The facts which are fundamentally necessary to a broad understanding of human nutrition pertain, first of all, to the materials out of which vegetable and animal tissues are constructed. It is important to know both what these are and what are their sources.

4. Number Of Elements Involved

About seventy substances are now believed to be chemical elements, i.e., substances that cannot be resolved into two or more simpler ones, and of which, so far as known, all forms of matter are composed, the variety of their combinations being almost infinite. It is remarkable that comparatively few of these fundamejital substances - about one-fifth are intimately related to the growth of plants; and those that occupy a prominent place in human nutrition are even less in number. It is necessary to mention only fifteen elements in this connection, some of which are of minor importance: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, chlorine, silicon, fluorine, potassium, sodium, calcium, magnesium, iron, and manganese.

5. Sources Of Elements

At ordinary temperatures, four of these, oxygen, hydrogen, nitrogen, and chlorine, are gases, and the remaining ones are solids. Four are constant and important ingredients of the atmosphere; viz., carbon, oxygen, hydrogen, and nitrogen, and they also exist in the soil in gases, as well as in combination in liquids and solids; the other eleven, though sometimes present in the air in minute quantities, are found to no appreciable extent except as fixed compounds in water and in the crust of the earth, or in plants and animals. Nearly all of these elementary substances are absolutely essential to man's existence. From the standpoint of necessity, they are, therefore, nearly all of equal value; but if we take into consideration the relative ease and abundance of the supply, certain ones rise to a position of supreme importance.