SILVER ORES. 



381 



tation until the process is finished. In the best arrangements, ac in 

 Saxony, this agitation is performed in revolving barrels, and the resul 

 s accomplished in a few hours ; but in Mexico it is effected by the 

 reading of mules or oxen, and requires two or three weeks or more. 

 The amalgam, separated from the muddy mass, by a current of water 

 or washing, is then filtered of the excess of mercury ; as a last step it is 

 subjected to heat in a distilling furnace, by which the silver is left be- 

 hind, the mercury passing off in a state of vapor to be condensed in a 

 condensing chamber or receptacle. The loss of mercury by the pro- 

 cess is often large. 



In case of the ordinary sulphurets and ars'eniurets of silver, or the 

 chlorid, in Mexico and South America, the poorer ores are first fusea 

 with a flux, and the result, (called the " matt") is then roasted to expel 

 the sulphur ; afterwards it is mixed with better ores, again fused, and 

 on cooling, again roasted. This fusion and roasting is again repeated 

 with the best ores. The result from this fusion is next mixed thorough- 

 ly with melted lead ; the lead separates the silver ; and the impurities 

 which float on the surface, are removed in plates as a crust cools, to be 

 again melted with new ores, as the slag is apt to contain some of the 

 silver. 



When the argentiferous galena is the ore, it is reduced by roasting in 

 a reverberatory furnace in the ordinary way for lead ore ; the resulting 

 lead contains also the silver. 



The accompanying sketch represents the essential characters of a 

 feverberatory furnace. It is a transverse section, a is the grate on 

 which the fire is made, 

 and from which the flame 

 proceeds through the hor- 

 izontal chamber or gen- 

 eral cavity of the furnace, 

 (usually very low,) to 

 the flue at e. b is the 

 sole of the hearth, for re- 

 receiving the ore or as- 

 say, having an elliptical or circular form according to the shape of 

 the furnace ; c is the fire bridge, separating the fire from the sole ; 

 d is the arched roof. The flame plays horizontally over the charge of 

 ore, and as the air may be made to pass freely with it, we may have in 

 such a furnace a combined effect derived from the heat and the pres- 

 ence of the atmosphere ; the ore, or its metal, if capable of uniting with 

 the oxygen of the atmosphere, may be oxydated by the process, pre- 

 cisely as in the outer or oxydating flame of the blowpipe. In an or- 

 dinary blast furnace, (page 233,) the ore and its flux are confined from 

 ihe atmosphere, (except the air that enters with the blast,) and the re- 

 sult is the reduction of an ore or its deoxydation, as in the inner or re- 

 ducing flame of the blowpipe. This latter effect may in many cases 

 be obtained also with a reverberatory furnace, when the atmosphere is 

 excluded except what is essential to feeding the fire. 



In the reverberatory furnace, there is a small door near the fire-grate, 

 m, for putting in fuel. There is also an opening either at top, or on the 



Describe a reverberatory furnace. 



