made. Many of these are insoluble in water, the chloride (CuCl)
being the best known. When suspended in dilute hydrochloric acid it is
changed into cupric chloride, the oxygen taking part in the reaction
being absorbed from the air:

2CuCl + 2HCl + O = 2CuCl_{2} + H_{2}O.

~Cupric compounds.~ Cupric salts are easily made by dissolving cupric
oxide in acids, or, when insoluble, by precipitation. Most of them are
blue or green in color, and the soluble ones crystallize well. Since
they are so much more familiar than the cuprous salts, they are
frequently called merely copper salts.

~Cupric oxide~ (CuO). This is a black insoluble substance obtained by
heating copper in excess of air, or by igniting the hydroxide or
nitrate. It is used as an oxidizing agent.

~Cupric hydroxide~ (Cu(OH)_{2}). The hydroxide prepared by treating a
solution of a copper salt with sodium hydroxide is a light blue
insoluble substance which easily loses water and changes into the oxide.
Heat applied to the liquid containing the hydroxide suspended in it
serves to bring about the reaction represented by the equation

Cu(OH)_{2} = CuO + H_{2}O.

~Cupric sulphate~ (_blue vitriol_) (CuSO_{4}.5H_{2}O). This substance,
called blue vitriol or bluestone, is obtained as a by-product in a
number of processes and is produced in very large quantities. It forms
large blue crystals, which lose water when heated and crumble to a white
powder. The salt finds many uses, especially in electrotyping and in
making electrical batteries.

~Cupric sulphide~ (CuS). The insoluble black sulphide (CuS) is easily
prepared by the action of hydrosulphuric acid upon a solution of a
copper salt:

CuSO_{4} + H_{2}S = CuS + H_{2}SO_{4}.

It is insoluble in water and dilute acids.

MERCURY

~Occurrence.~ Mercury occurs in nature chiefly as the sulphide (HgS)
called cinnabar, and in globules of metal inclosed in the cinnabar. The
mercury mines of Spain have long been famous, California being the next
largest producer.

~Metallurgy.~ Mercury is a volatile metal which has but little affinity
for oxygen. Sulphur, on the other hand, readily combines with oxygen.
These facts make the metallurgy of mercury very simple. The crushed ore,
mixed with a small amount of carbon to reduce any oxide or sulphate that
might be formed, is roasted in a current of air. The sulphur burns to
sulphur dioxide, while the mercury is converted into vapor and is
condensed in a se