The furnace designed by Kindler for preparing carbonic acid (Fig. 16) consists of a conical furnace, burning coal or coke upon the hearth, _a_. The passage, _c_, is divided by a vertical wall, in order to avoid obstruction from the piling up of fuel. The space, _K_, is filled with limestone, through which the carbonic acid passes; the tanks, _e_, filled with water, cool the limestone, and the gas, which then passes through the water in the washing vessel, _D_, is drawn off by a pump. In the old process at Clichy the apparatus depicted in Fig. 17 was used. The basic lead acetate was made in the wooden tub, A, provided with the stirrer, B C. The solution was run off from this vessel by means of the cock, b, into the settling tank, E, in which the mechanical impurities separated from the solution. The clear liquid ran into the decomposing vessel, a large shallow covered tank, holding 9,000 to 10,000 litres. In this tank opened 800 copper tubes, given off from the large pipe, S. The small furnace, D, in which limestone was burnt with coke, produced the carbonic acid; from the pipe at the top of the furnace the carbonic acid was brought into the Archimedean screw, h K, was washed with water, and pumped into the solution of lead acetate. The introduction of the carbonic acid was continued from 10 to 12 hours, after which the apparatus was left at rest until the liquid in the decomposing tank had become quite clear, through deposition of the white lead. The clear solution, now containing neutral lead acetate, was run off into the receiver, m, from which the pump, P, carried it back into the dissolving tub, A, where it was treated with fresh quantities of litharge. The solution of neutral lead acetate drawn off from the white lead had approximately a specific gravity of 1·0901. The white lead at the bottom of the decomposing tank was a tolerably thick paste. It was transferred to the tank, O, and washed several times with water. The first wash waters, which contained small quantities of lead acetate, were returned to the dissolving tub. The resulting white lead formed a very soft powder; it was at once placed in the drying pans. The white lead prepared by this process is a precipitate containing no coarse lumps, so that grinding is unnecessary. [Illustration: FIG. 17.] Theoretically, the quantity of acetic acid with which the process is commenced is sufficient to form an unlimited quantity of white lead, since all the acetic acid brought into the decomposing tank in the form of basic acetate is returned to the dissolving tub as neutral acetate. In practice, however, matters are somewhat different. Small quantities of acetic acid are lost in the wash waters; each time the solution of lead acetate is pumped back into the dissolving tub, a small quantity of acetic acid must be added to make up the loss. [Illustration: FIG. 18.]