60 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



The influence of added substances on the osmotic pressure of colloidal solutions 

 is of considerable interest to the biologist, for, whereas in the case of molecular solu- 

 tions this is purely additive, in the case of colloids the added substance may at one 

 time cause the osmotic pressure to increase, at another, to decrease. It has been found 

 that the osmotic pressure of gelatin solutions at first decreases, then rapidly increases 

 as the H-ion concentration is raised. The addition of alkali increases the osmotic 

 pressure until a maximum is reached, beyond which it begins to fall. Both acids and 

 alkalies lessen the osmotic pressure of egg albumin. Electrolytes always decrease the 

 osmotic pressure of gelatin and albumin solutions, and the degree to which they exert 

 this influence depends on the nature of the cation and anion composing the electrolyte. 

 In the order of their depressing influence the cations arrange themselves: 



Heavy metals > alkaline earths > alkalies; 

 and the anions: 



S0 4 > 01 > NO o > Br > I > CNS. 



The influence of a given electrolyte varies extraordinarily with the reaction of the 

 colloid, a fact which must be carefully regarded in all work in this field. 



