THEORIES OE ELECTROLYSIS. 



307 



and the driving force in any layer can be calculated from 

 a knowledge of the variation in osmotic pressure gradient. 

 If we replace the solution of sugar by one of hydrochloric 

 acid, a further complication ensues. We know from Kohl- 

 rausch's work, that the velocity with which the hydrogen 

 ions travel through a solution is, under an equal driving 

 force, greater than the velocity of the chlorine ions, so 

 that, at first, a separation goes on, positively charged hy- 

 drogen ions travelling quickly into the water, and negatively 

 charged chlorine ions remaining behind. This at once 

 explains the differences in potential observed when various 

 solutions are put in contact with pure water. But, as this 

 process of separation goes on, electrical forces, tending to pre- 

 vent further separation, come into play between the oppositely 

 charged ions, and presently a steady state is reached and 

 the ions travel forward together. We can then equate the 

 number of hydrogen ions crossing any layer in a given time 

 under the influence of both the osmotic and the electric 

 forces with the number of chlorine ions crossing the same 

 layer, and from this calculate the actual rate of diffusion of 

 the hydrochloric acid. The following table gives the 

 observed and calculated values of the " diffusion constant," 

 which may be defined as the number of gram-molecules of 

 the dissolved substance which crosses unit area in one day 

 when the osmotic pressure gradient is unity. 



In a similar way, the contact difference of potential 

 between solutions of the same electrolyte at different con- 



