For the production of white lead, the carbonic acid, by means of the pipe, _U_ (Figs. 19 and 20), enters the cylinder, _T_, provided with an agitator and containing a solution of basic lead acetate. By means of the pump, _V_, the lead solution is fed into the cylinder, _T_, through _W_. The absorption of the gas proceeds rapidly; the progress of the operation is followed by the observation of a pointer moving over a scale; as the gas holder sinks the pointer moves upwards. After the precipitation of white lead, the contents of _T_ are emptied into the tub, _b_, in which rotate rakes attached to a vertical axis of iron plated with copper. When the white lead has settled, the supernatant solution of neutral lead acetate is drawn off through the pipe, _c_, by means of the pump, _d_, and conducted into the water-tight vessel, _X_, containing a stirrer on the vertical axis, _W_, made of coppered steel. Here litharge is added, and the resulting solution of basic lead acetate is conveyed to the cylinders, _T_, by means of the pump, _V_, as already described. The white lead in the tub, _b_, by putting the stirrer in motion, is washed once with water which has been previously purified by a little lead acetate. It then goes into another tub provided with stirrers, where it is several times washed, sodium carbonate being added to the last wash water until a sample of the white lead is not coloured by a drop of potassium iodide solution. In this way the wash water is obtained free from lead, and the product is said to be of better quality. This, however, is not in accordance with the fact that good Dutch white lead generally contains some lead acetate. The two-cylinder pump, _h_, which is in connection with the gas holder, forces gas over the surface of the liquid in T in order to drive it into tubs which are not in the position shown for _b_ in the illustration, and into which there is no direct flow. The washed white lead is brought into bags which are pressed in a hydraulic press, dried, ground, sieved and packed in casks. These troublesome and often dangerous operations have been modified by Ozouf in the following manner. The pulp white lead runs from the tub, _b_, into the hopper, _g_, where it is kept mixed by a small stirrer, and from which it passes on to the cylinder, _f_, heated by gas from the inside. In its rotation the cylinder carries along the white lead and dries it, it is then removed by a knife below the hopper, and falls on to an inclined plane. The hopper and cylinder are in a room provided with a good draught. [Illustration: FIG. 20.] The lumps coming from the drying room are placed by workmen wearing respirators in buckets on an endless chain, are carried to the mills, ground and sieved; then, by means of an Archimedean screw, the white lead is conveyed to a cask in which it is evenly pressed by means of a special mechanism. A bell announces when a cask is full.