578 CONTRIBUTION TO OUR KNOWLEDGE OF THE ACTION OF RENIN. 



addition of chalk or zinc white. In milk, as we see, a somewhat 

 similar result is attained by a different mechanism. Free acid 

 will not be present until the phosphate has been entirely con- 

 verted to the di-hydrogen form. Experimentally this has been 

 confirmed by examination of milk for free acid. Only a small 

 quantity of free acid corresponding to 1 c.c. or 2 c.c. y^NaOH 

 was present when 100 c.c. of milk required 14 c.c. to 18 c.c. 

 y^-NaOH for neutralisation. 



As the milk becomes acid, the phosphate will be present partly 

 as soluble sodium di-hydrogen phosphate and partly as soluble 

 sodium di-hydrogen phosj)hate and partly as slightly soluble 

 calcium hydrogen phosphate. Much calcium hydrogen phosphate 

 is present in suspension, not in solution. When this suspended 

 calcium hydrogen phosphate was removed by filtering acid milk 

 through porcelain, the clear transparent filtrate required only 

 a small amount of y^-NaOH to render it alkaline. Thus a sample 

 of milk, 100 c.c. of which required 16 c.c. y^-NaOH was filtered 

 through porcelain, and 100 c.c. of the filtrate required 4 c.c. 

 — NaOH to turn phenolphthalein pink. In this sample the free 

 lactic acid in 100 c.c. milk was equivalent to only 0-8 c.c. — - NaOH. 

 The addition of quantities of yp-NaOH to 5 c.c. milk as described 

 in the experiments in Table i., would thus lead to free hydroxyl 

 ions in the milk in concentration sufficient to destroy the rennet 

 before the necessary conversion of caseinogen into casein. 



In conclusion we would render our thanks to Professor Ander- 

 son Stuart, in whose laboratory these experiments were carried 

 out. 



