"To sum up: For successful inside amalgamation in your mills it is necessary only to keep the mercury as nearly chemically pure as possible, and this can be accomplished by the introduction of any agent which will produce nascent hydrogen. As to quantities and proportions, this will vary according to conditions and the nature of the ore treated and is a matter which can be easily determined by any experienced amalgamator. Mercury should be fed to the mill as freely as possible but not in such quantity as to cause it to run on the copper plates. It is not our object to catch the amalgam inside the mills, as its recovery there is an expensive matter, but to prepare the amalgam so that it will readily adhere to the copper plates when it leaves the mill.

"In a plant at Jarbridge, Nev., where excessive losses between heads and tails could not be accounted for, an expert was brought in to check up results, who later reported that all he did was to open the mill, dump the contents, retort three separate balls of 50 per cent, amalgam, and hand the manager $16,000 worth of bullion.

"Here it was explained that the cause of the trouble was in too small a quantity of quicksilver being added to the charge in proportion to the gold content. As soon as a quantity of quicksilver sufficient to make a fairly liquid amalgam was introduced into the mills, the amalgam issued with the charge and was recovered on the plates." The system as practised at Tuolumne is outlined in the sketch, Fig. 63.

At the Incaoro mill, Bolivia, mercury is fed into the tube mill every hour, the mill being lined with flint pebbles 6 in. in diameter set in cement and lumps of hard quartz from the mine are used instead of pebbles for grinding. The clean-up takes place every 15 days, the amalgam being found distributed from the ball mill to the pebble mill but no loose balls of amalgam are found inside the mills. Three-quarters of the amalgam is collected from inside the mills and one-quarter from the plates.