34 REPORTS ON THE STATE OF SCIENCE.—1918. 
is the power of the solid to adsorb selectively the ions present in the 
liquid from the beginning, or produced by the solid dissolving.*° 
Frazer and Holmes"’, following a suggestion rejected by Whetham,* 
have advanced a distinctly different hypothesis based upon the 
solvate theory. If the hydration of the cation is greater than that 
of the anion, for example, liquid should be carried from anode to 
cathode ; and this is what the authors found to be the case with 
neutral salts of the alkalies against earthenware. Since the mobility 
of an ion might be regarded as an inverse measure of its hydration, 
one could expect a strong flow to the cathode in an alkaline solution 
and a strong flow to the anode in an acid solution. This deduction 
agreed with the facts as far as they were determined by Frazer and 
Holmes. Barratt and Harris, who favoured a similar theory, were 
rather overwhelmed by its consequences, for they calculated that 
anywhere between 18 and 370 molecules of water were transported 
through agar diaphragms for every molecule of solute decomposed 
electrolytically.* 
The solvate theory of Frazer and Holmes cannot be correct, for 
one would have to conclude, in consequence, that there is a funda- 
mental difference between electrical endosmose and Quincke’s dia- 
phragm currents. Moreover the theory assigns to the diaphragm a 
subordinate and purely mechanical réle. Neither does it explain 
why mere traces of lanthanum salts, for instance, reduce the flow 
of an alkaline solution, or why an acid solution flows to the cathode 
through powdered glass. If there is any relation between endosmose 
and ion mobility, it is better to consider it an indirect one, produced 
by a possible relation between ion mobility and ion adsorption, as I 
pointed out when discussing Perrin’s speculations. 
After careful consideration of the facts in the case, it would 
appear that the most satisfactory working hypothesis to account for 
contact electrification of this type (and hence to serve as basis to an 
understanding of electrical endosmose and cataphoresis) is the one 
proposed originally by Freundlich and emphasized recently by 
Bancroft.“ 
By virtue of their surface properties solids are able to adsorb 
substances from a liquid with which they are in contact. They may 
adsorb a particular ion preferentially, in which case we have selective 
ion adsorption and either a positive or negative charge on the solid. 
The adsorbing substance tends to be peptized by the adsorbed ion. 
40 The modified Freundlich theory of the ionization of colloid aggregates, 
according to which particles of a basic nature, such as ferric hydroxide, are believed 
to give off OH ions to the liquid and become positively charged, while particles of. 
acid nature, such as silica, give up Hicns to the liquid and become electro-negative. 
The suggestion has received the support of Zsigmondy and many others. While it 
is easy thus to account for the charge borne by substances such as silica and ferric 
hydroxide. it is by no means so simple to do so with substances such as sulphur, 
carborundum, and diamond, all of which are charged electrically when placed in 
contact with water or suitable aqueous solutions. Compare Burton, The Physical 
Properties of Colloid Solutions (1916). 
41 Am. Chem. Jour., 40, 319 (1908). 
42 Theory of Solution, 292 (1902). 
43 In connection with Frazer and Holmes’ theory, see Bethe und Toropoff, Zeit. 
Phys. Chem., 89, 597 (1915). 
44 Jour. Phys. Chem., 16, 312 (1912); Trans. Am. Electrochem. Soc., 21, 233 (1913). 
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