m m′, [fig. 664.], are basins of reception, distributed before the doors of each of the chambers f k f′ k′. The condensed mercury which flows out of the chambers is conveyed thither. n n′ is a trench into which the mercury, after being lifted into the basins m, is poured, so that it may run towards a common chamber o, in the sloping direction indicated by the arrows. o leads to the chamber where the mercury is received into a porphyry trough; out of which it is laded and packed up in portions of 50 or 100 lbs. in sheep-skins prepared with alum. p p′, [fig. 663.], are vaulted arches, through which a circulation may go on round the furnace a b c d, on the ground level, q q′ are the vaults of the upper stories. r r′, [fig. 665.], vaults which permit access to the tunnels e′ e′′, [fig. 663.]
s s′ and t t′, [fig. 665.], are the doors of the chambers, f k and f′ k′. These openings are shut during the distillation by wooden doors faced with iron, and luted with a mortar of clay and lime. u u′ is the door of the vaults 1. to 7. of the furnace represented in [fig. 663.] These openings are hermetically shut, like the preceding. v v′, [fig. 663.], are superior openings of the chambers, closed during the operation by luted plugs; they are opened afterwards to facilitate the cooling of the apparatus, and to collect the mercurial soot. x y z, [fig. 666.], are floors which correspond to the doors u u′ of the vaults 1. to 7., [fig. 665.] These floors are reached by stairs set up in the different parts of the building, which contains the whole apparatus.
On the lower arches the largest blocks of metalliferous rock are laid; over these the less bulky fragments are arranged, which are covered with the shivers and pieces of less dimension. On the middle vaults, the small ore is placed, distributed into cylindrical pipkins of earthenware, of 10 inches diameter and 5 inches depth. The upper vaults receive likewise pipkins filled with the sands and pastes called schlich.
In 3 hours, by the labour of 40 men, the two double sets of apparatus are charged, and all the apertures are closed. A quick fire of beech-wood is then kindled; and when the whole mass has become sufficiently heated, the sulphuret of mercury begins to vapourize; coming into contact with the portion of oxygen which had not been carbonated, by combustion, its sulphur burns into sulphurous acid, while the mercury becomes free, passes with the other vapours into the chambers for condensing it, and precipitates in the liquid form at a greater or less distance from the fire-place. The walls of the chambers and the floors, with which their lower portion is covered, are soon coated over with a black mercurial soot, which, being treated anew, furnishes 50 per cent. of mercury. The distillation lasts from 10 to 12 hours; during which time the whole furnace is kept at a cherry-red heat. A complete charge for the two double apparatus, consists of from 1000 to 1300 quintals of ore, which produce from 80 to 90 quintals of running mercury. The furnace takes from 5 or 6 days to cool, according to the state of the weather; and if to that period be added the time requisite for withdrawing the residuums, and attending to such repairs as the furnace may need, it is obvious that only one distillation can be performed in the course of a week.
In the works of Idria, in 1812, 56,686 quintals and a half of quicksilver ores were distilled, after undergoing a very careful mechanical preparation. They afforded 4832 quintals of running mercury; a quantity corresponding to about 81⁄2 per cent. of the ore. These smelting works are about 180 feet long and 30 feet high.
Upon the preceding three systems of smelting mercurial ores, I shall now make some observations.
It has been long well known, that quicksilver may be most readily extracted from cinnabar, by heating it in contact with quicklime. The sulphur of the cinnabar combines, by virtue of a superior affinity with the lime, to the exclusion of the quicksilver, to form sulphurets of lime and calcium, both of which being fixed [hepars], remain in the retort while the mercury is volatilized by the heat. In a few places, hammerschlag, or the iron cinder, driven off from the blooms by the tilting hammer, has been used instead of lime in the reduction of this mercurial ore, whereby sulphurous acid and sulphuret of iron are formed.