THE PROTEIN SUBSTANCES. 8/ 



pound separated is very different in the one case, however, 

 from what it is in the other. 



The free acids are but slightly soluble in water, but in the 

 salt form they are very soluble and these solutions do not coag- 

 ulate on boiling, as shown by the behavior of casein in milk. 

 The addition of weak acids to these salt solutions forms pre- 

 cipitates of the free nucleo-albumin acids. From very weak 

 solutions the precipitate may not separate until after heating. 

 A large number of bodies have been described as nucleo-albu- 

 mins, but only those will be mentioned here which are well 

 known. 



CASEIN. Of all the nucleo-albumins this is the best know r n 

 and most important. It occurs in milk as a neutral calcium 

 salt, and in the case of cow's milk makes up nearly 4 per cent 

 by weight. It may be readily separated from milk by the 

 addition of a little acetic acid. In precipitating, the fat is 

 usually carried down too, but may be removed after drying by 

 treatment with ether or petroleum spirit. Rennin, the peculiar 

 enzyme of the calf's stomach, causes a kind of coagulation in 

 casein solutions; if lime salts are present, which is practically 

 the case in milk, the coagulation extends to the formation of 

 a curd or cheesy mass which is very characteristic. The first 

 product formed by the rennin is known as paracasein and the 

 curd, or cheese, is the calcium combination of this. 



Casein was formerly considered as an alkali albuminate 

 because of its behavior with acids and alkali solutions. Many 

 of its alkali combinations are now produced in a technical way 

 as by-products in the butter and cream industries. Plasmon 

 and nutrose are apparently sodium-casein compounds. These 

 are used as foods, but some of the others find application in 

 other directions. Casein forms two series of salts with cal- 

 cium hydroxide and other bases and the amount of metal in 

 several of these has been found with considerable accuracy. 

 Most of these salts form opalescent rather than perfectly clear 

 solutions. The addition of sodium chloride or magnesium sul- 

 phate to these solutions in sufficient amount completely precip- 



