nd water, in a
thickness of not more than a quarter of an inch, pass over it slowly.
There are various methods of covering copper plates with quicksilver.
The first thing, in every case, is to wash the copper with diluted
nitric acid, so as to remove all dirt and grease. The quicksilver may
then be rubbed on with a rag; or, still better, it may be dissolved in
nitric acid, and the liquid nitrate of quicksilver may be applied with
a rag. The nitric acid will attack the copper, and leave the
quicksilver as an amalgam on the surface of the copper. This is the
most common process, but the nitrate of copper continues for a long
time to come up through the quicksilver and interfere with the catching
of the gold. When the nitrate of copper appears--it is a green
slime--it should be scraped off and the place rubbed over with
quicksilver. When a plate is once covered with mercury, the operation
need never be repeated; but more mercury must be sprinkled on as the
gold collects and forms a solid amalgam. The plate is usually three
feet wide and six feet long, and is set nearly level. In very large
sluices the stream should be divided so as to run over different
plates. The slowness of the current and the shallowness of the water
are important, for with a swift current or deep water many of the
particles of float-gold may escape without touching the quicksilver.
Wherever a speck of gold has fixed itself on the plate, there others
will collect about, evidently preferring to fix themselves in a
neighborhood rather than in a waste place. The more gold there is on a
plate, the better it is considered to be. The seasons for cleaning up
are usually determined by the danger of theft. Miners do not like to
leave their gold out in quantities so large as to attract thieves. The
amalgam is sometimes half an inch thick, and is usually, at cleaning-up
time, a hard mass, which must be loosened by heat. The plate is put on
a fire, and when it gets so warm that the hand can scarcely bear it,
the amalgam is softened and loosened, so that it can be scraped off
readily. The plate is then sprinkled anew with quicksilver, and is
ready for use again. Mercury does not amalgamate with copper so readily
as with gold or silver. A copper plate, the sixteenth of an inch thick,
may be used for at least five years, and perhaps for ten; whereas a
gold plate of equal thickness would, if exposed to the action of
quicksilver in the same manner, fall to pieces in a f