THE PRECIPITINS AND THEIR ANTIBODIES 267 



Upon these chemical properties Hammarsten founded 

 his method for the preparation of pure casein. Milk is di- 

 luted with three to four times its volume of water, and a pre- 

 cipitate produced by the addition of o. i per cent of acetic 

 acid. The precipitate is separated from the solution by 

 filtration through linen and then dissolved in a weak solu- 

 tion of caustic soda or better ammonia. The fat-drops 

 carried down by the precipitate separate upon the top of 

 the liquid, which is thereafter again precipitated and redis- 

 solved four or five times. The last traces of fat are 

 removed from the precipitate by extraction with alcohol 

 and ether, and the precipitate is after that dried to a white 

 powder, casein, which is very slightly soluble in water. Its 

 suspensions in water behave as an acid, and it drives the 

 carbonic acid out from the carbonates of alkali metals or 

 calcium, giving clear solutions of its salts with these metals. 



If a solution of the calcium caseate is neutralised by 

 the addition of a diluted solution of phosphoric acid, a 

 precipitate of calcium phosphate is formed, and a white 

 liquid remains which resembles very much a milk devoid 

 of cream. Probably we have here a pseudo-solution of 

 casein. It behaves like milk in being coagulated through 

 the action of rennet. 



The adherents of the chemical theory of coagulation by 

 rennet assume that this ferment exerts a decomposing 

 action on the casein, just as pepsin on protein, 1 after which 



1 According to recent investigations of Bang (Zeitschr. f. ph. Ch., 43. 358, 

 1905), Hemmeter (Berl. klin. Wochenschrift, E-wald-Nummer, 14, 1905), 

 and Schmidt -Nielsen (Zeitschr. f. physiol C/z., 48. 92, 1906), pepsin and 

 chymosin the milk-coagulating substance in neutral rennet are different 

 substances. Pepsin seems to coagulate milk in acid solution, wherein the 

 results of Pawlow and Sawjalow find their explanation (cf. p. 71). 



