as a Standard of Electromotive Force. 221 



The statement that " it may be due to the chilling- pro- 

 ducing a too highly concentrated amalgam in the outer 

 shell " scarcely removes the need for further explanation. 

 Effects of this kind are not confined to the chilled amalgams. 

 The 15, 17, and 19 per cent, amalgams of Table II. show 

 appreciable decline in the surface percentage of Cd during 

 the three months at 15" 0. 



Another possible cause* of a slow decrease of E.M.F., 

 which should be mentioned, depends upon the deduction 

 (from § 12 below) that the equilibrium concentration o£ Hg 

 salt in solution round an amalgam must be less than that 

 round pure mercury. In cells of the Weston type, Hg salt 

 must thus be diffusing continuously from cathode to anode 

 and the amount of Hg in the latter must be slowly increasing 

 by precipitation. Fortunately, the diffusion will generally 

 be so slow that its effect upon the E.M.F. of a two-phase 

 amalgam will remain unnoticeable for a very long time. 

 But if the anode surface consists of a thin single-phase skin 

 the percentage of Hg within it may increase perceptibly in a 

 comparatively short period. 



§ 11. The question of the horizontality of BC in fig. 2. — 

 The skin effects described in § 9 arise primarily from the 

 lightness of the solid grains and from their want of uniformity 

 of composition. As soon as such effects become appreciable, 

 departure from horizontality must ensue. A measure of the 

 importance of these effects is given by the difference 

 between the E.M.F.s of the richer amalgams, chilled and 

 slowly cooled, of fig. 4. 



But another question remains. If the amalgams (chilled 

 or otherwise) were of quite uniform composition between 

 B and C, differing only in the relative amounts of x per 

 cent, and of y per cent, amalgams present, would BC De 

 absolutely horizontal, i. e., would the E.M.F.s of all the 

 amalgams within this region be found to be identical, how- 

 ever refined the means of comparison might be ? 



The data of fig. 4 seem to establish the fact that there is 

 always a slight rise from B towards C. In the region near 

 B the amount of solid amalgam is so small that there is 

 little room for appreciable variation in the percentage of Cd 

 between the centres and surfaces of individual grains. 

 Hence, as the electromotive data show, chilled and slowly 

 cooled amalgams containing the same percentage of Cd must 



* Possible effects at the cathode are outside the range of the present 

 paper. 



