The composition of cow's milk varies widely. Dairy cows, by long domestication, breeding and feeding, have been brought to a high state of specialization. Some breeds have been so trained, fed, and bred as to produce large quantities of milk. Some Holsteins have been known to produce one hundred pounds of milk per day each, which of course is many times the quantity required for the nourishment of their young. Some Jersey stock have been so bred, raised, and fed as to produce large quantities of butter; in some cases the butter-fat of especially fed Jerseys has been known to run as high as 8 or 10 per cent, whereas the normal fat content of milk is not more than 3.5 or 4 per cent. The average composition of mixed milk from many cows runs about as follows: Water, 87 per cent; lactose or milk-sugar, 4.5 per cent; butter-fat, 3.5 per cent; ash, .7 per cent; proteids, 3.3 per cent, of which about 2.5 per cent are casein, and .8 per cent albumin. [Sidenote: Value of milk depends upon its nitrogenous content] The commercial value of milk is measured almost entirely by its content of butter-fat. This is because the public knows practically nothing about the food value, or the chemistry of milk, therefore its value is estimated upon that which can be seen, and upon that which tastes best. The chief value of milk as a food lies in the nitrogenous element it contains. Fat can be secured from many other sources. The nutritive elements of milk from various animals vary according to the natural requirements of the young of various species. Cow's milk contains too large a proportion of casein, and not enough milk-sugar to meet the natural requirements of the human infant. This subject, however, will be discussed at length in Lesson XVI on "Infant Feeding," Vol. V, p. 1154. [Sidenote: Coagulation of casein in milk] The casein in cow's milk is coagulated by the hydrochloric acid of the stomach, which forms into lumps or curds, rather difficult to digest. This can be overcome or counteracted in several ways. First, if milk is allowed to sour or clabber, the casein is coagulated by nature, which is really the first process of digestion. In this form it neither burdens the digestion nor causes the supersecretion of hydrochloric acid, which is likely to occur when sweet milk is too liberally used. Second, the sipping and thorough insalivation of milk, by taking it into the mouth with something that requires thorough mastication, insures better digestion and assimilation, and less liability to produce intestinal gas. Milk will harmonize chemically with all non-acid fruits, cereals and nuts. Milk is in chemical harmony with meat and eggs, but all of these articles being highly nitrogenous, when taken at the same meal, the portions should be limited to the minimum.