JACQUES LOEB 



245 



not combine or remain in combination with anions when pH > 4.7. 

 We chose for this purpose two anions, Br and CNS, for the volu- 

 metric determination of which we possess the convenient Volhard 

 method. Powdered gelatin was first treated with different concen- 

 trations of HNO3 and then washed free from the excess of acid. Then 

 the gelatin in each funnel was perfused three times with 25 cc. of 

 m/8 NaBr and this was followed by six perfusions with 25 cc. of H2O. 

 The swelling was plotted (upper curve in Fig. 4), the gelatin was 

 melted and made into a 1 per cent solution, and analyzed for Br by 

 the Volhard method. The lower curve gives the quantity of Br in 

 combination with 25 cc. of gelatin. Table II gives the amount of 

 Br found in combination with 0.25 gm. of gelatin. The table, as 

 well as the curve, shows that the treatment of gelatin with m/8 NaBr 

 did not lead to the formation of gelatin bromide when pH > 4.7; 

 while it led to the formation of gelatin Br when pH < 4.7. The 

 quantity of gelatin bromide formed increased with the hydrogen ion 

 concentration. 



TABLE II, 



In the experiment represented by Fig. 5 everything was the same 

 except that the treatment of gelatin with HNO3 was followed by three 

 perfusions with m/8 NH4CNS (instead of m/8 NaBr). The lower 

 curve shows that when pH > 4.7 the gelatin contains no CNS, 

 but that gelatin can combine with CNS as soon as pH < 4.7. 



The results are given in Table III. The reader's attention is 

 called to the sharp drop in the values for Br (Table II) as well as for 

 CNS (Table III) at pH = 4.7 and less. 



