The _rationale_ of the process therefore appears to be that the
chlorination of the ores is only partially effected during the roasting,
so as to prevent the formation of injurious salts, and is completed in the
vats, in which the chloride of copper is formed progressively as required,
by the gradual grinding away of the copper by friction between the bottom
copper plates and the stirrer; and this chloride subsequently becoming
incorporated with the boiling brine is considered to quicken the action of
the mercury upon the silver.
_Subliming_.--The subliming furnace, shown in Figs. 5 and 6, is a plain
cylindrical chamber, A, about 4 ft. diameter inside and 41/2 ft. high, lined
with firebrick, in the center of which is fixed the upright cast-iron
cylinder or retort, C, of 1 ft. diameter, closed at top and open at
bottom. The furnace top is closed by a cast-iron lid, which is lifted off
for charging the fuel. Round the top of the furnace is a tier of radial
outlet holes for the fuel smoke to escape through; and round the bottom is
a corresponding tier of inlet air-holes, through which the fuel is
continually rabbled with poles by hand. The fuel used is llama dung,
costing 80 cents, or 2s.
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