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



The excess over y per cent. Cd in the surface will only be 

 appreciable when the surface skin is relatively thick, i. e. 

 when the surface grains are relatively numerous. For much 

 of the under surface of the skin will be in contact with 

 liquid x per cent, alloy, and cannot therefore contain more 

 than y per cent. Cd. 



Thus it is only in the stronger slowly cooled amalgams 

 that the E.M.F. can be much greater than that of the two- 

 phase alloy — in agreement with fig. 4. 



In keeping with this view of the behaviour of the stronger 

 slowly cooled amalgams, the percentage of Cd in the surface 

 skin of any of them may increase within certain limits as 

 the temperature rises. Thus when the temperature of one 

 of these amalgams is raised from to 6', the solid grains 

 must partially liquefy. By this means the percentage of Cd 

 in the fluid alloy is raised from x to x\ and the percentage 

 of Cd in the new surfaces of the solid grains is y\ greater 

 than y. Electrolytic effects may ensue as before and cause 

 the surface skin percentage to rise above y' . 



As the temperature is raised the surface grains may 

 become relatively more numerous at first, since additional 

 grains may float up to the surface ; but in the end the 

 surface skin will grow thinner and the E.M.F. will approach 

 nearer to that of the two-phase amalgam corresponding with 

 the temperature of observation. 



§ 10. Skin effects in all-solid amalgams. — It will be obvious 

 that electrolytic skin effects of the kind described in § 8 

 may occur in amalgams which are already all-solid, but in 

 which ordinary diffusion effects are still proceeding with 

 appreciable velocity. In such cases the surface film may 

 acquire a larger share of the effects of diffusion than areas 

 just below the surface, with the result that these may be less 

 rich in Cd than the surface layer itself. 



Evidence of this condition of affairs is revealed when the 

 data of Tables II. and VIII. are compared as in Table A 

 above (see § 6), where it is shown that the surface begins to 

 change at a temperature lower than that at which it would 

 if no weaker amalgam were present near it. 



Similar considerations might explain otherwise puzzling 

 time effects exhibited by the chilled amalgams of Table VI. 

 (I. c. p. 266). In these, practically without exception, the 

 marked increase of E.M.F. which occurs in the first few 

 clays is succeeded by a small but unmistakeable decline. An 

 effect so general cannot be due to accidental irregularities 

 in the process of crystallization. 



