CASEIN. 189 



tash. But phosphoric and arsenious acid, according to Bracon- 

 not, do not precipitate casein, though, when we add prussiate 

 of potash to a solution of casein containing phosphoric acid, a 

 copious precipitate falls.* Alcoholic solutions of casein are not 

 precipitated by acids. And alcohol dissolves readily the preci- 

 pitates thrown down from water by acids. 



Casein combines with the alkalies without undergoing any al- 

 teration, unless the alkaline solutions be concentrated and heat 

 be applied. In that case the solution becomes brown, ammonia 

 is given out, and an alkaline sulphuret is formed. 



Casein combines with the alkaline earths. If the quantity of 

 earth be small, the compound is soluble, and the earth is not pre- 

 cipitated by exposure to the air, or by passing through the liquid 

 a current of carbonic acid gas. Indeed casein, as extracted from 

 milk, appears to contain caseate of lime. When casein is placed 

 in contact with an excess of hydrate of lime, a bulky compound 

 is formed very little soluble in water. When this compound is 

 boiled in water, the casein is gradually decomposed. A kind of 

 extractive, soluble in water, is formed, from which oxalic acid pre- 

 cipitates lime. 



If we heat sugar with a concentrated solution of casein, it loses 

 its consistence and becomes very fluid. But if we increase the 

 quantity of sugar considerably, the casein separates in curdy 

 masses or clots. But when washed these clots again dissolve in 

 water. When casein is mixed with gum-arabic, it loses its solu- 

 bility entirely ; owing, in the opinion of Braconnot, to a free acid 

 and earthy salts contained in the gum. 



Solution of casein in water is precipitated by all the earthy 

 and metallic salts capable of precipitating uncoagulated albumen. 

 Tannin throws it down both from its aqueous and alcoholic so- 

 lution. 



Like albumen it is capable of existing in two states, uncoagu- 

 lated and coagulated. The characters of uncoagulated casein 

 have been given. We must now state the properties of coagu- 

 lated casein. 



For coagulation it requires the boiling temperature or rennet. 

 When an aqueous solution of casein or skimmed milk is mixed 

 with rennet and gently heated, coagulation takes place. Rennet 

 is formed by digesting the innermost membrane of a calfs sto- 



Ann. de Chim. et de Phys. liii 344. 



