he floor, and a large flat stone is
fastened to the end of each bar by means of a rope. A horse is
hitched to one of the bars, which is purposely left longer than the
others. The ore is thrown into the arastra, and water is admitted,
a little at a time. As the horse is driven around the stones are
dragged over the circular depression, crushing the ore and setting
free the gold.

[Illustration: FIG. 103.--THE STAMPS IN A QUARTZ-MILL]

This way of separating the gold was too slow, and in a short time
the stamp-mill was invented. It has grown from a very simple affair
into the great mill which crushes hundreds of tons of ore in a
day. The iron stamps each weigh nearly half a ton. They are raised
by powerful machinery and allowed to drop in succession upon the
ore, which is gradually fed under them. The stamps crush the ore
to a fine sand more easily and rapidly than could be done by any
other method. Water is kept running over the ore, and as fast as
it is crushed sufficiently fine for the particles to pass through
a wire screen, the water with which they are mixed is allowed to
flow over large plates of copper which have been coated with
quicksilver. The latter mineral has an attraction for gold, and
so catches and holds most of the particles, no matter how small
they are.

The compound of gold and quicksilver is a soft white substance
known as amalgam, utterly unlike either metal. When the amalgam
is subjected to heat, the quicksilver is driven off in the form
of a vapor, and the gold is left pure. The quicksilver vapor is
condensed in a cool chamber and is used again.

The iron pyrites in the ore contains gold which cannot be separated
by the crushing process and a machine called a concentrator has
been invented to save this also. After passing over the copper
plates the crushed rock and pyrites are washed upon a broad, flat
surface, which is moving in such a way that the lighter rock waste
is carried away by the water. The pyrites now appears as a dark,
heavy sand. This sand is placed in a roasting furnace, where the
sulphur is driven off, and the gold and iron are left together.
Now the gold is dissolved by means of chlorine gas, with which it
unites in a compound called gold chloride. From this compound the
metallic gold is easily separated. All this may seem a complicated
process, but it is carried through so cheaply that the ore which
contains only two or three dollars to the ton can be profitably
worked.