206. This furnace consists of a horizontal, brick-lined, hollow cylinder, of boiler iron, with central openings at the ends, through which the flames from a fixed fireplace pass to a flue connecting with a dust chamber, while the cylinder rotates slowly on rollers. The ore is introduced and discharged, periodically, by means of trapdoors in the side of the cylinder. The latter is divided lengthwise by a reticulated, spiral partition, or diaphragm, composed of cast-iron plates supported by hollow cast-iron bars, which, passing through the walls of the cylinder, admit of a circulation of air through them, with the design of protecting them from too great heat.

The function of the diaphragm is twofold; firstly, to lift the ore, and shower it through the heated air within the furnace as the latter revolves; secondly, to move the ore from end to end of the cylinder, in order that all portions of the ore may be equally exposed to heat.

The diaphragm is liable to rapid destruction by the action of sulphur, arsenic, and antimony in the ore. It is stated, on excellent authority, that the loss of the diaphragm does not impair the efficiency of the furnace. In some cases the furnace has been found to work better without the diaphragm, forming fewer lumps in the roasting of leady ores.

In this, as in all rotating cylinder furnaces, the light and fine portion of the ore is carried away by the force of the draft. It is therefore necessary that extensive dust chambers be provided, through which the draft from the furnace must pass, on its way to the chimney, in order that the principal part of the dust may be deposited.

The writer superintended the erection and working of one of these furnaces in Inyo County, and obtained satisfactory results from it. About 15 per cent of the ore passed to the dust chamber, and was found to be but imperfectly roasted. This dust was collected, slightly moistened with a solution of salt and iron sulphate in water, and then returned to the furnace, and successfully roasted. About the same percentage of it again found its way to the dust chamber.

The cylinder is twelve feet long by six feet in diameter, and a charge for it is from 1 1/2 to 2 1/2 tons of ore, with the necessary salt. The time required for the roasting of a charge of ore in this furnace is from four to twelve hours, including the charging and discharging.

As the weight of the charge varies with the quality of the ore, and the degree of fineness to which it is crushed, and the time required for the roasting varies with the same circumstances, the capacity of the furnace may be said to range from four to twelve tons in 24 hours. One man on a shift can do all the work, or can attend to the firing of four furnaces.

This furnace has been extensively used in treating the silver ores of Colorado and Mexico, and one, without the diaphragm, is now, or was recently employed in the roasting of auriferous sulphurets in Nevada County, California.