The reaction of human milk is slightly alkaline; of cow's, neutral or slightly acid. The specific gravity of each is about 1.028 to 1.032. Human milk is sterile when secreted, but derives a few bacteria from the lacteal ducts. Cow's milk, as usually sold, contains large numbers of bacteria. Microscopically, human milk is a fairly homogeneous emulsion of fat, and is practically destitute of cellular elements. [Illustration: FIG. 127.--Holt's milk-testing apparatus.] Chemic examination of milk is of great value in solving {278} the problems of infant feeding. The sample examined should be the middle milk, or the entire quantity from one breast. The fat and proteid can be estimated roughly, but accurately enough for many clinical purposes, by means of Holt's apparatus, which consists of a 10 c.c. cream gage and a small hydrometer (Fig. 127). The cream gage is filled to the 0 mark with milk, allowed to stand for twenty-four hours at room temperature, and the percentage of cream then read off. The percentage of {279} fat is three-fifths that of the cream. The proteid is then approximated from a consideration of the specific gravity and the percentage of fat. The salts and sugar very seldom vary sufficiently to affect the specific gravity, hence a high specific gravity must be due to either an increase of proteid or decrease of fat, or both, and vice versâ. With normal specific gravity the proteid is high when the fat is high, and vice versâ. The method is not accurate with cow's milk. For more accurate work the following methods, applicable to either human or cow's milk, are simple and satisfactory. * * * * * [Illustration: FIG. 128.--Tube for milk analysis.] Fat.--_Leffmann-Beam Method_.--This is essentially the widely used Babcock method, modified for the small quantities of milk obtainable from the human mammary gland. The apparatus consists of a special tube which fits the aluminum shield of the medical centrifuge (Fig. 128) and a 5 c.c. pipet. Owing to its narrow stem, the tube is difficult to fill and to clean. Exactly 5 c.c. of the milk are introduced into the tube by means of the pipet, and 1 c.c. of a mixture of equal parts of concentrated hydrochloric acid and amyl-alcohol is added and well mixed. The tube is filled to the 0 mark with concentrated sulphuric acid, adding a few drops at a time and agitating constantly. This is revolved in the centrifuge at 1000 revolutions a minute for three minutes, or until the fat has separated. The percentage is then read off upon the stem, each small division representing 0.2 per cent. of fat. {280} Proteids.--_T. R. Boggs' Modification of the Esbach Method_.--This is applied as for urinary albumin (p. 74), substituting Boggs' reagent for Esbach's. The reagent is prepared as follows: (1) Phosphotungstic acid 25 gm. Distilled water 125 c.c.