194 THEORY OF COLLOIDAL BEHAVIOR 



grains of isolectric gelatin are put into water of pH 4.7, a consid- 

 erable swelling occurs. The swelling must be determined by 

 forces different from those set up by the Donnan equilibrium. 

 In the first place, there are those forces of chemical attraction 

 between the molecules of water and certain of the groups of the 

 gelatin molecule which cause the solution of gelatin in water when 

 the forces of cohesion between the gelatin molecules forming the 

 gel can be overcome. The absorption of water by dry grains of 

 isoelectric gelatin at pH 4.7 is, therefore, primarily but in all 

 probability not exclusively due to the residual valency forces, and 

 the swelling of solid isoelectric gelatin granules is primarily a 

 phenomenon of solid solution. 



