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GOLD. 22 1

slioiild l)c allowed to selllc to the 1)oUoni, where it ni.'iy be
Avashed after pouring" off the solution. It is ])etter to lilter
than decant in this case, as, frecjuently. ])articles of the gold
float on the surface, and would be lost in the w^ashings by
the latter process.
"
Minute traces of iron may adhere to the gold thus pre-
cipitated. These can be removed l)y digesting the gold in
dilute sulphuric acid ; and, when the process is properly
conducted thus far, the result is pure gold, which may be
melted, under carbonate of potash, in a crucible lined with
borax, and reduced to the required carat."
Assay or Testing by the Touchstone.—One of the
earliest methods of assaying or testing gold was by the use
of the touchstone. • The present touchstone is a black
jasper, the best specimens coming' from India. In testing
the fineness of a gold alloy, the metal is rubbed on the
touchstone, and the streak thus produced is compared with
those made by a series of alloys of known composition,
prepared for the purpose and called fouchnccdlcs. The
effect of a drop of nitric acid and dilute aqua regia on these
several streaks is also compared. The base metals will
be more readily acted upon, and the presence of copper will
give a more or less greenish color.
Recovery of Gold from Scrap.—When scrap gold is
not free from filings, fragments of solder, and platinum it
must be refined or recovered before it can be again worked
up into plate. This is accomplished as directed under re-
fining gold.
Recovery from Sweepings.—There is ahvays more or
less loss of gold about the laboratory work bench or the
operating chair in the ofiice. These fine metal particles
become contaminated with dust and other foreign matter,
varying considerably in composition.
To recover the gold from the sweepings a magnet is