270 



THEORY OF COLLOIDAL BEHAVIOR 



cohesion of particles of casein-acid salts. The forces of cohesion 

 in the case of casein sulphate and casein trichloracetate seem to be 

 so great that they cannot be overcome by the osmotic pressure 

 due to the Donnan equilibrium; and hence, no swelling (and as a 

 consequence no solution) of solid casein is possible in H 2 SO4 or 

 trichloracetic acid. The influence of valency on the Donnan 

 equilibrium is the same in the case of the swelling of casein and of 



5.5 



5.0 



4.5 



4.0 



3.5 



3.0 



X 



Depressing effect of 

 NaCl on swelling 

 and solution of 

 casein in acid 



\ 





1524 



Concentration of NaCl 



I 



FIG. 80. Depressing action of NaCl on swelling and solution of casein in acid. 



gelatin; what is different is the influence of certain ions on the 

 relative affinity of casein ions for water and for each other. 



Procter and Wilson have shown that the theory of the Donnan 

 equilibrium explains the depressing effect of a salt on the swelling 

 of solid gelatin. Microscopic measurements of the influence of 

 NaCl on the rate of swelling of individual grains of casein particles 

 in M/100 HC1 were made at 24C., and the results plotted in 



