192 THEORY OF COLLOIDAL BEHAVIOR 



from which it follows that: 



X = co 



It is clear from this that, as x increases from zero, e must increase to a 

 maximum and then decrease, approaching zero asymptotically, regard- 

 less of whether or not the ionization of the gelatin salt is appreciably 

 repressed." 1 



As far as the depressing action of salt on swelling is concerned, 

 Procter and Wilson do not accept the idea that it is due to the 

 repression of ionization. 



" Whilst the salt undoubtedly represses the ionisation of the gelatin 

 chloride to some extent, it would scarcely be sufficient to account 

 for the fact that salt reduces the volume of jelly almost to that of dry 

 gelatin. The chief action is probably that the addition of salt corre- 

 sponds with an increase in the value of #, and that this increase in x 

 must, according to the equation just discussed, produce a decrease in 

 the value of e, with a corresponding diminution of the volume of the 

 jelly." 1 



There can be little doubt that the osmotic theory of Procter 

 and Wilson accounts quantitatively for the process of swelling; 

 no other theory has thus far been offered which can claim the 

 same result. 



The force which opposes and limits the swelling is the cohesion 

 between the molecules or ions constituting the gel. When this 

 force is diminished the swelling should increase. Procter and 

 Wilson have pointed out that this is the case since the swelling 

 of gelatin increases when the gel is heated. 2 



The forces of cohesion depend not only on temperature but 

 also on chemical constitution. They are forces of the same kind 

 as the forces determining solution; and it is well known that, e.g., 

 the substitution of Na for H in oleic acid increases the solubility 

 of the substance in water, and that the substitution of K for Na 

 increases the solubility still more. We might a priori expect that 

 the forces of cohesion in a solid jelly of gelatin would also change 

 considerably with the nature of the ion in combination with 



1 PROCTER, H. R., and WILSON, J. A., /. Chem. Soc., vol. 109, p. 317, 1916. 



2 PROCTER, H. R., and WILSON, J. A., J. Chem. Soc., vol. 109, p. 315, 1916. 



