THE MILK. 413 



will increase the tendency of the lime-casein to coagulate upon the 

 subsequent addition of chymosin, while this is diminished if the 

 soluble salts are transformed into the insoluble form or if their 

 amount is diminished. It is for this reason also that boiled milk 

 does not coagulate so rapidly as fresh milk, as the free carbonic acid, 

 which holds a certain amount of calcium in solution, is thereby 

 removed. The common addition of lime-water to milk similarly 

 increases the tendency to coagulation, but does not render it more 

 digestible, as is generally supposed. 



The coagulation of the milk which occurs spontaneously on 

 standing is analogous to that which results upon the addition of a 

 mineral acid, and is referable to the formation of lactic acid from 

 lactose as a result of bacterial action. The phenomenon has 

 nothing in common with the coagulation which results from 

 chymosin, and is merely the outcome of the withdrawal of the lime 

 salts from the lime-casein and the liberation of the latter. 



From the fact that the coagulum which results in cows' milk 

 upon the addition of chymosin is much tougher and denser than 

 that which is obtained with human milk, it has been concluded that 

 the casein of the two is not identical. Soxhlet, however, has shown 

 that the density of the coagulum is primarily dependent upon the 

 concentration of the casein solution and the amount of soluble cal- 

 cium salts and acid phosphates present. As this is much greater in 

 cows' milk than in human milk, it follows that marked differences 

 must thus exist. There is evidence to show, nevertheless, that 

 different forms of casein occur. Elementary analysis of human 

 casein (Harnmarsten) and cows' casein (Wroblewski) has given 

 the following results : 



Human, 0, 52.96; H, 7.05; N, 15.65 ; S, 0.75; P, 0.84; O, 22.78 per cent. 

 Cows', C, 52.24; H, 7.32; N, 14.97 ; 8,1.11; P, 0.68 ; O, 23.66 " " 



The difference is here especially noticeable in the amount of sul- 

 phur. Human casein, moreover, is not so readily precipitated by 

 salting or by the addition of acids, and does not always coagulate 

 with chymosin. The gastric juice, it is true, can precipitate the 

 substance, but it readily dissolves in an excess without leaving any 

 residue of nuclein. From this observation Szontagh has concluded 

 that human casein is in reality no rmcleo-albumin. But aside from 

 these data we have abundant evidence that human casein and cows' 

 casein are not identical, in the fact that no modification of cows' 

 milk, however produced, is so readily digested by the infant as is 

 human milk. 



Like all albumins, casein is optically active ; its specific rotation 

 in neutral solution is 80 degrees. 



The isolation of the casein from milk will be described below, in 

 association with the isolation of the soluble albumins. These, as I 

 have already said, are lactalbumin and lactoglobulin. 



LACTALBUMIX. Lactalbumin is found both in human milk and 



