198 THE INCOME OF ENERGY 



(Rennin, 1 to 250, is a pastille of Hansen dis- 

 solved in 250 c.c. H, 2 0.) 



Keep both mixtures at 38 C. during forty 

 minutes. 1. Boil 25 c.c. from each solution. 

 Solution A coagulates, while solution B shows 

 no trace of coagulation. Hence the action of 

 rennin has rendered the casein in A coagulable 

 on boiling. 



2. To 25 c.c. from each solution add 8 c.c. of 

 a solution of calcium chloride capable of pre- 

 cipitating exactly, in equal volumes, the solution 

 of potassium oxalate. By this addition any ex- 

 cess of potassium oxalate is removed and the 

 calcium chloride remains in slight excess. 



A will coagulate ; B will not. Hence the casein 

 in solution A has been so changed by rennin that 

 it is precipitated on the addition of a small quan- 

 tity of calcium chloride. Solution A may also 

 be precipitated by restoring its original content 

 of calcium chloride, i.e. by adding 5 c.c. of the 

 above calcium chloride solution, which will exactly 

 combine with the 5 c.c. of potassium oxalate. 



If small quantities of rennin be added to 

 natural milk and equal portions of the milk be 

 tested from time to time by boiling, the amount 

 coagulated will be greater the longer the rennin 

 acts. An amount of calcium chloride too small 

 to produce coagulation in the early stages of 



