Foods that supply material for growth and repair are called nitrogenous foods, because some nitrogenous compound is found in them.

They are also called proteids, from a Greek word meaning " first," because in the living cells which are the first principle or form of life there is always nitrogen.

A common name is albuminous foods, because the most familiar form of these nitrogenous compounds is egg albumen, as found in the white of an egg. The word is derived from the Latin albus, meaning " white." Albuminous substances exist in many forms, and are called by different names in different things. They are found largely in meat, fish, milk, peas, beans, and grains. The albumen and fibrin in the juices and flesh of meat and fish, and in the juices and membranes of some vegetables and fruits, the casein in milk, the vegetable casein in peas and beans, and the gluten of grains, are all forms of nitrogenous substances, or proteids.

Nitrogenous foods are often classified as flesh-form-ing, and other foods as heat-producing; but these terms are misleading, because nitrogenous foods also contain carbon, and give out heat.

But there are foods containing carbon which do not contain nitrogen; and these, in which carbon is the chief element, are called Carbonaceous Foods.

Though the internal normal temperature of the body is only 98°, the amount of heat produced by the slow combustion going on in the body, as mentioned on page 19, is considerable. It has been estimated that this heat obtained during twenty-four hours, if confined within certain limits, would be sufficient to raise nine gallons of water from a temperature of 70° to 212°. A portion of this heat is derived from nitrogenous foods, but the greater part of it is obtained from carbonaceous foods. These are classed as fats and carbo-hydrates.

Fats. These include butter, the fat of meat and fish, oils, eggs, and some kinds of cheese.

Fats stand at the head of heat-producing foods, and are necessary in winter and in cold climates. They serve other and important uses in the body. A small amount is necessary in digestion, and indispensable to perfect nutrition.

" Fat forms the principal material of certain tissues, which, by filling the spaces between the bones, muscles, and the different organs of the body, give rotundity and beauty to the form, equalize external pressure, diminish the friction of the parts, and being non-conductors of heat, keep the body warm." An undue accumulation of fat is a species of disease, and disease just as surely results from a deficiency of fat.

Carbo-hydrates (Starch, Sugar, etc.). Other carbonaceous foods, called carbo-hydrates, are the starches found in grains, peas, beans, and some vegetables, and the sugars found in the sap and juices of plants, vegetables, and fruits. Sugar is also found in milk; but we take most of it in the form of pure sugar, made from the sugar-cane.

These carbo-hydrates form the larger part of our diet, and are very important and useful foods.