CASEIN." 373 



understand the importance and the uses of the globulin in the 

 blood until we have obtained an accurate knowledge both of its 

 chemical constitution, and of the function of the blood-corpuscles. 



CASEIN. 



Chemical Relations. 



Properties. In its dry state soluble casein occurs as an amber- 

 yellow mass, devoid of odour, insipid and viscous when tasted, and 

 having neither an acid nor an alkaline reaction ; it dissolves in 

 water, forming a yellowish viscid fluid, which on evaporation 

 becomes covered with a white film of insoluble casein which may 

 be readily drawn off. If a concentrated solution of casein be exposed 

 for a long time to the air, it rapidly passes into a state of putrefac- 

 tion^ developing a very large quantity of ammonia, and yielding 

 leucine, tyrosine, and similar substances. 



Alcohol renders casein opaque, and gives it the appearance of 

 coagulated albumen ; a part, however, of the casein dissolves in 

 alcohol, and on evaporation can be again obtained in an unchanged 

 state ; in boiling alcohol it dissolves more freely, but on cooling, the 

 greater part of the casein again separates ; this casein thus treated 

 with alcohol dissolves tolerably readily in water, especially with the 

 aid of heat, and has all the properties of non-coagulated casein. If 

 we add a little alcohol to a concentrated aqueous solution of casein, 

 a precipitate is thrown down which, however, dissolves again readily 

 in water ; if, however, the precipitation be effected by the free addi- 

 tion of strong alcohol, the casein is then difficult of solution or 

 even insoluble in water. By boiling it is not coagulated from its 

 solutions. 



Acids precipitate casein from its aqueous solution, and partially 

 combine with it, but they do not reduce it to the coagulated state, 

 for on neutralisation with alkalies or metallic oxides, the casein 

 again dissolves ; these combinations of casein with acids are readily 

 soluble both in pure water and in alcohol. Casein is especially 

 distinguished from albumen by the circumstances that it is preci- 

 pitated from its aqueous solutions by acetic and lactic acids, the 

 precipitate not being an acetate or a lactate, but pure casein. The 

 precipitate is only slightly soluble in an excess of acetic acid; 

 like all the other combinations of this class with acids, it is preci- 

 pitated by ferrocyanide of potassium. The alcoholic solution of 

 casein is not only not precipitated by acids, but alcohol even 

 possesses the property of dissolving those combinations of casein 



