a well-known fluid, constituting the sole food of mammals during a certain period after birth, is an opaque, white, emulsive liquid, with a bland, sweetish taste, and a faint peculiar odour. When examined by the microscope it is found to consist of myriads of remarkably minute globular particles suspended in a serous liquid. These particles are termed butter, and are readily separated from the surrounding liquor, on being allowed to stand, owing to their relative specific lightness. On rising to the surface they carry with them a portion of caseine, one of the main constituents of milk, retain some of the serum or whey in which they float, and thus form cream. The upper stratum of cream is richer in butter, while the lower abounds more in the albuminous substance, caseine. The milk from which the cream has been separated is termed skimmed milk, and that from which the butter has been extracted is termed butter-milk. The butter or fatty matter is obtained from the milk by the process of agitation called churning, and the curd or cheese, or caseine, as the technical phrase is, is separated from the milk by the infusion of rennet, a liquid obtained by macerating the stomach of a sucking animal (as of the calf) preserved by means of salt. The whey of milk from which the curd and butter have been completely separated, yields on evaporation a colourless, sweet sub- Milk, known by the name of sugar of milk, as well as one or more nitrogenous composition. The relative proportion of the constituents of milk vary considerably in all animals with the quality of the food, the age of the animal, and the period after parturition. Among the most elaborate experiments on the composition of several kinds of milk are those of MM. O. Henri and Chevallier, published in the Journal de Pharmacie, vol. xxv., and which are as follows:

| Milk of the | Cow | Ass | Woman | Goat | Ewe | |-------------|-----|-----|-------|------|-----| | Caseine | 4-18 | 1-82 | 1-53 | 4-02 | 4-50 | | Butter | 3-13 | 0-11 | 3-25 | 3-32 | 4-20 | | Sugar of milk | 4-77 | 6-68 | 6-50 | 5-28 | 5-60 | | Various salts | 0-60 | 0-34 | 0-45 | 0-58 | 0-68 | | Water | 87-02| 91-65| 87-99 | 86-90| 85-62 |

Total: 100-00 100-00 100-00 100-00 100-00

Solid substances: 12-98 8-85 12-02 13-20 14-38

Of the various saline constituents which enter into the composition of milk, Schwartz gives the following as the composition of the ashes of 100 parts of cow's milk. (See Gmelin, Handbuch der Theoret. Chemie, vol. ii.)

- Soda (in milk combined with lactic acid): 0-0115 - Chloride of potassium: 0-1350 - Phosphate of soda: 0-0225 - Phosphate of lime: 0-1805 - Phosphate of magnesia: 0-0170 - Phosphate of iron: 0-0032

The quality of the milk is also affected by the state of health of the female supplying it. Tubercular disease of the lungs of an animal has been found to increase the quantity of phosphate of lime in the milk. Labillardière states (Diet. Mat. Méd. iv.) that the milk of a cow affected by a species of tubercular phthisis contained seven times more phosphate of lime than usual; and Dupuy, according to Pereira, has also noticed the large quantity of calcareous matter in the milk of cows, in whose lungs the same substance was found in abundance. The morbid changes produced in the quality of the milk by the disease called cocote, so prevalent among the cows of Paris some years ago, attracted considerable attention among scientific men. The principal morbid changes recognised in the milk were a want of homogeneity, imperfect liquidity, a tendency to become viscid on the addition of ammonia, and on microscopic examination the presence of certain globules not found in healthy milk. And not only is this subject of great importance in reference to the frequency of disease in cows, and the consequent morbid character of their milk; it also requires very special attention in connection with the milk of the human subject. The deleterious effect to a child of being suckled by a female labouring under tuberculous disease must be too obvious to require exemplification.

According to Dr Jon. Pereira (Treatise on Food and Diet), it is possible by adding certain ingredients to the food to modify the colour, odour, taste, and medicinal effect of the milk of a female; and the influence which medicines taken by the parent exert over the child is known to every nurse. As the natural food of the young mammal of every species is the milk of its mother, we may accordingly regard milk not only as containing all the elements necessary for the nutrition and growth of the body during a certain period, but also as a kind of model food for the species to which the animal belongs. Milk appears to partake of the nature of both animal and vegetable food. The large proportion of caseine and butter which it contains represent the fibrin and fat of beef, while the equally large proportion of sugar represents the starch of wheaten bread. The curd of milk goes to form the albumen and fibrin of the blood. The butter serves to form fat, and with the sugar contributes, by yielding carbon and hydrogen to be burnt in the lungs, to support the animal heat of the body. The salts develop the osseous system; the iron is required by the blood and the hair, while its chloride salt goes to form the hydrochloric acid of the gastric juice.

Milk, as an article of food, is of great use and value, as well for the adult as for the child, and for healthy persons as well as for invalids. The chief objection often to its employment is the difficulty of digestion of its fatty matter. Whey, as an excellent diluent and nutritive, is often used in febrile and inflammatory complaints. It owes its slightly nutritive qualities to the sugar of milk which it contains. It gently promotes the action of the secreting organs, and thus proves useful in congestion of the liver. According to Pereira there are various establishments in Switzerland and Germany for the cure of chronic disorders by the use of pure or aromatised whey (Molkencureurs; Cares de Petit-Lait). The whey is obtained from the milk of the cow, the goat, or the ass; and is used as a drink, as a lavement, or as a bath, associated often with the employment of mineral waters. Butter-milk also forms a very agreeable cooling beverage in febrile and inflammatory diseases; and its nutritive qualities are owing to the caseine, the sugar, and the salts of milk which it possesses.

With respect to the distinctive properties of the different milks in most frequent use, valuable information will be obtained by referring to the table already given. While ewe's milk contains the largest proportion of nutritive matter, it is on that account less easy of digestion, and not suited for dyspeptics. The same holds of goat's milk, which ranks next in nutritive power. Ass's milk is the least nutritive, but the most easy of digestion. With the exception of woman's milk it is the richest in sugar. It is considered a most valuable aliment in consumptive cases, in chronic diseases of the digestive organs, and in convalescence from acute maladies. This seems to depend on the small quantity of butter and the large quantity of sugar of milk which it contains. Cow's milk holds a middle place in nutritive and digestible properties between goat's and ass's milk. According to Donné (Comptes Rendus, 1841), it is the only milk which is either very feebly alkaline, often neutral, or sometimes slightly acid. The milk of the ass and the woman are always obviously alkaline. The milk of women from fifteen to twenty years of age is said to contain more solid constituents than that of women between thirty and forty. Women with dark hair also are said to give a richer milk than women with light hair. (See Pereira's Treatise on Food and Diet, also Johnston's Chemistry of Common Life). For further information respecting the milk of cows see Dairy.