CHAP. II.] THE MILK-CURDLING ENZYME. 149 



extract of the stomach of the calf may be used, as Hammarsten first 

 shewed, instead of the solution prepared as stated above ; such a 

 glycerin-extract can be preserved permanently, and is very active. 

 Erlenmeyer 1 has shewn that a saturated aqueous solution of salicylic 

 acid extracts the milk-curdling ferment, very perfectly, from the 

 stomach of the calf; from this solution, the ferment, mixed with other 

 matters, can be precipitated by alcohol. The precipitate thus obtained 

 is soluble, in great part, in water, and the solution is active. 



The milk- Hammarsten separated the ferment free from pepsin 



curdling fer- by the fractional precipitation of a solution containing 

 ment separa- both enzymes, with lead acetate. He first prepared a 

 ted from pep- JJC1 infusion of the gastric mucous membrane by the 

 aid of dilute HC1, and this infusion he neutralised by 

 repeatedly shaking it with magnesium carbonate ; he then added, to 

 the neutralised solution, enough lead acetate to precipitate the whole 

 of the pepsin as completely as possible, so that when the filtrate 

 was acidulated and digested for 24 hours with a flake of fibrin 

 it was not perceptibly dissolved. To the liquid from which pepsin 

 had been precipitated there was added more acetate of lead and a 

 little solution of ammonia. The precipitate thus obtained was de- 

 composed with highly diluted sulphuric acid. Thus the milk-curdling 

 ferment which had been precipitated by the second addition of lead 

 acetate was set free. From the acid solution, the ferment was 

 obtained, by a modification of the cholesterin-process employed by 

 Bru'cke for the separation of pepsin. 



Characters A solution of the pure milk-curdling ferment, pre- 



of solutions pared as described above, possesses the following reac- 



of the pure (?) tions: it does not give the xantho-proteic reaction; it 



u^z is not coagulated by heat; it is not precipitated by 



alcohol, nitric acid, tannin, iodine, nor by sugar of lead, 



but by basic lead acetate. 



The milk-curdling ferment is not diffusible ; it does not pass 

 through porous earthenware unless the pressure be very high. 



Various 1. Alcohol slowly destroys the activity of the fer- 



j which ment the rap i(iity of the action being influenced by the 

 dimmish, or '. c r , , J , J 



destroy, the quantity of alcohol. 



action of the 2. Fixed caustic alkalies have a powerfully destruc- 



miik-curdiing tive action. If a solution rich in ferment contains as 

 much caustic soda as corresponds to 0'025 per cent, of 

 Na 2 0, it loses its activity entirely within 24 hours at a temperature 

 of 15 17C. 



3. Rennet ferment is also destroyed by 0'5 to 1 per cent, of 

 Na 2 C0 3 2 . Since rennet-zymogen is much less readily destroyed, 



1 Erlenmeyer, ' Darstellung der ungeformten Fermente.' Communicated to the 

 Acad. of Sciences of Munich in 1875 and abstracted in Mary's Jahresbcricht, Vol. v. 

 p. 267. 



2 Langley, Journal of Physiology, Vol. in. (1882), p. 259. 



