210 SOLUTION AND PRECIPITATION OF PROTEINS 



are true solutions consisting of isolated gelatin ions and molecules 

 which may contain, in addition, submicroscopic aggregates of gelatin 

 molecules and ions. The formation of new micella and a solution of 

 these already formed occur constantly. When the rate of formation 

 of such micella exceeds the rate of the solution, the viscosity of a 

 gelatin solution rises on standing and the rise is in proportion with the 

 difference in the rate of the formation and solution of these micella. 

 The reader must bear in mind that the micella in the case of gelatin 

 solutions are submicroscopic particles of a reversible jelly of gelatin 

 and not particles of an irreversible precipitate. 



SUMMARY. 



1. Colloids have been divided into two groups according to the 

 ease with which their solutions or suspensions are precipitated by 

 electrolytes. One group (hydrophilic colloids), e.g., solutions of gela- 

 tin or crystalline egg albumin in water, requires high concentrations 

 of electrolytes for this purpose, while the other group (hydrophobic 

 colloids) requires low concentrations. In the latter group the precipi- 

 tating ion of the salt has the opposite sign of charge as the colloidal 

 particle (Hardy's rule), while no such relation exists in the precipita- 

 tion of colloids of "the first group. 



2. The influence of electrolytes on the solubility of solid Na casei- 

 nate, which belongs to the first group (hydrophilic colloids), and of 

 solid casein chloride which belongs to the second group (hydrophobic 

 colloids), was investigated and it was found that the forces 

 determining the solution are entirely different in the two cases. 

 The forces which cause the hydrophobic casein chloride to go 

 into solution are forces regulated by the Donnan equilibrium; 

 namely, the swelling of particles. As soon as the swelling of a solid 

 particle of casein chloride exceeds a certain limit it is dissolved. The 

 forces which cause the hydrophilic Na caseinate to go into solution 

 are of a different character and may be those of residual valency. 

 Swelling plays no role in this case, and the solubility of Na caseinate 

 is not regulated by the Donnan equilibrium. 



3. The stability of solutions of casein chloride (requiring low 

 concentrations of electrolytes for precipitation) is due, first, to the 

 osmotic pressure generated through the Donnan equilibrium between 



