JACQUES LOEB AND ROBERT F. LOEB 211 



the casein ions tending to form an aggregate, whereby the protein 

 ions of the nascent micellum are forced apart again; and second, to 

 the potential difference between the surface of a micellum and the 

 surrounding solution (also regulated by the Donnan equilibrium) 

 which prevents the further coalescence of micella already formed. 

 This latter consequence of the Donnan effect had already been sug- 

 gested by J. A. Wilson. 



4. The precipitation of this group of hydrophobic colloids by salts 

 is due to the diminution or annihilation of the osmotic pressure and 

 the p.D. just discussed. Since low concentrations of electrolytes 

 suffice for the depression of the swelling and p.d. of the micella, it is 

 clear why low concentrations of electrolytes suffice for the precipita- 

 tion of hydrophobic colloids, such as casein chloride. 



5. This also explains why only that ion of the precipitating salt is 

 active in the precipitation of hydrophobic colloids which has the 

 opposite sign of charge as the colloidal ion, since this is always the 

 case in the Donnan effect. Hardy's rule is, therefore, at least in 

 the precipitation of casein chloride, only a consequence of the Donnan 

 effect. 



6. For the salting out of hydrophiUc colloids, like gelatin, from 

 watery solution, sulfates are more efficient than chlorides regardless 

 of the pH of the gelatin solution. Solution experiments lead to the 

 result that while CaCl2 or NaCl increase the solubility of isoelectric 

 gelatin in water, and the more, the higher the concentration of the 

 salt, Na2S04 increases the solubility of isoelectric gelatin in low 

 concentrations, but when the concentration of NaoS04 exceeds m '32 

 it diminishes the solubility of isoelectric gelatin the more, the higher 

 the concentration. The reason for this difference in the action of the 

 two salts is not yet clear. 



7. There is neither any necessity nor any room for the assumption 

 that the precipitation of proteins is due to the adsorption of the ions 

 of the precipitating salt by the colloid. 



