stirrers, similar to those in a pug-mill, which are driven by means of the pulley _l_, _m_ being a loose pulley; by shifting the strap on to this, the machine can be stopped at once. When the oil has been thoroughly incorporated with the pigment, the mixture is allowed to run through the spout _g_ on the roller _a_. Working against _a_ is a second roller _b_, and this in its turn bears upon a third roller _c_. In order to prevent the grooving of the faces of the rollers, which always takes place when they revolve in the same plane, there is an arrangement by which a slight lateral motion is communicated to _b_, in addition to the rotary motion. A pin fixed upon the rigid bracket _k_ works in the grooved cam _i_, which is keyed on the shaft of the roller _b_. The grinding power of the machine is considerably increased by this modification. The rollers are worked from the pulley _d_; the loose pulley _e_ receives the strap when a pause in the working of the machine becomes necessary. The details of the construction of the grinding machine are given in Fig. 48. The rollers _a b c_ are constructed of granite or porcelain; for fine grinding, the latter substance is preferable. They are adjusted by means of the screws _g h_. These are furnished with spiral springs, so that should a nail or other hard substance get between the rollers, these can rise in their bearings, letting the nail fall down at the back. The “doctor” or scraper _f_ removes the paint from the surface of the roller _c_; _a b_ are also provided with smaller scrapers, which remove any paint that may cake upon their surfaces. Where extreme fineness is requisite, the paint is again passed through the machine, and this operation is sometimes repeated several times. In working these or any other form of grinding rollers, great care must be taken to clean them thoroughly immediately after use. If the paint be allowed to dry upon the surface of the rollers it is difficult of removal, and interferes with the perfect action of the machine. Should the working parts become clogged with solidified oil, a strong solution of caustic soda or potash will remove it. By means of the same solutions, porcelain rollers may be kept quite white, even if used for mixing coloured paints. Although the colour of most pigments is improved by grinding them finely in oil, yet there are some which suffer in intensity where their size of grain is reduced. Chrome red, for instance, owes its deep colour to the crystals of which it is composed, and when these are reduced to extremely fine fragments, the colour is considerably modified.