The salts promote the growth of bone. The fat in milk is in small emulsified droplets within a thin albuminous sheath. When allowed to stand in a cool place it rises to the top. Besides casein, milk contains a certain amount of albumin—about one-seventh of the total amount—called lactalbumin. It maintains the fat in milk in emulsified form. In young babes the milk is curdled in the stomach, or the casein separated from the water and sugar, not by hydrochloric acid, but by a ferment in the gastric juice, known as rennin. Rennin, or rennet, from the stomachs of calves, is used in cheese and butter factories to coagulate the casein. This with other chemicals so hardens the casein that it is used in the manufacture of buttons. _Preserving milk._ If milk could be kept free from bacteria, it would keep sweet almost indefinitely. At the Paris Exposition, milk from several American dairies was kept sweet for two weeks, without any preservative except cleanliness and a temperature of about forty degrees. The United States Bureau of Animal Industry states that milk may be kept sweet for seven weeks without the use of chemicals. The importance of absolute cleanliness in the preparation and marketing of this important article of food is being recognized both by the producer and the consumer, and careful inspection has done away with many abuses. In the absence of an efficient health department, the consumer should ascertain in every case how the milk he uses is handled at every stage before it reaches him. Care in this regard may safeguard his family from disease and save him many dollars. The best method for the housewife to follow is to keep the milk clean, cool, and away from other foods, as milk will absorb a bad odor or flavor from any stale food or odorous vegetables, from fresh paint, or other substances. Milk must never be left exposed in a sick room or in a refrigerator unless the waste pipe and the ice chamber are kept scrupulously clean. _Milk Tests._ In testing the value of milk, or the value of a cow, butter makers and farmers gauge it by the amount of butter fat in the milk, while the cheese maker tests the milk for the proportion of protein (casein). The amount of butter fat depends on the feed and water, and on the breed. If the total nutrient elements fall below twelve per cent., it is safe to assume that the milk has been watered. In cheese and butter there is no sugar; it remains in the buttermilk and the whey, both of which the farmer takes home from the factories to fatten his hogs.