t decomposed matter will rise to the surface of the water, but
before many minutes it will settle to the bottom of the glass and leave
clear water above.

PURIFICATION AND SEPARATION OF METALS.--This electrolytic action is
utilized in metallurgy for the purpose of producing pure metals, but it
is more largely used to separate copper from its base. In order to
utilize a current for this purpose, a high ampere flow and low voltage
are required. The sheets of copper, containing all of its impurities,
are placed within a tank, parallel with a thin copper sheet. The impure
sheet is connected with the positive pole of an electroplating dynamo,
and the thin sheet of copper is connected with the negative pole. The
electrolyte in the tank is a solution of sulphate of copper. The action
of the current will cause the pure copper in the impure sheet to
disintegrate and it is then carried over and deposited upon the thin
sheet, this action continuing until the impure sheet is entirely eaten
away. All the impurities which were in the sheet fall to the bottom of
the tank.

Other metals are treated in the same way, and this treatment has a very
wide range of usefulness.

ELECTROPLATING.--The next feature to be considered in electrolysis is a
most interesting and useful one, because a cheap or inferior metal may
be coated by a more expensive metal. Silver and nickel plating are
brought about by this action of a current passing through metals, which
are immersed in an electrolyte.

PLATING IRON WITH COPPER.--We have room in this chapter for only one
concrete example of this work, which, with suitable modifications, is an
example of the art as practiced commercially. Iron, to a considerable
extent, is now being coated with copper to preserve it from rust. To
carry out this work, however, an electroplating dynamo, of large
amperage, is required, the amperage, of course, depending upon the
surface to be treated at one time. The pressure should not exceed 5
volts.

The iron surface to be treated should first be thoroughly cleansed, and
then immediately put into a tank containing a cyanide of copper
solution. Two forms of copper solution are used, namely, the cyanide,
which is a salt solution of copper, and the sulphate, which is an acid
solution of copper. Cyanide is first used because it does not attack the
iron, as would be the case if the sulphate solution should first come
into contact with the iron.

A sheet of copper, termed th