The actual quantities of mercury to be added in the grinding vat, and the times of its addition, are based entirely on practical experience of the process.  With ore assaying 150 oz. to 175 oz. of silver to the ton, 75 lb. of mercury are put in at the commencement, another 75 lb. at intervals during the middle of the process, and finally another lot of 75 lb. shortly before the termination.  When treating “pacos,” or earthy chlorides of silver, assaying only 20 oz. to 30 oz. of silver to the ton, 36 lb. of mercury is added to 21/2 tons of ore at three different stages of the process as just described.

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. 6d., per 250 lb.; it makes a very excellent fuel for smelting purposes, smouldering and maintaining steadily the low heat required for subliming the mercury from the amalgam.  Beneath the furnace is a vault containing a wrought-iron water-tank, B, into which the open mouth of the retort, C, projects downward and is submerged below the water.  For charging the retort, the water-tank is placed on a trolly; and standing upright on a stool inside the tank is placed the pina, or conical mass of silver amalgam, which is held together by being built up on a core-bar fitted with a series of horizontal disks.  The trolly is then run into the vault, and the water-tank containing the pina is lifted by screw-jacks, so as to raise the pina into the retort, in which position the tank is then supported by a cross-beam.  The sublimed mercury is condensed and collected in the water; and on the completion of the process the tank is lowered, and the spongy or porous cone of silver is withdrawn from the retort.  The subliming furnaces are ranged in a row, and communicate by lines of rails with the weigh-house.

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