278 THEORY OF COLLOIDAL BEHAVIOR 



diffusing while the other is free to diffuse. In this case a Donnan 

 equilibrium is established resulting in an unequal distribution 

 of the diffusible ions on both sides of the membrane. The 

 forces resulting from this distribution of crystalloidal ions are 

 the only cause of those phenomena which are designated as 

 colliodal. 



In general the block in the diffusion of one kind of ions can 

 be brought about by two kinds of conditions: first, by mem- 

 branes with a selective permeability; and, second, by the cohesion 

 between ions of one kind. An electrolyte which exists in a true 

 solution (and this is in all probability true for crystalline egg 

 albumin) will show colloidal behavior in regard to osmotic pres- 

 sure and to P.D. when separated from pure water by a membrane 

 which is permeable for all ions in the solution except one. More- 

 over, when ions of one type are attracted to each other with 

 greater force than they are attracted by the molecules of the 

 solvent, aggregates are formed, and when these aggregates are 

 permeable to other ions than those forming the aggregate, a 

 Donnan equilibrium is also established and colloidal behavior 

 follows again. This latter condition leads to the colloidal 

 character of swelling, viscosity, and of the stability of suspen- 

 sions. Since 1 per cent solutions of crystalline egg albumin are 

 very stable at room temperature and as long as the pH is not too 

 low, there are few or practically no aggregates in such a solution, 

 and the crystalline egg albumin behaves in regard to viscosity 

 and in regard to the stability of its solutions essentially as if it 

 were in the crystalloidal state. Of course, the proof has to be 

 furnished that crystalline egg albumin exists in the form of 

 isolated molecules in agneous solution. S0rensen has calculated 

 the molecular weight of crystalline egg albumin from measure- 

 ments of the osmotic pressure of this substance and has obtained 

 results which make this conclusion probable. 1 



The behavior of gelatin is especially interesting. Gelatin 

 solutions in water show colloidal behavior in regard to osmotic 

 pressure and P.D. when the solutions are separated from pure 

 water by a collodion membrane or by some other membrane with 

 similar selective permeability. Gelatin solutions in water show 



1 S0RENSEN, S. P. L., Studies on proteins: Compt. rend. trav. Lab, Carlsberg, 

 vol. 12, Copenhagen, 1915-17. 



