292 



COLLOIDS AND ELECTROLYTES 



of K2SO4 has about the same depressing effect as the addition of 



H2SO4. 



When gelatin-acid salts, e.g. gelatin chloride, are separated from 

 distilled water or a salt solution of the same pH by a collodion bag, 

 water diffuses through the membrane in the form of negatively- 

 charged particles which are attracted by the gelatin cation and 

 repelled by the anion. When we wish to replace the gelatin solution 

 by a crystalloidal electrolyte in the presence of which water diffuses 



q 

 "00 



1 



180 

 160 

 140 

 120 

 100 



80 



60 



40 

 ^ 20 

 



4000 2000 1000 500 250 125 62 31 15 



Concentration of salt added 



Fig. 14. 



through the membrane in the form of negatively charged particles, 

 we have to fulfill two conditions. First, we must use as electrolytes 

 salts with trivalent (or tetravalent) cation, e.g. CeCls or AICI3, or if 

 we wish to use salts with monovalent or bivalent cation the salts 

 must be rendered sufficiently acid; e.g., pH = 4.0 or less. Second, 

 the collodion membrane must previously receive a treatment with 

 gelatin. The gelatin will be rinsed out, but some gelatin obviously 

 adheres to the wall. As I shall show in another paper this treatment 



