re 177.

[Illustration: FIG. 176. The copper and the nickel cube ready to hang in
the cleansing solution.]

[Illustration: FIG. 177. Cleaning the copper in acids.]

Pour over the piece of copper enough of the cleansing solution to
cover it.[9] _The cleansing solution contains strong acids. If you get
any on your skin or clothes, wash it off immediately with ammonia or
soda._ As soon as the copper is bright and clean, take it out of the
cleansing solution and suspend it by the _negative_ wire in the green
nickel solution. You can tell if you have it on the negative wire, for
in that case bubbles will rise from it during the experiment. The copper
should be entirely covered by the nickel solution, but should not touch
the bottom or sides of the bowl. Pour the cleansing solution from the
evaporating dish back into the bottle. Suspend the nickel, in the same
way as the copper, from the _positive_ wire crossing the bowl. When
set up, the apparatus should appear as shown in Figure 178.

[Footnote 9: The formula for making the cleansing solution is as
follows:

1 cup water.

1 cup concentrated sulfuric acid.

1 cup concentrated nitric acid.

1 teaspoonful concentrated hydrochloric acid.

The sulfuric and nitric acids must be measured in glass or china cups,
and the hydrochloric acid must be measured in a silver-plated spoon
or in glass--not in tin.]

[Illustration: FIG. 178. Plating the copper by electricity.]

Turn on the electricity. If the copper becomes black instead of
silvery, clean it again in the cleansing solution, and move the two
bare wires much farther apart,--practically the full width of
the bowl. If the copper still turns black, it means that too much
electricity is flowing. In that case use fewer batteries.

The electricity has started two chemical changes. It has made part of
the piece of nickel combine with part of the solution of nickel salt
to form more nickel salt, and it has made some of the nickel salt
around the copper change into metallic nickel. Then the negative
electricity in the copper has attracted the positive bits of nickel
metal made from the nickel salt, and made them cling to the copper. If
there is no dirt or grease on the copper, the particles of nickel get
so close to it that they stick by adhesion, even after the electric
attraction has ceased. This leaves the copper nickel-plated, but to
make it shiny the nickel plating must be polished.

Silver plating and gold plating