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 are not equal in this respect. Pound for pound, when burned in the body, the fats yield 2¼ times as much heat as protein or the carbohydrates.

Mineral Matters.—Finally there are in all foods the mineral matters, a group containing a number of salts which are indispensible because they are constituents of every part of the body. Phosphate of lime, for instance, makes up one-half of the substance of the bones, and the sulphates and chlorides of potash and soda, iron, etc., are everywhere present in smaller quantities. No food in which any of them is lacking is complete.

Relation.—The value of a food depends largely upon the relation of one group of nutrients to another. Proper nutrition can only be obtained when a sufficient amount of flesh-forming as well as of heat-producing nutrients are present, when the "nutritive ratio" is properly balanced for the particular purpose in view, be it the growth of the child, the maintenance of the body, the restoration of matter consumed by labor of body or brain, or the supply of heat to make up for cold surroundings.

Milk contains all of these groups of nutrients. The protein is represented in milk by the casein and albumin, the fats by the butter-fat, the carbohydrates by milk-sugar, and the mineral matters by the milk-*ash. Human milk contains them in a perfect proportion for infants, and for all purposes of nutrition cow's milk may be used to make up a "balanced ration," if not alone, then in connection with other food.

Nutritive Ratio.—As we said before, the "nutritive ratio" of a food means the ratio of its flesh-forming constituents—proteins—to its heat-producing nutrients—carbohydrates and fats. Cow's milk, containing