[Illustration: FIG. 6.] [Illustration: FIG. 7.] _White Lead Mills._—Before the white lead is subjected to the real process of grinding, it is generally first ground dry, or, more properly, pressed or crushed. This crushing is accomplished by means of vertical or edge-runner mills, which consist of mill-stones running round upon a stone bed about a vertical axis. The mill is surrounded by a wooden casing to prevent the escape of dust. Wet grinding, which is done between mill-stones, may be carried out in two ways, with the production of hard or soft white lead. The former is obtained when the lead acetate is not removed, the latter when all the lead acetate is washed out. [Illustration: FIG. 8.] _Hard White Lead_ presents a shining mass, broken with difficulty. This appearance is a guarantee against adulteration with barytes; white lead, which contains this adulterant, does not give a smooth, but an uneven earthy fracture. Hard white lead is rather difficult to grind, and requires very careful treatment to be brought into that state of fine division in which it is usable. The dry white lead, after powdering under the edge-runners, must pass through a sieve, which retains the particles of metallic lead, before it is subjected to wet grinding. The mills used for wet grinding differ little, or not at all, from the ordinary pattern. [Illustration: FIG. 9.] A mill designed by Richter of Königsee, in Thuringia, for wet grinding, is constructed as follows (Figs. 8 and 9). The shaft, _F E_, is driven by a water-wheel and the necessary gearing at 60 revolutions per minute; the bevelled cog-wheel _G_, by means of the cog-wheels _H_ and _J_, drives the shafts _K_ and _L_, which, by means of the bevelled cog-wheels _a_ and _b_, communicate to the runner stones of the mills, _M_, _N_ and _O_, a speed of 140 revolutions per minute. _C D_ is the framework for the support of the storied arrangement of mills, _f_ and _g_ are bearings with cast-iron cups to receive the escaping oil, _i_ is a clamp fastening the axle _d_ to _k_. A steel cup in _k_ rests upon the hardened top of the spindle, _l_; the bed stone rests upon the beam _n_ (80 millimetres thick) and is prevented from lateral movement by the surrounding _oo_; _p_ is a wooden support in the opening in the bed stone, carrying the stuffing box, _q_, which prevents the white lead from running out, and connects the spindle, _l_, with the bed stone. The runner stone, _r_, is surrounded by the box, _s_, cemented with white clay inside and out and fastened to the bed stone. The lever, _t_, turning about _w_, moved by the screw, _u_, and handle, _v_, regulates the position of the runner. The diameter of the stones is 95 centimetres.