209. The White furnace consists of a hollow rotatory brick-lined cylinder of cast-iron, with open ends. The brick lining is so arranged, in the short segments of which the cylinder is composed, as to form "grooves, cavities or projections," by which the ore is lifted and showered through the flames, which pass through the furnace from end to end. The cylinder is slightly inclined, either downward from the fireplace, or the reverse, and the pulverized ore is poured automatically and continuously, together with the salt, into the higher end. The rotation, aided by the lifting and dropping of the ore, causes the latter to traverse the entire length of the cylinder, and to fall continuously from the lower end. By an ingenious arrangement, the inclination of the cylinder is rendered adjustable, so that the ore may occupy a longer or a shorter time in passing through it.

When the furnace is so arranged that the cylinder is inclined upward from the fireplace, and the ore consequently enters at the cooler end, it results that the lighter dust does not pass through the furnace, but is carried by the force of the draft in the opposite direction, toward the flue, and into a series of chambers in which the greater part of it settles, while the smoke, etc., passes on to the chimney. To effect the roasting of this portion of the ore, an auxiliary fire is necessary, through the flames of which the dust may pass on its way to the chambers. The coarser portion of the ore passes along the cylinder, under continually increasing heat, and, on its exit at the lower end, falls into a pit, situated between the end of the cylinder and the main fireplace, where it is exposed to the heat of the flames passing over it. At stated periods, the accumulated ore is removed from the pit and spread on the cooling floor.

This arrangement of the cylinder - the ore entering at the cooler end - fulfils, better than the reverse plan, the requisite conditions of a proper roasting. The writer is not aware of an instance in which the inclination was downward from the fireplace, and the ore entered the furnace at the hotter end, in which satisfactory results were obtained.