SURFACES OF DISCONTINUITY 685 



the latter case the solution would have to be so dilute round 

 the electrode that a quantity of it as large as the earth would 

 contain two palladium ions at most! With such a huge solu- 

 tion pressure zinc would have to part with over one gram of 

 ions per sq. cm. in order to attain equilibrium when placed in 

 an ordinary solution of a zmc salt; to avoid such an obviously 

 impossible result one has to make ad hoc hypotheses concerning 

 the extreme slowness with which equilibrium is reached. It is 

 true that, by abandoning the assumption that ionic atmospheres 

 obey the gas laws, Porter and others have shown that more 

 moderate values for p^ can be obtained; but investigators have 

 of late considered other possible explanations of metal-solution 

 pressure. References to these will be found in Newman's book 

 Chapter VI and Rideal's Surface Chemistry. A feature of 

 Nernst's formula is its logarithmic form, in which it resembles 

 the contact potential formula obtained above — indeed Nernst's 

 formula could be obtained by somewhat similar statistical argu- 

 ments provided the physical environm.ent of the metal were as 

 simple as in the case of contact potentials. Now Rideal (Trans. 

 Faraday Soc, 19, 667 (1924)) has observed that the order of 

 different metals as regards electron affinities is much the same 

 as the ordinary electromotive order. Nevertheless, the fact 

 that an electrode P.D. depends upon the concentration of the 

 electrolyte shows that it is impossible to interpret such a P.D. 

 entirely in terms of a quantity such as is adequate to account 

 for contact potentials. However, Rideal has derived a formula 

 in which the difference between the electrode potential and the 

 electron affinity of the metal is dependent on its atomic volume. 

 Its form is 



kt 

 F. - * = -f(A), 



where A is the atomic volume of the metal. Schofield (Phil. 

 Mag., [7], 1, 641 (1926)), by an argument based on Gibbs' 

 chemical potential of an ion, derives a formula 



J. _ kt( \og c — {km — ke}) 



Ve — - ) 



ve 



