32 REPORTS ON THE STATE OF SCIENCE.—1918. 
4. Theories of Electrical Endosmose. Contact Electrification and 
Adsorption. 
Wiedemann believed that the electric current exerted a tractive 
action upon the liquid in a capillary tube and that the liquid was 
carried thus from anode to cathode, regardless of the substance 
composing the walls. In opposition to this view, Graham, Quintus 
Icilius and Breda and Logemann,** showed conclusively that no 
transport of liquid occurred unless a diaphragm or its equivalent 
were present. As I observed in the earlier pages of this paper, we 
owe to Quincke and to Helmholtz the electrical double layer theory 
of contact electrification. The Coehn rule is an interesting develop- 
ment of this theory. But both Quincke and Helmholtz contributed 
little to further our understanding of why and how an electrical 
double layer or its equivalent may be formed when solid and liquid 
are placed in contact. 
For the special case of a metal in contact with its own ions in 
solution the Nernst theory holds. It is only natural that there 
should have been proposed an analogous explanation of the contact 
potentials of non-metallic solids. Every solid dissolves in water to a 
certain extent and the electrical double layer might be supposed to 
be produced by differences in the rates of ion diffusion—if the 
cation diffused faster, a separation of charges would tend to occur 
and the solid (or liquid immediately in contact with the solid) 
would become negative. Bredig*t has given vague expression to 
this idea by observing that the Coehn rule points to some relation 
between the solubility of ions in different media and the dielectric 
constant. 
As a matter of fact, the explanation outlined above cannot be the 
correct one for the case of a solid immersed ina pure liquid, unless 
one postulates that the liquid remains unsaturated with respect to 
the solid or that fresh liquid is being supplied constantly. The 
electrical charge does not disappear when the liquid is saturated with 
the particular solid, although its sign and intensity may change, 
while the potential difference at the liquid interface in a concentration 
cell exists only so long as a difference in concentration is maintained. 
No permanent potential difference can possibly be produced as the 
result of unequal ion mobilities. Perrin has offered an explanation 
of his acid-alkali rule by postulating that, since hydrogen and 
hydroxyl] ions are abnormally mobile, they are correspondingly small, 
and are able thereby to crowd to the surface of a sclid more closely 
than the other ions. This might account for the solid being positive 
in acid and negative in alkaline solutions. Perrin was unable, 
nevertheless, to reconcile his theory with all the facts, for he 
observed that lithium bromide failed to charge a chromic chloride 
diaphragm negatively although bromine ions are twice as mobile as 
lithium ions. 
Haber*® has suggested for the particular case of glass against 
water that the solid is essentially a hydrogen electrode, and that the 
8 (2) Phil, Mag. (4) 8, 151 (1854). 
33° (3) Pogg. Ann., 100, 149 (1857). 
4 Zeit. Elektrochemie, 9, 738 (1903). 
35 Haber and Klemensiewicz, Zeit. Phys. Chem., 6'7, 413 (1909). 
