708 PROTEIDS. [Apr. 



does not contain any sulphur ; and the acid-albumin, prepared by the 

 solution in an acid of the neutralisation precipitate from such an alkali- 

 albumin solution, is similarly free from sulphur. 



3. Casein. 



This is the well-known proteid existing in milk. When freed from 

 fat, and in the moist condition, it is a white, friable, opaque body. In 

 most of its reactions it corresponds closely with alkali-albumin ; thus it 

 is readily soluble in dilute acids and alkalis, and is re-precipitated on 

 neutralisation; if, however, potassic phosphate is present, as is the 

 case in milk, the solution must be strongly acid before any precipitate 

 is obtained. 



Various reactions have at different times been assigned to casein as distinguishing 

 it from the closely allied body alkali- albumin. Later researches have however in 

 most cases cast so much doubt on these differences that the identity or non-identity 

 of casein and alkali-albumin must still be left an open question, the discussion of 

 which would be out of place here. 



Casein, as occurring in milk, has had several reactions ascribed to it, as character- 

 istic; but these lose their importance on considering that milk contains, in addition 

 to casein, other substances such as potassic phosphate, and a number of bodies 

 which yield acids by fermentation. The presence of potassic phosphate has an 

 especial influence on the reactions of casein. In the entire absence of this salt, 

 acetic acid in the smallest quantities, as also carbonic anhydride, gives a precipitate; 

 but if this salt is present, carbonic anhydride gives no precipitate, and acetic acid 

 one only when ihe solution is acid from the presence of free acid, and not from 

 that of acid potassic phosphate 1 . 



When prepared from milk by magnesic sulphate (see below), freed 

 by aether from fats, and dissolved in water, casein possesses a specific 

 rotatory power of - 80 for yellow light ; in dilute alkaline solutions, of 

 -76; in strong alkaline solutions, of 91; in dilute hydrochloric acid, 

 of -87. 



Casein has been asserted to occur in muscle, in serous fluids, and in 

 blood-serum (Serum-casein). In many cases it has probably been 

 confounded with globulins (see Class III.); but blood-serum and muscle- 

 plasma undoubtedly contain an alkali-albumin in addition to whatever 

 globulin may be present, but the usual doubt exists as to the identity- 

 of this with true casein. Its presence may be shewn by adding dilute 

 acetic acid to blood-serum which has been freed from globulin by a 

 current of carbonic anhydride ; a distinct precipitate is thrown down. 

 A substance similar to casein has also been described as existing in 

 unstriated muscle and in the protoplasm of nerve-cells. 



Preparation. Dilute milk with several (10 to 15) times its bulk of 

 water, add dilute acetic acid till a precipitate begins to appear, then pass 



1 See Kiihne, Lehrb. d. Physiol. Chem., 1868, S. 5G5. 



