244 AMPHOTERIC COLLOIDS. II 



swelling of the gelatin was measured while it was in the cylindrical 

 funnel and the height of the column of gelatin in mm. expresses the 

 degree of swelling, all the funnels having the same diameter. The 

 gelatin was then melted, made into a 1 per cent solution, and the con- 

 ductivity, osmotic pressure (in mm. of the height of the gelatin solu- 

 tion), alcohol number (cc. of 95 per cent alcohol required to cause a 

 definite degree of precipitation in 5 cc. of 1 per cent gelatin solution) 

 were determined. There is an abrupt fall in all the curves near pH 

 4.6; i.e., at about the isoelectric point, and a rise on both sides. On 

 the right from the isoelectric point (pH > 4.7) the gelatin exists as 

 silver gelatinate, while on the left, more acid side from the isoelectric 

 point (pH < 4.7) the gelatin exists as gelatin nitrate. At the iso- 

 electric point it exists as non-ionized gelatin. This was proved by 

 exposing the gelatin solutions to the light after completion of the 

 experiment which was of course carried out in a dark room ; all those 

 gelatin solutions whose pH was from 5.3 to 6.4 turned black when 

 exposed to Hght, while none of the others was affected by light even 

 after an exposure of more than 2 weeks. Since the original treatment 

 of the gelatin with m/16 AgNOs near the point of neutrality was the 

 same for all the solutions, it is obvious that the silver gelatinate 

 when its pH was 4.7 or less gave off part if not practically all its silver. 

 This was actually proved to be the case by the analysis of the filtrate 

 of such gelatin during and after the acid treatment. 



When we treat the gelatin first with HNO3, wash the acid away, and 

 then treat the gelatin with m/16 AgNOs and wash away the excess of 

 salt, we get an identical system of curves and an identical effect of 

 exposure to light, proving that gelatin can only combine with Ag 

 when its pH > 4.7, while it cannot combine with gelatin when its 

 pH ^ 4.7. 



The writer would like to call attention to the fact shown in Figs. 1 

 and 3 that for pH > 4.7 the curves for swelling and for conductivity 

 are practically parallel to the curve for the amount of silver gelatinate 

 formed. 



The Combination of Anions with Gelatin. 



In this chapter we intend to furnish the data necessary for proving 

 that gelatin combines with anions only when pH < 4.7, while it can- 



