374 PROTEIN-COMPOUNDS. 



with acids, which are insoluble in water. When treated with 

 concentrated nitric, hydrochloric, or sulphuric acid, casein yields 

 the same products of decomposition as albumen and fibrin. 

 Tannic acid precipitates it from very dilute aqueous and alcoholic 

 solutions. 



Casein combines very readily with bases, turbid solutions of 

 this substance becoming clear on the addition of caustic alkalies; 

 alkaline earths dissolve in solutions of casein, and can only with 

 difficulty be separated from that body ; with larger quantities of 

 these earths casein forms insoluble compounds. Hence its solutions 

 are precipitated by chloride of calcium and sulphate of lime, as well 

 as by sulphate of magnesia, on the application of heat, which thus 

 afford a reaction very characteristic of casein. It resembles albu- 

 men in being precipitated by metallic salts, and forming with them 

 two combinations, namely, one of casein and the acid, and the other 

 of casein and the metallic oxide. Ferrocyanide of potassium does 

 not throw down casein from alkaline solutions, and only induces a 

 slight turbidity in neutral solutions. 



These are the properties of casein, as it occurs in its ordinary 

 state of solution in the milk ; if, however, we obtain it perfectly 

 free from alkaJi, according to Rochleder's* method, which we shall 

 presently give, it presents some characters different from those 

 which we have just described. For instance, it dissolves only very 

 slightly in pure water, rather better in hot water, and not at all in 

 alcohol 5 it reddens blue litmus without, however, communicating 

 this property to water, but it forms solutions with carbonate and 

 phosphate of soda, which no longer exhibit an alkaline reaction ; it 

 dissolves very readily in solutions of hydrochlorate of ammonia, 

 nitrate of potash, and other neutral alkaline salts, does not coagulate 

 on boiling, like albumen, but forms on evaporation a film of casein 

 as we have already described. It dissolves in dilute mineral acids, 

 but is precipitated on the addition of an excess of the acid ; the 

 solutions of casein in dilute acids become covered on evaporation 

 with this colourless, transparent, and somewhat tough membrane ; 

 the solution of this substance in acids or in alkalies is completely 

 precipitated by neutralisation, and mineral acids throw it down 

 from its acetic acid solution. The precipitated hydrochlorate of 

 casein is, like the hydrochlorate. of albumen, soluble in pure water ; 

 before dissolving, however, it swells, like the latter, into a jelly-like 

 mass ; both acids and alkalies precipitate it from this solution ; the 

 deposit thrown down by hydrochloric acid swells and finally 

 dissolves in alcohol, but is precipitable from this fluid by ether, this 

 * Ann. d. Ch. u. Fharm. Bd. 45, S. 253. 



