1836.) 



On Assaying Silver. 



74 



each pound troy of silver refined by him, and if there be a 

 surplus of gold, he returns it to the proprietor of the bullion. 

 It is said that the value of 5 or 6 grains of gold covers the 

 expenses of refining a pound of silver. Refining by sulphuric 

 acid is termed the humid process in contradistinction to the 

 method by fire, which is denominated the dry process. 



6. The defective nature of the assay by fire, besides its 

 inadequacy to denote the quantity of gold, arises from the diffi- 

 culty in practice of imparting a perfectly uniform heat to the 

 muffle, or small oven, in which the assay samples are pla- 

 ced. I know cases in which regularly educated assayers have 

 deviated from their own reports as much as one dwt. on silver 

 of the same standard fineness ; that is, to the extent of near- 

 ly a half per cent. And although this deviation may be 

 greater than may usually occur, it is a simple fact that it has 

 occurred, and proves the defectiveness of the process ; assay- 

 ers do indeed say that not more than one fourth of that, or a 

 ~ of a dwt. is the common deviation, but their practice is, not 

 to report nigher than i a dwt. or about i per cent., which of 

 itself implies that a nearer approximation to truth is not to 

 be expected. In fact were their reports to distinguish i dwts. 

 they could not be depended on, as I shall presently shew ; but 

 this seems to be far less accurate than the present state of 

 chemical analysis would seem to indicate as attainable. 



7. The following diagram of experiments obligingly made 

 at my request at the Madras Assay Office, will further explain 

 the defective nature of the process by fire. The general figure 

 of the diagram is supposed to represent the assay muffle, and 

 the squares the positions which the assay musters occupied in 

 the muffle. It is only necessary to notice that the silver 

 assayed was of the same known standard fineness, and that 

 where S occurs it denotes that the assay came out standard 

 or correct ; that where the sign of negation — is put, or 

 the sign -f- plus, the assay varied from the true fineness of 

 the silver and came out minus, or plus, under or above its 

 true quality as the sign denotes. The notation of the dif- 

 ferences is in lOths of a penny weight ; the whole muffle 

 was not filled with musters, which accounts for the blank 

 squares . 



