as a Standard of Electromotive Force. 207 



But there are certain much less obvious differences of 

 which, in view of the importance of the AVeston cell as a 

 standard (if for no other reason), it is desirable to know the 

 cause. 



An all-fluid amalgam, of course, yields the same E.M.F. 

 whether previously chilled or cooled slowly. An all-solid 

 alloy, on the other hand, always gives a higher E.M.F. when 

 solidified by chilling. 



The E.M.F. of the chilled amalgam thus in general equals 

 or exceeds that of the slowly cooled amalgam. 



But in the range of the two-phase alloys, over which the 

 E.M.F. varies comparatively little with the total percentage 

 of Cd, the opposite is true. The E.M.F. of the slowly-cooled 

 amalgam now either equals or exceeds that of the chilled 

 amalgam. 



The excess never amounts to more than a few hundred- 

 thousandths of a volt ; but is important in measurements of 

 the highest precision and requires explanation. 



It is shown that this phenomenon may be due to electro- 

 lytic surface effects arising from the lightness and want of 

 uniformity of composition of the solid grains in the slowly 

 cooled amalgams. 



Instances of similar effects, liable to escape notice, are to 

 be found amongst the data for the all-solid alloys. 



Finally, the question as to whether there is any range 

 over which the E.M.F. is absolutely independent of the 

 percentage of cadmium is discussed. 



Theory and experiment alike suggest that the E.M.F. 

 must rise as the percentage increases ; but the variation 

 frequently does not amount to more than a few millionth- 

 of a volt for one per cent, variation of the cadmium content. 



§ 2. A mode of crystallization of binary alloys. — The 

 various ways in which fluid mixtures of two metals can 

 freeze have been carefully studied within recent years. One 

 of these is shown in fig. 1. The abscissas represent per- 

 centages of one metal (B) in the mixture, reckoned from a 

 zero at which the other metal (A) alone is present. The 

 ordinates represent temperatures. The melting point of A 

 is O . The " freezing point curve " xa gives the tempera- 

 tures at which different alloys begin to solidify, and shows 

 that the freezing point rises continuously as the percentage 

 of B in the mixture increases. The u melting point curve ,? 

 6 yb gives the composition of the solid which deposits from 

 any particular liquid when it begins to freeze. Thus a cooling 

 fluid containing cc per cent, of B begins to freeze at 6 and /; 



