in diameter, and 6 in. broad.
The driving engine is placed at one end of the building. Each vat requires
from 21/2 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
ton of ore. Into this brine a jet of steam is then directed, and the
stirrer is set to work for about half an hour, until the liquid is in a
thoroughly boiling condition, in which state it must be kept until the end
of the process.

As soon as the liquid reaches boiling point, the stamped and roasted ore
is run into the vat, and at the end of another half-hour about 1 cwt. of
mercury is added, further quantities being added as required at different
stages of the process. The stirring is kept up continuously for eight to
twelve hours, according to the character and richness of the ores. At the
end of this time the amalgam is run out through the stop-cock at bottom of
the vat, is washed, and is put into hydraulic presses, by means of which
the mercury is squeezed out, leaving behind a thick, pulpy mass, composed
mainly of silver, and locally termed a "pina," from its resembling in
shape the cone of a pine tree. These pinas are then carefully weighed and
put into a subliming furnace, Figs. 5 and 6, in order to drive off the
rest of the mercury, the silver being subsequently run into bars. About
four ounces of mercury are lost for every pound of silver made.

The actual quantities of mercury to be added in the grinding vat, and the
times of its addition, are based entirely on practical experience of the
process. With ore assaying 150 oz. to 175 oz. of silver to the ton, 75
lb. of mercury are put in at the commencement, anoth