the
alloy with a smooth hammer on an anvil, and then boil it in strong
nitric acid in a glass flask for about ten minutes. Carefully pour off
the acid into a vial, and wash the powder in the flask (which is fine
gold) with water and dry. To the liquid in the vial add a solution of
common salt. The white powder which falls should be removed, washed
with water, and fused with powdered chalk or iron filings; a button of
pure silver is the result.
Any
substance supposed to be or to contain gold may be tested by dissolving
it powdered in aqua regia aud then pouring in a solution of copperas;
if there is gold, the reddish-brown precipitate, by rubbing, assumes a
bright metallic lustre.
To
tell whether a globule of silver has any gold it in, put it on a white
porcelain dish and moisten it with a drop of nitric acid ; if it is
pure silver, it will dissolve and retain its white color; if mixed with
gold, it will soon turn gray or black.
To
test the purity of gold, rub some of it off on a hard black flint
slate, and apply to the mark a drop of aqua fortis. If the gold is
pure, the yellow streak remains unchanged, but if alloyed it partly
disappears; if it is only an imitation of gold, it vanishes altogether.
A
ready method of finding the amount of gold in a quartz rock with
considerable accuracy, is by taking the specific gravity of the rock
(well cleaned) as given on page 13. If the gravity is not over 2*7, it
contains little or no gold. If it is 3, it very likely is gold-bearing,
although pyrites may be present. But if it is over 5, it is undoubtedly
auriferous, and if 12, it is very rich in gold.
It
is generally considered that the sand of any river is worth working for
the gold it contains, provided it will yield twenty-four grains to the
hundred weight.
Assay of Silver Ore.—Pure silver is easily recognised. But lead and copper ores often contain a large percentage of the precious metal.
To
detect silver in lead ore, dissolve the powdered ore in strong nitric
acid ; pour off the liquid and insert a piece of pure copper. If silver
is present, it will go to the bottom. Or add to the liquid a solution
of common salt, and it will instantly become cloudy or white. If lead
ore yields three ounces of silver to a ton, it may be worked for the
silver as well as the lead. In Colorado, the average value of
silver-bearing galena is f 100 per ton.
To
test the copper ores for silver, dissolve them in nitric acid; then add
a few drops of muriatic acid, and if silver is present, a white curdy
precipitate will fall to the bottom. Native copper, when polished,
often shows white spots of silver.
To
estimate the proportion of silver in lead ore, reduce a known quantity
of the clear ore to powder, mix with a little dry soda and a few nails
and heat in a round bottomed iron pot or crucible. The lead which is
obtained should then be put in a cup having ashes at the bottom, and
strongly heated in an open furnace. A globule of silver will be left,
if any is present, and being weighed, the percentage can be found. v
Rich
silver ores may be reduced by mixing them with ten parts of common
salt, and exposing the mass for hours in an open furnace, stirring it
frequently. When cold reduce to powder and mix with an equal quantity
of quicksilver and enough Water to make a paste, and agitate the
mixture for two days, when the amalgam will fall to the bottom. The
amalgam is then squeezed in a leather bag and washed.
Silver glance will yield its metal by heating it before a blow-pipe.
Assay of Coiter Ore.—When
the ore is native copper and rock, as at Lake Superior, it should be
pounded and the earthy matter washed away. Then mix with a little
potash or soda a'nd bring to a high heat in a crucible.
Other
copper ores may he tested by dissolving them powdered in dilute aqua
regia. The presence of silver will be shown by a white powder on the
bottprn. Then add considerable ammonia. If there is any copper a blue
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