352 ELECTRICAL CHARGES OF COLLOIDAL PARTICLES 



suspensions by low concentrations of a salt was caused by an annihila- 

 tion of the charge of the colloidal particle. The problem of the sta- 

 bility of the colloidal suspension then developed into the problem of 

 accounting for this peculiar behavior of the electrical charges of 

 colloidal particles. 



Hardy's original idea was that the H ions of the acid or OH ions of 

 the alkali were adsorbed by the colloidal particle in preference to the 

 other ions on account of their greater rapidity of migration; and this 

 idea was also accepted by Perrin in his experiments on electrical 

 endosmose, where it was necessary to account for the fact that certain 

 membranes become positively charged in the presence of acid and 

 negatively in the presence of alkali.^ Those who accept this adsorption 

 hypothesis explain the fact that the electrical charges of the particles 

 are apparently diminished or destroyed by the addition of a salt on 

 the assumption of a preferential adsorption of one of the ions of the 

 salt; yet such an assumption is incompatible with the purely 

 stoichiometrical behavior of proteins. It is also difficult to account 

 for the fact that the addition of little acid increases while the addition 

 of more acid depresses the electrical charge of micellae on the basis 

 of the adsorption hypothesis. 



A second possibility was pointed out by the writer in 1904; namely, 

 that Hardy's migration experiments might be explained in the case 

 of proteins by the fact that proteins are amphoteric electrolytes which, 

 in the presence of alkali, dissociate electrolytically by giving rise to a 

 protein anion and, in the presence of acid, by giving rise to a protein 

 cation while at the isoelectric point no protein ion would be formed.^ 

 While this idea is correct if applied to the migration of isolated 

 protein ions in the electrical field, it cannot explain why the addition 

 of a salt in low concentration should diminish the charge of aggre- 

 gates of molecules and ions, the micellae, except by assuming that 

 in this case the electrolytic dissociation of the protein salts should 

 be repressed. The concentration of salts required for the precipi- 

 tation of colloidal suspensions is, however, much too small to make 

 such a suggestion acceptable. 



2 Perrin, J., /. chim. physique, 1904, ii, 601; 1905, iii, 50. 



3 Loeb, J., Univ. California Pub., Physiol., 1903-04, i, 149. 



