218 Dr. S. W. J. Smith on the Weston Cell 



only x per cent, Cd. Electrolytic action will ensue. Cad- 

 mium will enter solution round the surface grains and will 

 be deposited upon the adjacent fluid. 



The electrolyte will thus act as a distributor, over the 

 whole surface, of the cadmium diffusing from the surface 

 grains. The surface will thus tend to acquire a thin and 

 probably solid skin which, on account of its lesser density, 

 will have no tendency to sink below the underlying fluid. 



In the presence of CdS0 4 solution, the whole of the 

 surface film may therefore become solid and of uniform com- 

 position, although, on account of the extreme slowness of 

 ordinary intermolecular diffusion, there may still be an 

 appreciable quantity of fluid alloy underneath. 



§9. "Minor irregularities" of Weston cells. — Regarding 

 the surface behaviour of partially fluid amalgams in the 

 above way, it is possible to account for peculiaiities, at first 

 sight perplexing, of the horizontal branches of the curves for 

 the chilled and the slowly cooled amalgams. The vertical 

 scale of fig. 3 is not sufficiently open to show these peculiar- 

 ities clearly. They are exhibited in fig. 4 which represents 

 the behaviour of the amalgams, at various temperatures, over 

 the range represented by the branch BC of fig. 2. 



The data for the chilled amalgams are marked by dots and 

 for the slowly cooled amalgams by crosses. 



In the slowly cooled amalgams, as in the chilled, the rise 

 of E.M.F. near B is at most very gradual. But, towards C, 

 the former amalgams behave differently. The rise is much 

 greater than in the chilled amalgams, and the E.M.F. of a 

 slowly cooled amalgam is now considerably greater than that 

 of the corresponding chilled amalgam. 



This effect is exhibited numerically in Table A, and has 

 already been alluded to in § 6. It may be explained as 

 follows : — 



We assume, for reasons given in § 5, that the solid grains 

 in partially fluid chilled amalgams are of more uniform 

 composition than those in similar slowly cooled amalgams. 

 A chilied amalgam which is nearly all solid at any temper- 

 ature will consist mainly of solid y per cent Cd amalgam, 

 together with a small amount of fluid containing x per cent. 

 Cd. 



A slowly cooled amalgam, of the same average composition, 

 will contain a smaller proportion of solid grains ; but these 

 will be richer in Cd. Their surfaces will contain y per cent. 

 Cd ; but their interiors will contain more. If a sufficient 

 number of these grains be present in the surface originally, 



