JACQUES LOEB 



241 



treated. Under each concentration is put the pH found for the gelatin 

 solution at the end of the experiment. The ordinates of the lower 

 curve are the numbers for the cc. of 0.01 n silver in combination with 

 0.25 gm. of gelatin. The curve shows that at the isoelectric point 

 (pH = 4.7) the gelatin is free from ionic silver and that the same is 

 true for all the gelatin on the acid side from the isoelectric point (pH 

 3.6 to 4.7). On the alkaline side from the isoelectric point the 



Fig. 2. Photograph of the gelatin solutions whose curves are contained in Fig. 

 1, taken a week after the experiment was made. The gelatin with a pH > 5.0 

 turned dark brown in a few hours when exposed to diffused daylight, while the 

 gelatin with a pH < 4 . 7 was not affected by light after an exposure of over a week. 

 The photograph also shows that owing to the precipitate in suspension the tubes 

 containing the gelatin at the isoelectric point, pH = 4.6 and 4.7, were more 

 whitish than the rest, which were slightly yellow. 



amount of ionic silver in combination with gelatin rises in a regular 

 curve increasing in height with the increase in pH. 



20 cc. of each gelatin solution were put into a test-tube and exposed 

 to the light. The five test-tubes that had been treated with m/1024 

 or less concentrated HCl turned rapidly dark or black in the light, 

 while none of the others did, even when made slightly alkaline and 

 though the exposure lasted for more than a week. The gelatin treated 



