ydrochloric acid, dilute sulphuric acid, and fused alkalis are almost
without action upon it; nitric acid and hot, concentrated sulphuric
acid, however, readily dissolve it. In moist air it slowly becomes
covered with a thin layer of green basic carbonate; heated in the air it
is easily oxidized to black copper oxide (CuO).

~Uses.~ Copper is extensively used for electrical purposes, for roofs and
cornices, for sheathing the bottom of ships, and for making alloys. In
the following table the composition of some of these alloys is
indicated:

COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES

Aluminium bronze copper (90 to 97%), aluminium (3 to 10%).
Brass copper (63 to 73%), zinc (27 to 37%).
Bronze copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%).
German silver copper (56 to 60%), zinc (20%), nickel (20 to 25%).
Gold coin copper (10%), gold (90%).
Gun metal copper (90%), tin (10%).
Nickel coin copper (75%), nickel (25%)
Silver coin copper (10%), silver (90%).

~Electrotyping.~ Matter is often printed from electrotype plates
which are prepared as follows. The matter is set up in type and
wax is firmly pressed down upon the face of it until a clear
impression is obtained. The impressed side of the wax is coated
with graphite and the impression is made the cathode in an
electrolytic cell containing a copper salt in solution. When
connected with a current the copper is deposited as a thin
sheet upon the letters in wax, and when detached is a perfect
copy of the type, the under part of the letters being hollow.
The sheet is strengthened by pouring on the under surface a
suitable amount of molten metal (commercial lead is used). The
sheet so strengthened is then used in printing.

~Two series of copper compounds.~ Copper, like iron, forms two series of
compounds: in the cuprous compounds it is univalent; in the cupric it is
divalent. The cupric salts are much the more common of the two, since
the cuprous salts pass readily into cupric by oxidation.

~Cuprous compounds.~ The most important cuprous compound is the oxide
(Cu_{2}O), which occurs in nature as ruby copper or cuprite. It is a
bright red substance and can easily be prepared by heating copper to a
high temperature in a limited supply of air. It is used for imparting a
ruby color to glass.

By treating cuprous oxide with different acids a number of cuprous salts
can be