ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 39 
magnitude of the potential difference depends upon the concentration 
of hydrogen ions in solution. Guided by Haber’s theory, Cameron 
and Oettinger*® performed some rather inconclusive experiments on 
the potential produced by acid and alkaline solutions forced through 
a capillary of glass. Haber’s theory,as Freundlich points out, is 
open to the serious objection that electrical endosmose does not 
depend entirely upon hydrolysis of the dissolved solute, as it should 
do if the concentration of hydrogen ions is the only factor. Acidity 
and alkalinity, while of very great influence, are not the only factor 
determining electro-osmotic effects. 
Freundlich*” was the first to point out clearly the intimate 
relations existing between adsorption and electrical endosmose. 
This development may be said to be the result of his own work on 
adsorption and the speculation of Perrin*®* regarding the analogies 
between the behaviour of suspensions and the peculiarities of 
electrical endosmose. Freundlich went still further than Perrin. 
He had already shown the relation between adsorption and the 
stability of suspensions and had pointed out the validity of the 
Schulze valence rule for adsorbed light-metal caticns and the 
ordinary anions. He had discovered, too, that many organic ions as 
well as the heavy metal cations were exceptions to the Schulze rule. 
He was able, as a result, to emphasize the similarity between the 
Schulze rule and the valence rule of Perrin, and, from Elissafoff’s 
data, he showed that exceptions to Schulze’s rule were exceptions 
likewise to Perrin’s. From this it was only a step forward to 
apply to electro-osmutic phenomena a definite theory based upon 
adsorption, or more specifically, upon the selective or preferential 
adsorption of ions. 
Freundlich called the difference of potential between solid and 
solution, the “adgorption potential.” If a cation is adsorbed to a 
greater extent than the accompanying anion, the solid becomes 
positively charged, and if it is employed as a diaphragm, the 
electrical endosmose will occur from cathode to anode. For the case 
of a solid against pure water it is only necessary to postulate 
selective adsorption, either of the ions already present in water or 
produced by the solid dissolving. When preferential ion adsorption 
occurs, the number of ions actually adsorbed is very small, because 
the charge on a single ion is relatively large and the electrical double 
layer that is established opposes any further spatial separation of 
positive and negative ions. 
Freundlich*®® has recently modified his original theory of the 
the adsorption potential, without improving matters appreciably and 
not without adding several complicating hypotheses. He suggests 
that the contact potential of the diaphragm depends upon the nature 
of the material of which it is composed (upon differences in solution 
tension of the ions thrown out) and that it is affected only indirectly 
by adsorption. In my opinion contact potential does undoubtedly 
_ depend upon the nature of the solid itself, but the important thing 
36 Phil. Mag. (6), 18, 586 (1909). 
37 Kapillarchemie, 245 (1909) ; Zit. Phys. Chem. "79, 407 (1912). 
38 Jour. Chim. Phys., 3, 85, (1905). 
39 Freundlich and Elissafoff, Zeit. Phys. Chem., 79, 407 (1912). 
20895 B 
