=Average Composition of Milk.=--Perhaps there is no food substance which has been subjected to so many and such severe analytical tests as milk. Hundreds of thousands of analyses have been made in all civilized countries, not only of the milk of the individual cow but of herds of greater or less size. There is a great variation in the composition of milk in different breeds of cattle and also in different individuals of the same breed. For instance, the Holstein breed of cattle affords a milk with a very low content of fat, sometimes as low as 3.25 percent, and in individual cases lower. On the other hand the Jersey breed of cattle affords a milk of a very high content of fat, sometimes reaching as high as 6 percent, and in individual cases very much higher. The content of the nitrogenous element in milk is more stable than that of fat and the common content of casein in milk ranges from 2¹⁄₂ to 3¹⁄₄ percent. The sugar in the milk is usually the complementary substance with the fat, diminishing in relative proportions as the fat increases and vice versa. The average content of sugar in cow’s milk is approximately 4 percent. The content of mineral substances in milk is also quite constant, being about 0.70. The ash contains the phosphoric acid which is one of the essential food components of milk. A milk of fair average quality contains 12 percent of solids and 88 percent of water. This is an expression for milk during the various seasons of the year and from all breeds and kinds of cows. The influence of season has much to do with the quantity of milk produced. It is always greater in the spring and summer months, when the cows are turned out to pasture and the growth on which they feed is unusually succulent. The increase in volume is not attended with a proportionate increase of solids, and thus the percentage of solids in spring and summer milk is less than that in the winter milk unless the cows are particularly well fed during the winter on a generous diet, including large quantities of roots. [Illustration: FIG. 13.--COW STABLES, MAPLETOWN FARM, SUMNER, WASHINGTON.]