190 ANIMAL AMIDES. 



raach in cold water. A small quantity of a peculiar substance 

 is dissolved, to which the name pepsin has been given, because it 

 has the extraordinary property of dissolving food and converting 

 it into chyme in the stomach of living animals. A very minute 

 quantity of pepsin is sufficient to coagulate a great quantity of 

 milk. Berzelius evaporated a quantity of rennet to dryness in 

 a gentle heat He mixed one part of the dry residue with 1800 

 parts of milk, and heated the whole to 122. The whole casein 

 was so completely coagulated, that scarcely a trace of it could 

 be detected in the whey. The dry rennet being separated was 

 found to weigh 0*96. So that one part of pepsin coagulated 

 45000 parts of milk.* 



Coagulated casein, when pure and dried, is hard, translucent, 

 and yellowish. Unless it be well freed from all traces of butter, 

 it has a resinous lustre. This may be removed by digesting the 

 coagulated casein in ether, which dissolves the butter without al- 

 tering the casein. When put into water it softens and swells ; 

 but does not dissolve. When strongly heated before it has been 

 quite dried, it is rendered soft without melting, and becomes 

 elastic like caoutchouc. If the temperature be increased, it 

 swells, melts and burns with flame. The products obtained when 

 it is distilled are the same as those given by albumen. The com- 

 pounds of coagulated casein with acids and alkalies are similar 

 to those of uncoagulated albumen with the same bodies. But 

 when the acid is withdrawn by means of carbonate of barytes or 

 carbonate of lime, the casein does not dissolve in water, as hap- 

 pens with uncoagulated casein. 



Coagulated casein (or cheese, as it is called in common lan- 

 guage,) is soluble in concentrated sulphuric acid, from which it 

 is precipitated by water. It dissolves in nitric acid of 1*29, to 

 which it communicates a yellow colour. Muriatic acid dissolves 

 it very slowly, requiring to be continued for several days. The 

 solution, like that of albumen and fibrin, becomes blue, if the 

 temperature has exceeded 60. By degrees the colour changes 

 to a dirty violet When the acid is saturated with potash, the 

 colour disappears, and the cheese is precipitated greyish-white. 

 With concentrated acetic acid it forms a jelly, and dissolves when 

 we add water and apply heat But a great deal of acid is neces- 

 sary. It is very soluble in the hydrates and in the carbonates of 



* Traite de Chimie, vii. 601. 



