Our bodies are made up of different materials: skin, flesh, blood, bone, etc. These consist of a large number of substances, called compounds; the compounds contain two, three, or four elements, united chemically in definite proportion. Some of these compounds are: water, which forms more than two thirds of the whole body, and is the common carrier of food into and through the system; fibrine, which is the chief solid material of flesh, and forms one tenth of the body; fat, a mixture of three compounds, distributed all through the system; albumen, and other nitrogenous substances, of which osseine in bones, keratin in the hair, nails, and skin, cartilagin in cartilage, and hemoglobin, which contains iron and gives the red color to the blood, are the most important. Chloride of sodium, or common salt, and other mineral salts of polassa, lime, and magnesia, are found in all parts of the body. These compounds are made up of two or more separate and distinct elements.

There are sixteen elements in the human body, - oxygen, carbon, hydrogen, nitrogen, phosphorus, sulphur, chlorine, fluorine, silicon, calcium, potassium, sodium, magnesium, iron, manganese, and copper. Compounds only, not the separate elements, are capable of nourishing the body. Oxygen only exists as an element. Its office is to support combustion.

Combinations of two or more of these elements are found in the various articles used as food. It is quite important that our food should consist of various materials, containing elements similar to, or capable of being changed into, the elements of our bodies, of which the most important are: water, to help digestion and assimilation of food, and to help carry away refuse material; salts, to renew the mineral parts and replenish certain tissues; carbon, to furnish fuel for warmth; nitrogen, to build up and repair the whole structure.

For convenience the elements are classified chemically into Non-Combustibles, or inorganic compounds, including water and salts; Combustibles, or organic compounds, including carbonaceous and nitrogenous foods. Some foods contain no carbon, some contain no nitrogen, and some have all the elements in various proportions. Food has been classified into gaseous, of air; liquid, or water; and solid, including animal and vegetable foods. But the division into carbonaceous, or heat-producing, and nitrogenous, or flesh-forming foods, answers every purpose.