1904.] 



Constant- Standard Silver Trial- Plates. 



125 



of the results were satisfactory, and eventually, after no less than 

 31 plates had been cast without success, recourse was had to the 

 method adopted in 1873, of casting a large mass of metal and detaching 

 a particular portion which proved by assay to be of approximately 

 uniform standard."* 



The casting of the standard silver into thin instead of into the thick 

 moulds ordinarily employed having been attended with such excellent 

 results,! I was induced to believe that it must be due to the more rapid 

 cooling of the metal, by which liquation was arrested ; so that if the 

 standard silver were cast into moulds sufficiently cooled, liquation 

 might be induced to disappear altogether. In order then to overcome 

 the difficulty of obtaining the standard trial-plates in larger sizes, I 

 commenced by casting quantities of not less than 8 kilogrammes into a 

 mould cooled externally by ice, and also by freezing mixtures as low as 

 10° C, and by these means I obtained most encouraging results — 

 results which confirmed me in the supposition that by cooling with 

 rapidity there is less time for liquation, which appears to be the direct 

 converse of what has been supposed hitherto, viz. : " that a uniformity 

 of standard was best attained by slow and uniform cooling. "% But 

 although I thus obtained exceedingly good results (two of these are 

 subjoined, see p. 126), I was not satisfied that this was the best way of 

 producing a constant-standard plate. 



I therefore adopted a different method of casting the standard silver. 

 Instead of pouring the melted alloy into a mould from the top, I 

 poured it into a mould by which the skillet was produced from the 

 bottom, thus : — 



A 



Rough Section of Cast-iron Mould employed. 



By pouring the alloy into the gate A the metal passes by B into the 

 space C. And instead of cooling the mould by ice or freezing mixture 

 I used the mould simply cold. I have obtained excellent results by 



* Thirtietli Annual Report, Deputy Master of Mint, 1899, pp. 69, 70. 



f Vide my paper of February 16, 1894, before referred to. 



X Vide memorandum already referred to in Mint Report, 1873. 



