PROPERTIES OF GELATINE AND GLUE 213 

 maximum will in many cases be determined largely by the 

 value of a^i a^. This factor, therefore, demands 



particular consideration. 



Now, unfortunately, the adsorption law constants for 

 the different ions have not yet been numerically determined, 

 so that we are still somewhat in the dark as to the operation 

 of ionic adsorptions. It is possible, however, to form con- 

 clusions of a qualitative or relative order, and these are such 

 as to throw much light upon the question at issue. In the 

 first place, we know that in general the various ions are not 

 usually very widely different in the extent to which they are 

 liable to be adsorbed. If this were otherwise, the valency 

 rule would hardly operate so well in endosmosis, kata- 

 phoresis, and precipitation. In consequence we must expect 

 the differences between the ions to appear in small rather 

 than in large concentrations, the amounts adsorbed being 

 under those conditions more affected by changes in the 

 volume concentration. At the larger concentrations, there- 



fore, the value of a^ a 2 cn* is small, and the force causing 

 swelling often tends to zero. 



There are, however, noticeable differences at lower con- 

 centrations. Thus we know that if a substance be primarily 

 a positive colloid, it will absorb kations more readily than 

 anions. As gelatine falls into this class, we may therefore 

 conclude that usually x > a 2 . Further, it often happens 

 that very adsorbable substances are less affected by concen- 

 tration changes, and in the case under consideration, there- 

 fore, we should expect that HI > n 2 . Moreover, we know 

 that the hydrion and hydroxyl ion are much more readily 

 adsorbed than other ions, i.e. have a large value for a. 



Hence in the case of gelatine we expect that a^ a^ will 



have a comparatively large value when one of the ions is 

 H + or OH ~. Also we know that organic anions are usually 

 much more strongly adsorbed than inorganic anions, and 

 hence that in such cases a\ is more nearly approached by the 

 value of a 2 . It should be emphasized perhaps, at this point, 



