52 IONISATION 



this gives rise to a difference of electrical potential, which may 

 explain some of the anomalies of diffusion through living animal 

 membranes. 



Of course there exists an enormous electrostatic attraction 

 between ions of opposite sign. The introduction of other electro- 

 lytes into a solution may therefore alter not only the rate of 

 migration of the original ions but the nature of the surface charge. 

 The addition of HC1 to a solution of KC1 would increase the 

 diffusion potential that would be produced at the boundary 

 between solutions of KC1 at different concentrations ; the more 

 HC1 present, the greater the diffusion potential. 



This is due of course to the relatively greater speed of the 

 hydrogen ion. The K ions move at about the same speed as 

 the Cl ions, while the H ions move about five times as fast. The 

 boundary surface previously charged negatively with a low E.M.F. 

 would take on a positive charge with a higher E.M.F. 



It is imperative to note that unless the electrostatic force 

 mutually exerted between anion and cation is overcome, these 

 ions though separated will never be far apart. 



In ordinary solutions the " metal ' ion, no matter what its 

 relative speed, cannot be separated from its " acid " ion by mere 



diffusion. The disturbance of electrical equi- 

 librium caused by the introduction of elec- 

 trodes into the solution will produce a 

 separation of the salt into metal and acid. 



Now, if there exist equal and opposite 

 charges on an- and cat-ions, tending to draw 

 them together, why, in the first instance, did 

 they separate, and what keeps them apart ? 

 This brings us to the discussion of the 

 dielectric constant. To put a name on a 

 c/vnoN * NION thing or on a process does not explain it. 



FIG. 6. Model of anion Neither is it sufficient to say that the dielec- 



and cation. Two pith balls J 



suspended by siik threads trie constant or specific inductive capacity 



attract one another if carry- . > 



ing opposite charges when o f an y medium is a measure of the capacity 



the charges are of the same A 



sign the bails diverge, i.e. o f that medium to act as a dielectric (non- 



repel one another. 



conducting) substance of an electric con- 

 denser. The higher the value of the constant, the greater is the 

 value of the condenser. 



If two pith balls (Fig. 6) are hung by a dry silk thread and 

 given equal and opposite charges of electricity, they may be 

 regarded as representing ion-hydrates. 



