There being such great difference in the purity of fuels, and this irregularity not allowing of uniform results, Lewis prefers to use a furnace in which the flame and heat are produced by burning gaseous fuel with air, which is forced into the furnace with the fumes which have been collected in a previous process. Fig. 25 represents a furnace which may be advantageously employed for the purpose when gas is used as fuel. _a_ represents a blower, into which the fumes are fed from the hopper _b_. The fumes, being thoroughly mixed with air in this blower, are forced into a chamber _c_, and then through a series of tuyeres _d_. At the same time, gas from a generator or producer is admitted by the flue _e_, and is burned by the incoming blast from the tuyeres _d_; the volatile fumes produced in the furnace _f_ pass through it and out of the flue _g_, and are collected in bags or screening chambers. By using gas fuel, which is easily and fully burned, and clean to handle, a fine white pigment is produced. _MacIvor’s Processes._--The name of MacIvor is a familiar one in improvements in manufacturing chemistry, and not the least in connection with the subject of pigments, sometimes in conjunction with other inventors. In 1889 MacIvor and others introduced some modifications in the process of producing white lead or carbonate of lead, by the treatment of oxide of lead (litharge or massicot) with a solution of acetate of ammonia, whereby the oxide of lead is transformed into hydrate and acetate, which are subsequently converted into carbonate of lead by the injection of carbonic acid. The hydrate and acetate of lead, in presence of free ammonia formed in the reaction, are quickly decomposed by [Illustration: Fig. 25.--LEWIS’S WHITE LEAD PLANT.] the carbonic acid, yielding the final product, namely carbonate of lead, and re-forming the acetate of ammonia. The rapidity with which the conversion of oxide of lead into hydrate can be effected depends upon the strength of the acetate of ammonia solution employed, that is to say, the weaker the solution the slower will be the conversion. It has been found that, for commercial purposes, the solution of acetate of ammonia may be used with advantage of a strength which need not be more than 25 per cent. nor less than 5 per cent. A strength of 25 per cent, operates in a comparatively short space of time; but a strength as low as ½ per cent. will effect the hydration if there be a sufficient quantity of the weak solution, the lead oxide being in a fine state of division, and time of no object. This ½ per cent. strength, however, or any strength below 5 per cent., is not recommended for commercial purposes, having regard to the time required for completing the operation.