as a Standard of Electromotive Force. 225 



complete equilibrium. With any one of these electrolytes 

 the E.M.F. would be independent o£ the relative amounts of 

 the two phases present. 



§ 13. The effect of surf ace energy. — If, as above, the surface 

 energy variations of the liquid and solid amalgams are 

 regarded as identical, the conditions of equilibrium assume a 

 simplicity which there is no doubt they do not possess. 



We do not know the relative importance of the energy 

 per unit area of the surface separating the amalgams ; but 

 the difference between the energies of the surfaces separating 

 the respective amalgams and the electrolyte is perceptible. 

 The liquid amalgam appears always to spread over the 

 surface of the solid amalgam in the presence of the electro- 

 lyte. We may therefore assume that the surface energy 

 between solid amalgam and electrolyte is greater than that 

 between liquid amalgam and electrolyte. 



There are phenomena which show that the relation between 

 surface energy and potential difference is often very complex 

 and § 12 is put forward merely as a method of representing 

 the general nature of what occurs. It can be seen, however, 

 that even if the conditions of equilibrium were as simple 

 as those already given, the difference of surface energy 

 would cause ]i{ to exceed Jl z . The two amalgams could not 

 then, subject to the condition V — V" = V' — Y", be in 

 equilibrium with the same electrolyte. 



The value of V" — V satisfying the second pair of equations 

 would be greater than that of Y n —Y satisfying the first two, 

 i. e., the potential of a given electrolyte with respect to the 

 solid amalgam would exceed that of the same electrolyte 

 with respect to the fluid amalgam. Hence, supposing 

 V = V / , the amalgams could not coexist in contact with the 

 same electrolyte. 



Such considerations are enough to show that there is no 

 theoretical necessity for horizontality of BO. It is much 

 more likely that the equilibrium at the anode is between a 

 variable phase and the electrolyte, and that the E.M.F. electro- 

 lyte/electrode is distinctly greater when the amalgam is one 

 which is just on the point of partial liquefaction at than 

 when it is one in which the last traces of solid have just 

 disappeared. 



There is therefore every probability * that the E.M.F. of a 

 cell of the Weston type must always be to some extent 

 dependent upon the percentage of (M in the amalgam. 



* Mr. F. E. Smith has kindly supplied me with further details of some 

 of the measurements recorded in his Table VIII. which confirm this 

 view. 



Phil Mag. S. 6. Vol. 20. No. 115. July 1910. Q 



