unless gas is freely evolved from the surface of the article upon
which the deposit is taking place. As this solution is used hot, a
considerable evaporation takes place, which requires that additions
be made to the solution from time to time. If water alone be used
for this purpose it will precipitate a great quantity of the
copper as a white powder, but this is prevented by dissolving a
little cyanide of potassium in the water at the rate of 4 ounces
to the gallon. The vessels used in factories for this solution are
generally of copper, which are heated over a flue or in a sand-bath,
the vessel itself serving as the positive electrode of the battery;
but any vessel will suit if a copper electrode is employed when the
vessel is not of copper.


20. PREPARATION OF IRON FOR COATING WITH COPPER

When it is required to cover an iron article with copper, it is
first steeped in hot caustic potash or soda to remove any grease or
oil. Being washed from that it is placed for a short time in diluted
sulphuric acid, consisting of about one part acid to 16 parts of
water, which removes any oxide that may exist. It is then washed in
water and scoured with sand till the surface is perfectly clean,
and finally attached to the battery and immersed in the cyanide
solution. All this must be done with despatch so as to prevent the
iron combining with oxygen. An immersion of five minutes duration in
the cyanide solution is sufficient to deposit upon the iron a film
of copper, but it is necessary to the complete protection of the
iron that it should have a considerably thick coating, and as the
cyanide process is expensive, it is preferable when the iron has
received a film of copper by the cyanide solution, to take it out,