The function of fat in the body is to yield heat and energy primarily. Each ounce of fat yields two hundred sixty-four calories of heat, making the group two and one fourth times as active as either protein or carbohydrate in this respect. Fats ordinarily supply from twenty-five to thirty per cent of the total calories of a well balanced dietary. On the basis of two thousand five hundred total calories a day, about seven hundred fifty should be fat. At two hundred sixty-four calories to an ounce, we have about three ounces as our daily need of this food element. Fats are also stored in the body as a reserve of energy. Every one has more or less of this sort of reserve, unless he has been starving for some time, or is suffering from a wasting disease. This reserve of fat also acts as a protection, and gives shape and symmetry to the body. Recently methods have been devised for changing the unstable vegetable oils into stable, lardlike, solid fats. This process is called hydrogenation, so named because the process is really one of adding hydrogen until the fat becomes saturated and less likely to undergo decomposition into fatty acid and glycerin. The fats thus formed seem to be equal to the animal fats so far as digestion and utilization are concerned, and hence are of considerable economic value at the present time. Certain fats, including those of butter and milk, are rich in the so-called vitamines, and have been shown, by recent experiments upon animals, to be efficient growth stimulants. _Carbohydrates_ The carbohydrates are made up of the chemical elements carbon, hydrogen, and oxygen. By noticing the name, one readily sees that the first part stands for the carbon. The latter half, "hydrate," indicates that water might be present; and in fact, nearly all of these bodies have hydrogen and oxygen present in the proportion to form water, that is, two parts hydrogen to one of oxygen. Carbohydrates ordinarily make up about sixty to sixty-five per cent of the total number of calories of our diet. Most carbohydrates, when pure, are either white powders or white crystalline solids. Many of them are sweet to the taste. The starches and the celluloses are not soluble in cold water, but the sugars are readily soluble. The classification of the carbohydrates is comparatively simple; and part of it is given here, as it will help in our discussion of the properties of the group: / 1. Cellulose / 1. Starch Group | 2. Starch | \ 3. Dextrin | / 1. Cane Sugar Carbohydrates | 2. Cane Sugar Group | 2. Malt Sugar | \ 3. Milk Sugar | / 1. Glucose \ 3. Glucose Group | 2. Levulose \ 3. Invert Sugar