1904.] Properties of the Alloys of Silver and Cadmium. 219 



the shape and temperature of the mould in which it is cast, the 

 temperature of casting, and the rate of cooling. In the case of ingots 

 weighing about 125 ozs. troy (4 kilos.), the ordinary weight of the 

 Imperial trial plates, the centre of the mass is usually richer in silver 

 than the outside to the extent of from 1 3 parts per 1000. 



When a pure metal such as silver is solidified (Curve A, fig. 1) no 

 pasty stage occurs. The part first solidified is of the same composi- 

 tion as that still remaining liquid, and the temperature remains 

 constant until the whole is solid. 



FIG. 1. 



1,000' 



-800 



200 



600- 



400- 



200* 



50 



2O 



Time in minutes. 



The solidification of an alloy containing 91'66 per cent, of gold and 

 8-33 per cent, of silver shows similar characteristics,* and remains 

 uniform in composition under all conditions. Having regard to these 

 considerations and to the experience that has been gained of the 

 difficulties attending the preparation of a homogeneous standard silver 

 trial-plate, it seemed more likely that such a plate would be success- 

 fully prepared by using a different alloy than by making fresh experi- 

 ments with silver and copper. The first alloy to be tried consisted 

 of silver 925 parts, gold 75 parts, but although the plates prepared 

 were, doubtless, uniform in composition, the difficulties connected with 

 the assay of the plates were insuperable. It is obvious that the 

 fineness of the plate could not be compared with that of silver coin 

 by the cupellation process, for the reason that the gold would not be 

 separated from the silver in that case. Nor were the wet processes 

 * 'Koy. Soc. Proc.,' vol. 71 (1903), p. 161. 



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