210 Dr. S. W. J. Smith *oh the Weston Cell 



In this way the general form, either of ABCD or of 

 ACEF, could be accounted for ; but the cause of the dif- 

 ference between the two curves is not immediately obvious. 



§ 4. The freezing of slowly -cooled amalgams. — To under- 

 stand the exact significance of fig. 2, it is necessary to consider 

 how a mixture, to which fig. 1 applies, freezes. 



According to this figure, an alloy containing a per cent. 

 Cd should begin to freeze at 6 X and should apparently become 

 solid at 2 . But, because of the extreme slowness of diffusion 

 in solids, this will happen only when the rate of cooling is so 

 slow that it cannot be dealt with in practice. 



At the temperature 0^ a fluid amalgam containing a per 

 cent. Cd and a solid amalgam containing b per cent. Cd are 

 in equilibrium. When the temperature is lowered slightly 

 a fluid amalgam containing slightly less than a per cent. CM 

 will be in equilibrium with a solid amalgam containing 

 slightly less than b per cent. A small quantity of the 

 a per cent, amalgam may therefore solidify. 



As the temperature falls the percentages of Cd contained 

 by fluid and solid amalgams in equilibrium become continu- 

 ously lower. The amount of solid material will therefore 

 increase ; but the percentage of Cd in the newest crystals 

 will always be less than in those previously formed. 



The fluid existing at any given stage of the cooling will 

 be in equilibrium with the solid with which it is in direct 

 contact, but, since most of the successive growths will take 

 place around earlier crystals, this solid will in general enclose 

 older solid, richer in Cd. 



There must thus be a continuous diffusion of Cd in the 

 crystallized part of the material from within towards the 

 surface. 



In any practical case, where the rate of cooling is not 

 infinitely slow 7 , the diffusion outwards will be very gradual 

 and will not keep pace with the lowering of temperature. 



Thus, although (for true equilibrium) an amalgam con- 

 taining a per cent. Cd should be all-solid just below 2 , a 

 considerable quantity of liquid, containing c per cent. Cd, 

 will remain. The solid in contact with this liquid will contain 

 a per cent. Cd, but will envelop a considerable quantity of 

 solid richer in Cd than itself, and there will be, in conse- 

 quence, neither so little c per cent, liquid nor so much a per 

 cent, solid as true equilibrium would imply. 



A superior estimate of the amount of liquid remaining at 2 

 can, however, be found. Thus we may imagine that the 

 cooling from 0± to 0%, of an amalgam containing a per cent. 



