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 are done substantially in the way that you have done the nickel plating, only gold salt or silver salt is used instead of nickel salt. Just as electricity helps chemical changes in plating, it helps changes in a storage battery. But in the storage battery the new compounds formed by "charging" the battery change back again and generate electricity when the poles of the battery are connected with each other by a good conductor. _APPLICATION 75._ Explain how spoons can be silver plated; how water can be changed into hydrogen and oxygen. INFERENCE EXERCISE Explain the following: 471. Clothes dry best in the sun and wind.