taining the gold
is washed away from its natural location into the sluices by powerful
streams of water delivered under pressure from pipes. This is called
hydraulic mining. In vein mining the gold-bearing quartz is mined from
the veins, stamped into fine powder in stamping mills, and the gold
extracted by one of the processes to be described.

~Extraction.~ 1. _Amalgamation process._ In the amalgamation process the
powder containing the gold is washed over a series of copper plates
whose surfaces have been amalgamated with mercury. The gold sticks to
the mercury or alloys with it, and after a time the gold and mercury are
scraped off and the mixture is distilled. The mercury distills off and
the gold is left in the retort ready for refining.

2. _Chlorination process._ When gold occurs along with metallic
sulphides it is often extracted by chlorination. The ore is first
roasted, and is then moistened and treated with chlorine. This dissolves
the gold but not the metallic oxides:

Au + 3Cl = AuCl_{3}.

The gold chloride, being soluble, is extracted from the mixture with
water, and the gold is precipitated from the solution, usually by adding
ferrous sulphate:

AuCl_{3} + 3FeSO_{4} = Au + FeCl_{3} + Fe_{2}(SO_{4})_{3}.

3. _Cyanide process._ This process depends upon the fact that gold is
soluble in a solution of potassium cyanide in the presence of the oxygen
of the air. The powder from the stamping mills is treated with a very
dilute potassium cyanide solution which extracts the gold:

2Au + 4KCN + H_{2}O + O = 2KOH + 2KAu(CN)_{2}.

From this solution the gold can be obtained by electrolysis or by
precipitation with metallic zinc:

2KAu(CN)_{2} + Zn = K_{2}Zn(CN)_{4} + 2Au.

~Refining of gold.~ Gold is refined by three general methods:

1. _Electrolysis._ When gold is dissolved in a solution of potassium
cyanide, and the solution electrolyzed, the gold is deposited in very
pure condition on the cathode.

2. _Cupellation._ When the gold is alloyed with easily oxidizable
metals, such as copper or lead, it may be refined by cupellation. The
alloy is fused with an oxidizing flame on a shallow hearth made of bone
ash, which substance has the property of absorbing metallic oxides but
not the gold. Any silver which may be present remains alloyed with the
gold.

3. _Parting with sulphuric acid._ Gold may be separated from silver, as
well as from many other metals, by heating the alloy with conce