Milk is one of our most perfect types of food, containing water and solids in such proportions as are known to be needful for the nourishment of the body. A proof of this is seen in the fact that it is the only food of the young of the _Mammalia_ during the time of their greatest growth. It contains those food principles in such amounts as to contribute to the rapid formation of bone and the various tissues of the body, which takes place in infancy and childhood; but after this growth is attained, and the individual requires that which will repair the tissues and furnish warmth and energy, milk ceases to be a complete food. =Composition of Cow's Milk.= The composition of cow's milk varies with the breed and age, care and feeding, of the animals. Cows which are kept in foul air in stables all the year, and fed upon bad food such as the refuse from breweries and kitchens, give a quality of milk which is perhaps more to be dreaded than that from any other source; for such animals are especially liable to disease, and are often infected with tuberculosis, pneumonia, and other fatal maladies. Cows are particularly susceptible to tuberculosis, and may convey it to human beings either in their milk or flesh. According to Dr. Miller, cow's milk contains the following ingredients: Water 87.4% Fat 4.0% Sugar and soluble salts 5.0% Nitrogenous matter and insoluble salts 3.6% Another analysis is that of Uffelmann: Water 87.6% Albuminoids 4.3% Fat 3.8% Sugar 3.7% Salts .6%[15] =Characteristics.= Milk from healthy, well-nourished cows should be of full white color, opaque, and with a slightly yellowish tinge sometimes described as "cream white." It should vary but slightly in composition from the above analyses. The fat should not be less than 2.5%. The amount of fat may be easily determined with a Feser's lactoscope (Eimer and Amend, New York), directions for the use of which come with the instruments. It will generally vary from 3% to 4% in good milk. Should it fall below 2.5% the milk should be rejected as too poor for use. Such milk has probably been skimmed, or comes from unhealthy or poorly fed cows. The specific gravity of milk should be from 1.027 to 1.033. This may be found with a Quevenne's lactometer. If it falls below 1.027, one has a right to claim that the milk has been watered or that the cows are in poor condition.[16]