As the vapour must be cooled down as far as practicable, the natural draught is very slight, and a tall chimney, generally with a small fire in it, is needful to draw in the requisite amount of air through the grate. The working is very simple. The fire fa gradually increased from the start, until, after 3 to 4 days, the mass of ore has come to n moderately bright-red heat, plenty of air being admitted all the time. When this point has been reached, the distillation is considered complete, and the furnace is closed up and allowed to cool. As soon as it is cold, the exhausted ore is removed through suitable openings, and the furnace is re-charged. The necessity of allowing the furnaces to cool down before re-charging involves great loss of time and fuel.

The Idrian furnace is shown in Fig. 151: a is the grate upon which the fuel (wood) is burned; it is surmounted by 3 perforated firebrick arches b c d, which carry the ore, the largest pieces being stacked on the lowest, and the size decreasing upwards, while the tope, filled with ore-dust and " soot." Air is admitted to the heated ore through passages f in the walls. The sulphurous acid and mercurial vapour pass by 2 chimneys g into 6 condensing-chambers h on each side, connected alternately at top and bottom (as at i), and each provided with a movable lid k. The sixth chamber on each side is furnished with means of maintaining a shower of water, to complete the condensation of the vapours. The first 3 chambers on each side catch the bulk of the mercury in a liquid state; this finds its way by a subterranean channel to a reservoir, 3 whence it is ladled into cloth filters, and bottled for export. The last 3 chambers on each side collect solid matters,consiting of a mixture of minute mercury globules, vaporized mercury sulphide, and soot from the combustion of bituminous matters in the ore.

The Working-off of a charge, and cooling the furnace ready tor another, occupies about a week.

The first furnace devised to allow of a continuous working was the Hahner furnace. In this the ore and fuel are charged together into a shaft, the bottom of which consists of a grats with movable bars. By moving one or more of these bars, the lumps of exhausted ore can be allowed to drop from the furnace, while fresh material is added at the top. The condensation-chambers are arranged as in the Almaden furnace.-

The continuity of the Hahner furnace is a great advantage, but not the mixing of fuel and ore. It has consequently been modified so as to resemble furnaces long in use for ores of other metals, by allowing the flame from one or more outlying fireplaces to strike into the shaft filled with ore alone. An important advantage is gained, as in the similar Swedish furnace, by placing the openings for the entrance of the flame some distance from the bottom of the shaft, and allowing the air for roasting to enter at the opening intended for withdrawing exhausted ore. In this way the air is heated by the hot worthless material, and the heat is thus returned to the upper portion of the furnace. The "Knox and Osborne" furnace of California, and the latest furnace of the works at Idria, Southern Austria, correspond to this general description. The former is provided with a single fire, which draws straight across the column of ore in the shaft, and is built of heavy brick walls. The Idria furnace, of which a copy is now putting up at New Almaden, is provided with 3 fireplaces, which draw through nearly 1 of the height of the column, the gases passing out at 2 openings close to the top of the furnace.

The walls are of a 2 c construction first adopted In England for iron blast-furnaces, and since applied with great success to a variety of other furnaces; a comparatively thin lining of firebrick, surrounded by an annular space filled with air, answers admirably as a non-conductor of heat, outside of which in another thin wall of brick enclosed in a shell of boiler-iron. The charging apparatus is also borrowed from blast-furnace construction, and is of the kind called " Parry's bell and hopper." The mouth of a conical hopper is closed by drawing up into it an upright suspended cone. When the hopper is filled, the cone is lowered, and the ore enters the furnace; but its fall is broken by the presence of the cone, and it is thus necessarily distributed in all directions in the furnace. The cone is instantly raised again, and thus shuts off the escape of gas.

Fig. 152 illustrates the improved furnace used at Idria for ores continning less than 1 Per cent. of mercury: a la the hearth of the reverberatory furnace, whence the vapours pass through a condensing chamber b, thence by an iron pipe c (kept constantly cool by water trickling from the gutter d) successively into other chambers e f,back by another similar pipe g into a chamber A, and so into the chimney i. The hearth is divided into 3 sections, that farthest from the fire receiving the crude ore by means of the hopper k, provided with an adjustable damper l; the ore it turned over, as the roasting proceeds, in the direction of the fireplace, thus experiencing a growing heat, till, exhausted, it it raked into the well m. The liquid mercury is run into suitable receptacles, while the "soot" which collects in the chambers is dried, raked over a sloping table to liberate as much metal as will run free, and finally distilled. The fuel used is wood.

At the Siele works. in the Amiata mountain, Tuscany, the ore is reduced by women into small pieces, which are kept as dry as possible. The works have 12 half-moon retorts of cast-iron for treating the rich ore, and 3 vertical furnaces for the poor ore. The retorts are arranged horizontally, 3 by 3, in 4 ovens. The mouth of the retort is closed by a cover, luted. with clay, and kept in place by a screw and strap. The charge of a retort consists of 2 to 2 1/4 cwt. of ore and quicklime, in the proportion of 1 of the latter to S of the former. The retorts are charged every 8 hours, and this operation mutt be performed very quickly, as distillation begins at once. If the retort be not closed directly, there is great loss of metal, and the men's health suffers through the mercurial vapours. The distilled mercury escapes by the neck of the retort, and passes into a receiver in which cold.water is constantly circulating. The mercarial vapours from the 6 retorts are condensed in one and the same tank filled with cold water; and the metallic mercury is drawn off from inclined channels at the bottom. The retorts, fired with wood, of which a large quantity is consumed, only serve for the rich ore; the poor is treated in the 3 Vertical fnrnacss, which are 13 ft. high by 3 1/4 ft, in diameter.