1870.] On the Method o f Assaying Silver . 381 



refineries), it became necessary here to adapt and introduce a 

 system more likely to fulfil all the objects required of it. 



In the volumetric system it has been seen that the proportion of 

 silver present in a mixed metal is estimated by ascertaining the 

 exact amount of salt which it took to precipitate it in the form of 

 chloride of silver ; the same end can be attained by collecting, 

 drying and weighing the chloride itself. 



100 parts of it represent 75*3 of pure (metallic) silver. 



Hitherto this process when resorted to at all seems to have been 

 restricted to a very limited application, such as a solitary analysis 

 for some special purpose, possibly the examination of a standard 

 f trial-plate" where the greatest accuracy was required, or perhaps 

 an assayer would resort to it as a delicate confirmatory test of one 

 or two of his assays by the volumetric method. 



Some books which go into the principles and details of the 

 assaying of silver, make no mention of it whatever, others allude 

 to it, merely to dismiss it, as " tedious and less exact" (than the 

 Trench process). In theory, the process is allowed by all to have 

 the merits of accuracy and simplicity, but it is implied that the 

 tediousness and difficulties of the manipulations (supposed to be) 

 necessary to the carrying out of the theory, detract materially 

 from its practical value : certainly the few details of the manipu- 

 lations occasionally given, such as the weighing the chloride of 

 each assay (after collecting on a filter and fusion)* in a porcelain 

 capsule, previously counterpoised, were calculated to deter from 

 the idea of this process being ever made available for the assay of 

 silver on a large scale. 



The credit is due to Mr. J. D o d d, a former assay master of 

 the Calcutta Mint (and a Surgeon in the Madras army) of having 

 encountered those difficulties of manipulation, and of having over- 

 come them inasmuch as he modified and simplified them, and in 

 short so systematised the whole practical working of the process, 

 as to render its application to the assaying of silver, to any amount, 

 easy, accurate and economical. 



* Of course in practice it would be necessary periodically to recover the 

 silver (by reducing it to the metallic state) from the closely attached fused 

 chloride in each capsule, — a very tedious measure. 



