ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 255 



That the magnitude and even the sign of the charge represented by 

 the double layer depend upon the chemical nature of the phases was 

 indeed not completely overlooked by the earher investigators, but the 

 question could not have been satisfactorily answered before the 

 advent of the ionic theory. With the development of modern physical 

 chemistry this problem attracted much attention, and we are in- 

 debted chiefly to the work of Perrin for the description and inter- 

 pretation of the fundamental laws underlying these relationships. 



The basic idea of correlating all of these phenomena with the ionic 

 theory originated with Nernst, and the conception is really the same as 

 that which we appHed for the explanation of phase boundary po- 

 tentials. Let us imagine that both of our adjacent phases contain 

 ions. Distribution equilibrium is not reached, for the tendency 

 towards equilibrium, because of opposing electrostatic forces, results 

 in the appearance of a potential difference instead of equilibrium. 

 Thus the two strata of the double layer consist of free ions] Helmholtz 

 did not leave us a material conception of the nature of the electric 

 particles of the double laj^er. At present we can say that they con- 

 sist either of electrons or of ions. The possibiUty that electrons alone 

 constitute at least one of the two layers must be admitted in the case 

 where one of the phases is a metal. For the time being we shall deal 

 only with double layers consisting of free ions, and thus the phe- 

 nomena of endosmosis and cataphoresis become a part of our pres- 

 ent discussion of ''Ions as Sources of Electric Potential Differ- 

 ences," 



This renders the phenomenon of motion in electrophoresis entirely 

 comprehensible. But in addition the following assumption must be 

 made. The point of attack of the forces of the electric field is the 

 motile ionic layer, which in its motion carries along the water. Earlier 

 in our discussion of the conductivity of dissolved electrolytes we 

 have seen that the ions cannot move in water without carrying along 

 with them large numbers of water molecules. It is this strong attrac- 

 tion between ions and water molecules which is responsible also in 

 this case for the fact that the displacement of an ionic layer becomes 

 externally apparent through the displacement of a water layer. 



It is pertinent to draw an analogy between the carrjdng along of 

 water molecules bj^ ions in the simple conductance of an electric 

 current by an electrolyte in solution and that occurring in the process 

 of electroendosmosis, or, to conceive of the common electrolytic 



