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 ther aci Is y bhe n n of various micro-organ- 
isms, until finally considerable amounts of acid are produced, which 
are responsible for the souring and curdling of the milk ordinarilv 
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 . p. — . 
Milk consists chiefly of water. In addition to this it contains fat. 
lactose, several proteids (see Halliburton (10a , such as caseinogen, 
1 act albumin, 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 (111, 
cholesterol (12 . citric acid 13 . lacrosin. a new carbohydrate 14 . 
and orotic acid (15). This substance has the composition C 5 H 4 0 4 X,. 
H,0. and is believed by its discoverers to have the constitution: 
XH.CH-.CO XH. CO.CH, 
CO< or CO< 
XH.CO.CO XH.CO.CO. 
Sherman, Berg, Cohen, and Whitman (16) found small amounts ol 
ammonia in fresh milk. According to Trillat and Santou (17) the 
presence of ammonia in fresh milk is usually indicative of contamina- 
tion. According to Schondorf (18) human milk contains small 
amounts of urea. Jolles (19^ and others have called attention to 
the relatively large amounts of iron which woman's milk normally 
contains, and to its influence on the health of the child. Cameron 
(19a) found 21 mi lligrams of iron oxid in 100 cubic centimeters of 
human milk from the third to the twelfth day of lactation. 
According to Jolles and Friedjung (19b) the quantity of iron in 
human milk decreases with bad environment and poor condition of 
the mother. 
In certain diseased conditions milk may contain still other sub- 
stances not ordinarily present in the milk of healthy animals. For 
example Tan der Marck (20) has detected bile in the milk of a woman 
who had developed jaundice after confinement, and Desmouiieres 
