GOLD. 269 



673. GOLD FROM STJLPHURETS OF IRON, 



How is qold ci i i f e 



obtained from &c. Sulphurets of iron, copper, dec., 

 certain ml- sometimes contain gold, in small quantity, 

 and so completely disseminated that it can- 

 not be readily extracted by mercury. It has been found 

 advantageous to heat such ores with nitrate of soda, 

 previous to amalgamation. The sulphurets are thus 

 partially converted into sulphates, which can be washed 

 out. What remains of the pulverized material is at 

 the same time thoroughly opened to the action of mer- 

 cury. 



Describe the 674. THE GOLD ASSAY. Gold to be as- 

 method of 'as- sa y e( j contains commonly, only silver and 



saying gold ? . . . ' ' / 



Why is siher copper, as impurities. By msing the sam- 

 ple with lead, and then removing this 

 metal by cupel! ation, it carries with it the copper, into 

 the cupel. A globule, containing only gold and silver, 

 remains. The silver is then dissolved out by nitric 

 acid. The remaining sponge of pure gold being 

 weighed, and its weight compared with that of the orig- 

 inal sample, the assay is completed. More silver is 

 added in the process, for reasons stated in a. previous 

 paragraph. 



What is the 675. ASSAY OF GOLD BY THE TOUCH- 



touckstone? STONE. Any hard and somewhat gritty 



and how ts it J J 



used in assay- stone, of a dark color, which is not acted 

 on by acids answers the purpose of a touch- 

 stone. The assay consists in marking upon the stone 

 with the alloy, and judging of the purity of the metal 

 from the color of the mark, and the degree in which 

 it is affected by an acid. Nitric acid, to which a very 



