The flagging should be of tough, coarse rock; granite, basalt or compact quartz are all good. This flagging should be at the very least a foot thick. When the arms of a 10-foot arrastra are revolving 14 times a minute, the outer stone is traveling 400 feet a minute. Round holes closed by wooden plugs, or a side gate, lets the liquid mud out. Some mill men use chemicals in the arrastra; potassium cyanide, and wood ashes or lye are probably the most useful, as the latter cuts grease and the former gives life to the quicksilver. Rich silver ores are treated with blue stone and salt. When the pulp has been ground sufficiently, quicksilver is added, sometimes 250 pounds being put in a single charge. A 12-foot arrastra will never treat more than two tons a day, and often no more than one-half that. One man a shift can look after a couple of arrastras, and the owner, in case of one arrastra that is working on tailings, often does everything himself. Overshot wheels, or turbines, or hurdy-gurdies, furnish the power in many cases. A simple mule-power arrastra may be built for $150. A side hill should be chosen for the site of a battery. Ample water power is necessary, though provision may be made for saving it in catch basins should such a course be desired. Moreover, there must be plenty of room below the mill for the tailings, as it may be desirable at some future time to put them through a second course of treatment. [Illustration: STAMP BATTERY.] Automatic ore-feeders are always put in by good mill men. In cold climates the water that goes through the mill should be heated, and this may be done by the exhaust steam, but care is necessary that no grease get into it, as it would prevent the gold from amalgamating. The stamps for a light mill may be 3 or 5 in number, and weigh from 700 to 850 pounds. Tables must be water-tight, with half an inch to one inch drop to the foot, according to the fineness of the gold. Below them tables, having the same inclination and covered with blanketing, are used to retain specks of gold that have passed over the plates without amalgamating. [Illustration: THREE STAMP BATTERY.] After the concentrated materials, always spoken of as the concentrates, have passed over the tables, they are often roasted to get rid of the sulphur, arsenic, etc., and afterwards treated with quicksilver in the pan, or tin, with chlorine or cyanide. These processes belong, however, to the domain of the professional chemist and metallurgist, and require the knowledge and experience of an expert to stand a chance of success.