152 THEORY OF COLLOIDAL BEHAVIOR 



available at that time and this was probably the reason that his 

 suggestion was not accepted. 



We may consider a protein solution inside a collodion bag and 

 surrounded by a watery solution as a model of a protein micella 

 suspended in a watery solution. In that case it can be shown 

 that the electrical charge of such a model varies in exactly the 

 same way as the charges of colloidal particles in suspension, 

 e.g., coagulated egg albumin. 



1. The electrical charge of the micella model (i.e., gelatin solu- 

 tion in a collodion bag) is zero at the isoelectric point. 



2. The charge of the model is positive on the acid side and 

 negative on the alkaline side of the isoelectric point of gelatin and 

 of crystalline egg albumin. 



3. The charge of the model increases with the addition of little 

 acid and diminishes with the addition of more acid to isoelectric 

 particles. 



4. The charge of the model is diminished by the addition of low 

 concentrations of neutral salts, and the depressing action of the 

 salt increases rapidly with the valency of that ion of the neutral 

 salt which has the opposite sign of charge to that of the micella. 



It has been shown in the preceding chapter that these facts 

 can be explained not only qualitatively but quantitatively from 

 the theory of Donnan's membrane equilibrium. This quantita- 

 tive agreement leaves no doubt that the electrical charge of this 

 micella model is caused exclusively by the Donnan equilibrium. 



2. THE ELECTRICAL CHARGE OF SUSPENDED PARTICLES OF 

 POWDERED GELATIN 



We may ask whether it is justifiable to consider a gelatin solu- 

 tion enclosed in a collodion bag and surrounded by an aqueous 

 solution as a model of a micella. This is theoretically correct 

 since the colloidal behavior of both a true micella as well as the 

 protein solution in a collodion bag is due to the same condition, 

 namely, that the protein ion is prevented from diffusing into the 

 outside aqueous solution while no such block exists for the diffu- 

 sion of the crystalloidal ions. The block which prevents the 

 diffusion of the protein ions in the model is the collodion mem- 

 brane, while in the case of the micella (or a solid jelly) it is the 



