318 
With the higher powers of the microscope various forms of bacteria 
can be distinguished, some of which at least play an important part 
in the changes which take place when milk is kept for some time at 
ordinary temperatures. The perfectly fresh milk of carnivorous 
animals is as a rule acid in reaction. According to Leach (9) the 
acidity of fresh milk is due to carbon dioxide and acid phosphates, 
and according to Richmond (10) to mono- and di-phosphates. 
Human milk and that of herbivora is slightly alkaline and cow’s 
milk has been described as amphoteric; that is, it is alkaline to red 
litmus, acid to blue litmus. Yogel (11) states that he has never yet 
found perfectly freshly drawn cow’s milk to show a decidedly alka- 
line reaction to litmus. In the greater number of instances the reac- 
tion of freshly drawn milk was either neutral or transiently acid. On 
standing exposed to the air for some time all forms of milk become 
more or less acid in reaction in consequence of the conversion of milk 
sugar into lactic and other acids by the action of various micro-organ- 
isms, until finally considerable amounts of acid are produced, which 
are responsible for the souring and curdling of the milk ordinarily 
observed. 
For further information relative to the reaction of human and 
cow’s milk and for a theoretical explanation of the acidity and 
alkalinity shown by these milks, see Courant, pp. 319-350. 
Milk consists chiefly of water. In addition to this it contains fat, 
lactose, several proteids (see Halliburton (12)), such as caseinogen, 
lactalbmnin, lactoglobulin, opalisin. and lactomucin, and a number of 
salts. It also contains certain dissolved gases, such as oxygen, nitro- 
gen, and carbon dioxide. The oxygen and nitrogen are carried into 
the milk mechanically in the process of milking. Carbon dioxide is 
present in milk to the extent of 3 or I per cent by volume and partly 
escapes into the air when milk is drawn from the udder. Besides the 
substances already mentioned still others have been found in milk 
in small quantities. Among these may be mentioned lecithin (13), 
cholesterol (II), citric acid (15), lactosin, a new carbohydrate 
(16), and orotic acid (IT). This substance has the composition 
C-H 4 0 4 X 2 .H 2 0, and is believed by its discoverers to have the con- 
stitution : 
NH.CH 2 .CO NH. co.ch 2 
CO< | or CO< 
ira.co.co NH.eo.co. 
Sherman, Berg. Cohen, and Whitman (18) found small amounts of 
ammonia in fresh milk. According to Trillat and Sauton (19) the 
presence of ammonia in fresh milk is usually indicative of contamina- 
tion. According to Schondorf (20) human milk contains small 
amounts of urea. Jolles (21) and others have called attention to 
the relatively large amounts of iron which woman’s milk normally 
