REFINING METALS. 20,3 



not remain united with reguline metals, and as 

 these latter sink to the bottom when in fusion, on 

 account of their greater specific gravity, they may 

 be easily parted from the scoriae. 



The silver to be purified by nitre is to be first 

 granulated, and then mixed with a fourth part of its 

 weight of dry nitre, an eighth part of potash, and 

 a little common glass, all in powder. This mixture 

 is to be put into a good crucible, two-thirds of 

 which only must be full. This crucible is to be 

 covered with a smaller crucible inverted, in the 

 bottom of which a small hole has been made, and 

 luted to the former. The crucibles, thus disposed, 

 are to be placed in a furnace capable of drawing 

 air sufficiently to make the fire intense enough only 

 to melt the silver. Then charcoal is to be put into 

 the furnace to such a height, that only the top of 

 the inverted crucible shall be uncovered. The coal 

 is then to be kindled, and the vessels to be made 

 moderately red ; a hot coal ought to be put upon 

 the small hole in the bottom of the inverted cruci- 

 ble. If a shining light be observed round this 

 coal, and a slight hissing noise at the same time 

 heard, we may know that the operation proceeds 

 well. The fire must be sustained at the same 

 degree till these appearances cease, when it must 

 be increased, so that the silver be well melted, and 

 then the crucibles are to be taken out of the fur- 

 nace. The larger crucible is to be broken when it 

 is cold, and the silver will be found at the bottom 

 covered with green alcaline scoriae. If the metal 

 be not sufficiently ductile, the operation must be 

 again repeated. 



Some silver is apt to be lost in this operation, by 

 the swelling and detonation of the nitre, which 

 often forces it through the hole in the upper cru- 



