OEGANIC PRINCIPLES. 87 



from the liquid, then washed, is redissolved in a solution of 

 carbonate of soda ; this solution separates from the fat which 

 floats on the top, and can be completely removed at the end 

 of twelve hours of repose. The liquid thus freed from fat is 

 acidified by a few drops of hydrochloric acid, and the caseine 

 is precipitated perfectly pure." Obtained by this process, it 

 is perfectly white, and insoluble in water, resembling pot 

 cheese. 



Caseine has certain marked properties by which it is dis- 

 tinguished from albumen. It is not coagulable by heat; 

 is coagulable by the feeble vegetable, as well as the mineral 

 acids, and by rennet. This latter substance is obtained from 

 the fourth stomach, or abomasus, of sucking ruminating ani- 

 mals, and is the milk almost reduced to caseine, and mixed 

 with the gastric fluids. It is salted and dried, and in this con- 

 dition used in making cheese. Added to the milk in the pro- 

 portion of fifteen to twenty grains to a quart, it produces com- 

 plete coagulation. According to Robin and Yerdeil, caseine 

 is precipitated by the metallic salts, with which it forms com- 

 binations not to be distinguished from like combinations of 

 albumen. 1 It is a curious fact that caseine is sometimes 

 coagulated almost instantly during thunder storms. This 

 phenomenon we cannot fully explain; but the immediate 

 cause of the coagulation is the transformation of some of the 

 sugar of milk into lactic acid. Caseine retains its fluidity 

 in the milk by union with the carbonate of soda ; and when 

 coagulated spontaneously, it may be restored to its liquid 

 condition by the addition of this salt, which does not render 

 the fluid alkaline, but seems to enter into combination with 

 the organic substance. 



Caseine has its origin in the albumen of the blood, by a 

 catalytic process which takes place in the mammary glands. 

 In its liquid condition it constitutes the important organic 

 element of the milk. It is taken into the stomach of the 



1 Loc. tit. 



