Within the vat, and working on the top of the copper plates, there is a
heavy copper stirrer or muller, B, Figs. 1 and 2, caused to revolve by the
shafting, C, at the rate of 45 revolutions per minute. At Huanchaca this
stirrer has been made with four projecting radial arms, D D, Figs. 1 and
2; but at Guadalupe it is composed of one single bell-shaped piece, Figs.
3 and 4, without any arms, but with slabs like arms fixed on its
underside; and this latter is claimed to be the most effective. The
stirrer can be lifted or depressed in the vat at will by means of a worm
and screw at the top of the driving shaft, Fig. 3.

The bevel gearing is revolved by shafting connected with pulley wheels and
belting, the wheels being 3 ft. and 1½ ft. in diameter, and 6 in. broad.
The driving engine is placed at one end of the building. Each vat requires
from 2½ to 3 horse-power, or in other words, an expenditure of 1
horse-power per ton of ore treated.

At the bottom of the vat, and in front of it, a large wooden stop-cock is
fitted, through which the liquid amalgam is drawn off at the end of the
process into another shallow-bottomed and smaller vat, Figs. 1 and 2.
Directly above this last vat there is a water hose, supplied with a
flexible spout, through which a strong stream of water is directed upon
the amalgam as it issues from the grinding vat, in order to wash off all
impurities.

The following is the mode of working usually employed. The grinding vat or
tina is first charged to about one-fifth of its depth with water and from
6 cwt. to 7 cwt. of common salt. The amount of salt required in the
process depends naturally on the character of the ore to be treated, as
ascertained by actual experiment, and averages from 150 lb. to 300 lb. per