solution, and may be collected and preserved for use again.  If the solution of cyanide of potassium used to dissolve the precipitate is dilute, it will be necessary to condense the liquor by evaporation to obtain the yellow prussiate in crystals.  The remaining solution is the coppering solution; should it not be convenient to separate the yellow prussiate by crystallization, the presence of that salt in the solution does not deteriorate it nor interfere with its power of depositing copper.

19.  Peculiarities in working cyanide of copper solution

The true composition of the salts thus formed by copper and cyanide of potassium has not yet been determined, but their relations to the battery and electrolyzation are peculiar.  The solution must be worked at a heat not less than from 150 to 200 degrees Farenheit (that is not quite as hot a boiling water, which is 212 degrees Farenheit.) All other solutions we have tried follow the laws, that if the electricity is so strong as to cause gas to be evolved at the electrode, the metal will be deposited in a sandy or powdered state, but the solution of cyanide of copper and potassium is an exception to these laws, as there is no reguline deposit obtained 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