?32 METALS. 



tide, for introducing the charge ; also there may be cne or more doora 

 oii each side for working the charge while exposed to the heat. There 

 may also be a tap hole for drawing off the reduced metal into one or 

 more pots attached for the purpose ; another in some cases for the es- 

 cape of slag as in cupellation, and where there is a vaporizable ingre- 

 dient to be condensed, one or two flues leading to a condensing cham- 

 ber. In large establishments several of these reverberatory furnaces 

 connect with a single chimney. They are actually like *arge elliptica 

 or circular ovens, of brick or stone, communicating with a common 

 flue. 



In reverberatory furnaces adapted for melting metals, the hearth is 

 a gently inclined plane, sloping to a spot towards one end, in order that 

 the fused metal may flow down together and be convenient for drawing 

 off. For many other purposes, the sole is flat, and the depth is greatei 

 than in the above figure. 



To separate the silver from the lead, the lead is heated in a reverbe- 

 ratory furnace, the hearth of which is covered with wood ashes anc 

 clay, so as to give it the nature of a cupel. The air received through 

 an aperture on one side, passes over the metal in fusion, in a constant 

 current, oxydizing it and changing it to litharge, which is from time to 

 time drawn out ; finally the lead is thus removed, and the silver remains 

 nearly pure. The completion of the process is known by the metal be- 

 coming brilliant. It is again subjected to another similar operation, and 

 thus rendered quite pure. The litharge from the latter part of the pro- 

 cess is also subjected to another operation for the silver it usually con- 

 tains. 



According to Pattinson's new process, adopted in England, the silvei 

 is separated by melting the lead, and, as it begins to cool, straining out 

 the crystals with an iron strainer. The portion left behind contains 

 nearly all the silver. This is several times repeated, each time the re- 

 maining lead becoming richer in silver. This is then cupelled. Ar 

 ore containing only 3 ounces of silver to the ton of lead, (or but 1- 

 10,000th part,) may thus be profitably worked, and with little loss of 

 lead. 



When the ore containing silver is a copper ore, as is often the case 

 with gray copper ore, the calcined ore is mixed with lead or lead ore, 

 and fused and calcined, and the resulting products are either liquated to 

 sweat out the silver or cupelled. In liquation, the coppei is run into 

 oigs, (called liquation cakes,) and kept above a red heat for two or three 

 days ; the lead first melts and flows in drops into cast iron troughs, car- 

 rying with it the silver, which is afterwards obtained by cupelling. 

 The copper still contains some of the lead. 



In trials by cupellation, a piece of lead of known weight is placed in 

 a cup of bone-ashes, and this is subjected to heat in a small air cham- 

 ber or oven, and placed in a furnace so that the air shall have free ac- 

 cess. The lead is oxydized, and the oxyd sinks into the cupel, leaving 

 a globule of silver behind. The globule being then weighed, and com- 

 pared with the weight of lead, the proportion of silver is ascertained 

 Silver .may thus be found in almost any specimen of the lead of com- 



What is the process of amalgamation with an argentiferous lead ore * 

 What is the mode of trial by cimellation 1 



