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



place at a temperature at which Cd and Hg can exist to- 

 gether in equilibrium only as a homogeneous mixture *. It 

 would therefore occur without redistribution of the Cd with 

 respect to the Hg. What " differential " crystallization there 

 was would be on a scale relatively so minute that the process 

 of equalization by diffusion and the approach to true equili- 

 brium would take place comparatively rapidly as the tempera- 

 ture rose. 



There is, therefore, no difficulty in finding a satisfactory 

 general interpretation of fig. 2, as a consequence of fig. 1. 



We may now proceed to examine the data more minutely. 



§ 6. Quantitative comparison of the " chilled " anal " slowly 

 cooled " amalgams. — It is reasonable to suppose that two 

 amalgams have the same surface composition when they give 

 the same steady E.M.F. at the same temperature, even 

 although their average compositions may be different. 



Hence, if we assume as a first approximation that the 

 values of x and y at any temperature 6 (when fig. 1 is taken 

 to represent cadmium amalgams) can be deduced from the 

 thermo-electromotive properties of the series of chilled 

 amalgams of Table VIII. (/. c. p. 268), we can determine 

 the state of the surface of any slowly cooled amalgam when 

 the E.M.F. which it gives is known. 



For this purpose, some of the data for the slowly-cooled 

 amalgams (Tables I. and II., I. c. pp. 256, 257) were plotted, 

 as in fig. 3, along with the data for the chilled amalgams. 

 The dotted curves refer to the former. 



Table A, below (p. 214), summarizes the inferences which 

 can be drawn from the figure when the data are interpreted 

 in the way above described. 



Each row of numbers gives, for a particular temperature, 

 the surface constituent or constituents of the slowly cooled 

 amalgams (11 to 20 per cent. Cd) deduced by interpolation 

 from the curve, at that temperature, for the chilled amal- 

 gams. The table begins with the results at 15° C. after the 

 amalgams had stood for three months at this temperature. 



(i.) At 15° C. (according to Table VIII.) any amalgam 

 containing less than about 4*3 per cent. Cd should be all 

 liquid, and any containing more than about 12*85 per cent. 

 Cd should be all solid (if in equilibrium). According to 

 Table A, the surface of the 11 per cent, slowly cooled 

 amalgam is a mixture of these alloys. The surfaces of the 

 12 per cent, and 13 per cent, amalgams appear to be solid 

 alloys containing slightly above 12*85 per cent. Cd. Each of 



* See § 14 below. 



