84 



DONNAN EQUILIBRIUM AND VISCOSITY 



SO rapidly that the viscosity measurements were no longer possible 

 for a concentration of 3.5 per cent gelatin solution, and for this 

 reason the value of the viscosity of a 2 or 3 per cent solution is already 

 too high on account of the mechanical hindrances of the flow of the 

 solution through the viscometer owing to partial solidification. 



From these experiments we may therefore draw the conclusion 

 that since the viscosity measurements of gelatin solutions agree 

 fairly well with Arrhenius's viscosity formula the variation of the 

 viscosity of gelatin solutions must be due to variations in the relative 

 volume occupied by the gelatin in solution ; and since the gelatin used 

 in these experiments was isoelectric, these variations cannot possibly 



TABLE III. 



be ascribed to a hydration of gelatin ions. The only other way by 

 which the isoelectric gelatin particles can increase their volume is 

 through the occlusion of water by submicroscopic particles of solid 

 jelly. Since the amount of water occluded by solid jelly is regulated 

 by the Donnan equilibrium this will explain why the influence of 

 electrolytes on the viscosity of a gelatin solution is similar to that 

 of electrolytes on the viscosity of suspensions of powdered gelatin 

 in water. 



The connection between the tendency of gelatin solutions to set 

 to a jelly and their high viscosity lies therefore in this, that the setting 

 to a jelly is preceded by the formation of submicroscopic solid particles 

 of gelatin capable of swelling and of occluding comparatively large 



