266 LECTURES ON IMMUNITY 



strong solution even the salts of Ca, Sr, and Ba retard 

 the action of rennet. This action of stronger solutions 

 is probably only due to a change in the solubility, just 

 as the precipitation of casein by means of alcohol, and 

 possesses, therefore, only a secondary interest. 



Acids precipitate the casein, even in rather minute quan- 

 tity. Salts which have an acid reaction accelerate the 

 action of rennet just as acids do. This may be due to 

 the presence of free acid in these solutions. On the other 

 hand, alkalies hinder or retard the precipitation of the 

 casein, and in the same manner behave salts with an alka- 

 line reaction, as the carbonates or bicarbonates of the alkali 

 metals. These last facts speak very much in favour of 

 the chemical theory first advanced by Hammarsten, who 

 regards the casein as an acid, which is very slightly solu- 

 ble, whereas its salts with alkali metals are soluble in water. 



Laqueur and Sackur 1 have determined its equivalent 

 weight to be 1135. The conductivity of the sodium salt 

 in solution seems to indicate, according to a rule of Ost- 

 wald, that the acid is tetra- or hexavalent, so that its 

 molecular weight is computed to be 4540 or 6710. 



With this last number agrees very well another (6600), 

 found by Hedin, Blum, and Vaubel 2 from a study of the 

 products of the decomposition of casein. 



By the prolonged action of weakly alkaline solutions on 

 casein, another stronger acid is formed, termed isocasein, 

 possessing the equivalent weight 960. Its molecular 

 weight is four or six times greater. 



1 Laqueur and Sackur : Hofmeisters Beitrdge, 3 (1902); Sackur: Zeitschr.f. 

 ph. Ch., 41. 672 (1902). 



2 Hedin, Blum, and Vaubel: Journ. /. praktischc CA., 60. 55 (1899). 



