Lewis’s process, however, is based upon the discovery that by subliming unroasted galena, or the native or raw sulphuret of lead, and then oxidising the volatile products, cooling the fumes, and collecting them by means of a textile fabric, a superior basis of a pigment can be obtained. The admixture of carbon with the raw ore will facilitate the subliming process, or it may be carried on in a muffle or reverberatory furnace without the previous admixture of carbon. The furnace, which has been found to answer well for the purposes above mentioned, is commonly known as the Wetherill furnace, and is represented in plan, in Fig. 21; in front and back elevations, in Figs. 22, 23; and in central longitudinal section, in Fig. 24. _a_ represents the main chamber, the bottom _b_ of which is composed of iron bars perforated with small holes of about ¼ inch in diameter and about 1 inch apart, and preferably made slightly conical, with the larger diameter downward, that is to say, the said holes are of such a size as to prevent [Illustration: Fig. 21. Fig. 22. Fig. 23. Fig. 24. LEWIS’S WHITE LEAD PLANT.] the escape of the crushed ore and coal. These perforated bars are suitably sustained at the ends on the front and back walls of the furnace. The ash pit below the perforated bottom is of equal area therewith, and is provided with a door _e_ in front, and with a hole _f_ at the back for the reception of a pipe from suitable blowing apparatus. The walls _g_, and arch on the top should be built of some refractory substance, such as fire-brick. The front of the furnace is entirely open, and is provided with sliding doors _i_, by which it can be closed when working, and opened to remove the residuum. At the back of the furnace there are two slides _j_, to permit access to the main chamber, for stirring the charge and for inspection. At the back, near the arch, there is a hole _k_, governed by a sliding damper leading to a chimney, for carrying off smoke and impure gases at the commencement of the operation upon a new charge. In the centre of the roof there is an aperture _l_, governed by a damper or sliding door _m_, the said aperture leading to a suitable apparatus for the collection of the oxidised vapours of lead. The exterior walls _n o p q_ may be built up to form two feeding troughs _r_, one on each side of the arch or roof, and each provided with an aperture or passage _s_, communicating with the inside or main chamber, and each aperture or passage is provided with a cover to be put on after the furnace has been charged.