SILVER ASSAY. 67 



When frequent quantitative determinations of silver 

 are made, as in mints, the test is made either by cupel- 

 lation, which consists in fusing the weighed alloy with 

 several times its weight of pure lead in a small bone 

 earth cupel in a current of air, when the lead and cop- 

 per are oxidized and absorbed by the cupel, while the 

 pure silver remains as a fused button. Or, more ac- 

 curately, by volumetric analysis. 



In order to prepare pure silver, it is precipitated 

 from the solution by hydrochloric acid or chloride of 

 sodium, in the form of chloride which is well washed 

 and fused in a porcelain capsule. A fragment of zinc 

 is placed upon the fused mass, and some dilute hydro- 

 chloric acid poured over it. After twenty -four hours, 

 the chloride of silver is completely reduced ; the 

 spongy masses of silver are rinsed out, rubbed to a 

 fine powder under water, and digested with dilute 

 hydrochloric acid, to remove any zinc. It is then 

 thoroughly washed and fused with borax to a reguline 

 mass. 



Or the dry chloride of silver may be mixed with an 

 equal quantity of anhydrous carbonate of soda, and 

 the mixture introduced into a crucible, the bottom and 

 sides of which are coated with as thick a layer as 

 possible of carbonate of soda. The crucible is then 

 heated for a length of time to low redness, and after- 

 wards to the fusing-point of silver. 



42. SILVER ASSAY. 



From argentiferous galenite tetrahedrite, chalcopy- 

 rite, &c., even when intimately mixed with gangue, 

 the whole of the silver, concentrated in a small quan- 

 tity of lead, may be extracted in the following manner : 



