troplating can continue as long as any positive electrode remains.

[Illustration: FIG. 209.--Plating spoons by electricity.]

Practically all silver, gold, and nickel plating is done in this way;
machine, bicycle, and motor attachments are not solid, but are of
cheaper material electrically plated with nickel. When spoons are to
be plated, they are hung in a bath of silver nitrate side by side with
a thick slab of pure silver, as in Figure 209. The spoons are
connected with the negative terminal of the battery, while the slab of
pure silver is connected with the positive terminal of the same
battery. The length of time that the current flows determines the
thickness of the plating.

294. How Pure Metal is obtained from Ore. When ore is mined, it
contains in addition to the desired metal many other substances. In
order to separate out the desired metal, the ore is placed in some
suitable acid bath, and is connected with the positive terminal of a
battery, thus taking the place of the silver slab in the last Section.
When current flows, any pure metal which is present is dissolved out
of the ore and is deposited on a convenient negative electrode, while
the impurities remain in the ore or drop as sediment to the bottom of
the vessel. Metals separated from the ore by electricity are called
electrolytic metals and are the purest obtainable.

295. Printing. The ability of the electric current to decompose a
liquid and to deposit a metal constituent has practically
revolutionized the process of printing. Formerly, type was arranged
and retained in position until the required number of impressions had
been made, the type meanwhile being unavailable for other uses.
Moreover, the printing of a second edition necessitated practically as
great labor as did the first edition, the type being necessarily set
afresh. Now, however, the type is set up and a mold of it is taken in
wax. This mold is coated with graphite to make it a conductor and is
then suspended in a bath of copper sulphate, side by side with a slab
of pure copper. Current is sent through the solution as described in
Section 293, until a thin coating of copper has been deposited on the
mold. The mold is then taken from the bath, and the wax is replaced by
some metal which gives strength and support to the thin copper plate.
From this copper plate, which is an exact reproduction of the original
type, many thousand copies can be printed. The plate can be preserved
an