378 On the Method of Assaying Silver. [No. 4, 



In general terms then, it may be said, that the particular duty 

 of an Assayer is to ascertain the proportion of pure gold or silver 

 present in any specimen of mixed metal submitted to him for exami- 

 nation, so that from his report the value may be 1 by cal- 

 culation to the mass which the sample is supposed to fairly 

 represent. 



This is done by the separation of the precious metals from the 

 coarser ones with which they may happen to be alloy 



The most ancient plan for thus separating silver is that by 

 u Cupellation" which attains the end in view, owing to the fact 

 that silver resists the action of air, at a high temperature, whilo 

 the baser metals under identical circumstances become oxidized, 

 and if a certain proportion of lead be present, its very fusible 

 oxide unites with the other oxides produced during the operation 

 and renders them capable of soaking with it into a porous littlo 

 vessel (made of bone-ash), called a cupel, leaving behind on the 

 surface of each cupel, a glistening button of pure metallic* silver, 

 whose weight can be accurately ascertained. 



A certain weight of the specimen of the metal to be assayed is 

 folded up in a certain proportion of thin lead and placed on a 

 cupel. The operation is conducted in a suitable oven (called 

 a " muffle") and furnace. When the remaining little button of 

 silver has cooled, it is weighed and the loss of weight of the - 

 men operated on represents the baser metals that have been re- 

 moved : thus if the specimen weighed 20 grains and the resulting 

 bead of pure metal weighs 15 grains the mass would be reported to 

 contain 75 per cent, of pure silver. 



Several contingencies, however, and collateral circumstances 

 (known to assayers) tend to modify the result of an assay by cupel- 

 lation, and the assayer has to consider them all in arranging his 

 compensation, failing this the report would be most erroneous ; 

 eveiything, therefore, depends on his skill and experience ; but even 

 in the hands of the most experienced and the most skilful the re- 

 sult will fall short of accuracy, and a margin for error must be left, 

 owing to unavoidable imperfection in the assay : the average 



* Should gold or platinum happen to be present in the specimen assayed, 

 as they also resist oxidation, they remain behind, included in the " button," 

 and are under ordinary circumstances estimated as silver. 



