion of phosphorus compounds. Yet, while the brain contains phosphorus, and the tissues nitrogen, the brain cannot be built up by eating elementary phosphorus, nor the muscles by pure nitrogen, but compounds rich in phosphorus or nitrogen may be utilized. Plants use the simple compounds of the earth, air, and soil, and, within their own cells, build them up into such complex substances as starch, sugar, protein, fat, and salts, putting them in condition for man and other animals to appropriate to their use. All plant life is compounded from the elements in the soil, air, and water, by the action of the sun’s rays. The rays of heat and light store something of their power in latent heat and energy in these plant compounds. The end of plant life is the completion of its compounds—when it has matured them, the plant dies. All organic matter is thus formed by the action of the sun’s rays on inorganic matter.