SILVER. 665 



leaves, is separated from the gangue or accompanying rock, by 

 amalgamation) a process which is also followed in the treatment 

 of the most frequent ore of silver, the sulphuret, when it is riot 

 accompanied by sulphuret of lead. The last ore, ground to 

 powder, is roasted in a reverberatory furnace with 1 per cent 

 of chloride of sodium, by which the silver is converted into 

 chloride. It is then introduced into barrels, with water, iron 

 and a quantity of metallic mercury, and the materials kept in 

 a state of agitation for eighteen hours by the revolution of the 

 barrels on their axes. The chloride of silver, although insoluble, 

 is reduced to the metallic state by the iron, and chloride of iron 

 is produced, while the silver forms a fluid compound with the 

 mercury. By adding more water and turning the barrels more 

 slowly, the fluid amalgam separates and subsides. It is drawn 

 off and subjected to pressure in a chamois leather bag ; the 

 mercury passes through the leather, while a soft amalgam of 

 silver remains in the bag. The mercury is afterwards separated 

 from this amalgam, by a species of distillation, per descensum, 

 and the silver remains. Where machinery cannot be applied 

 and iron is not used, the waste of mercury in the amalgamation 

 is considerable. Mr. P. Johnston proposes to diminish the loss 

 of mercury, as soluble chloride, which then occurs, by using an 

 amalgam of zinc and mercury, instead of pure mercury. 



Silver is obtained free from other metals and in a state of 

 purity, for chemical and other purposes, in two different ways. 

 1. The metal is dissolved in pure nitric acid, slightly diluted, 

 and precipitated by a solution of chloride of sodium ; the salts 

 of the other metals present remain in solution. The insoluble 

 chloride of silver, thus obtained, is washed well upon a filter with 

 hot water and dried. A quantity of carbonate of potash, equal 

 to twice the weight of the silver, is fused in a crucible, and the 

 chloride of silver gradually added to it ; chloride of potassium 

 is formed, and carbonic acid and oxygen escape with efferves- 

 cence. The crucible is then exposed to a sufficient heat to fuse 

 the reduced silver, which subsides to the bottom. 2. The mode 

 of separating silver from the common metals, in the ordinary 

 practice of assaying, is like many metallurgic operations, an 

 exceedingly elegant and refined process. A portion of the silver 

 alloy, the assay, is fused with several times its weight of pure 

 lead (an alloy of I copper and 15 silver with 96 lead for instance) 

 upon a bone-earth cupel, which is supported in a little oven or 



