as a Standard of Electromotive Force. 217 



proceeds a solid net-work will develop within the material. 

 The later crystallization will take place from fluid entangled 

 within the meshes of this net-work. Consequently the 

 material need not in general separate into upper, all-solid, 

 and lower, all-fluid, parts. 



In an amalgam which is still partially fluid, but would he 

 all-solid in true equilibrium, at the temperature of obser- 

 vation, the mean concentration of a layer near the surface 

 will no doubt be greater than that of a layer near the bottom. 

 But the difference need not be very marked, and there may 

 be an appreciable quantity of fluid in the spaces between 

 individual grains near the surface, which will take a lono- 

 time to disappear. If the density effects are appreciable, a 

 greater proportion of the earliest formed crystals will be 

 present in the upper layers. The centres of the grains 

 nearest the surface may then be richer in Cd than the centres 

 of those lower down ; but even when the amalgam has stood 

 long enough for the upper portion to become solid through- 

 out, there will be graduations in the percentage of Cd, from 

 point to point in that portion, of w T hich the existence will 

 become obvious (as described below) when the temperature 

 is raised. 



§ 8. An effect of the pretence of the electrolyte at the surface 

 of the amalgam. — The surface layer of a partially crystallized 

 amalgam will consist of a number of grains between which 

 are spaces filled with liquid amalgam. Some of the solid 

 grains will in general project slightly above the mean surface 

 level. The layer of fluid amalgam covering these will be, at 

 most, very thin. 



Under the ordinary process of diffusion from the grains 

 the liquid surrounding them will gradually diminish in 

 amount, more or less uniformly in all directions. But it is 

 important to notice that the process of equalization of distri- 

 bution of Cd will be accelerated, in the surface, when (as in 

 the case of the Weston cell) the amalgam is covered by a 

 solution of cadmium sulphate. 



The very thin layers of x per cent, fluid in immediate 

 contact with the surface grains* will soon receive enough Cd 

 by diffusion to convert them into solid containing more than 

 y per cent. Cd (fig. 1). In consequence they will no longer 

 be in electromotive equilibrium with the neighbouring, 

 relatively thick, layers of surface fluid which still contain 



* When a saturated solution of CdS0 4 is poured over the fresh 

 surface of a partially fluid amalgam, the positions of these surface grains 

 can easilv be seen. 



