This is the latest method, and that now in use. To a solution of sulphate of copper, add a solution of ferrocyanide of pottasium, so long as a precipitate continues to be formed. This is allowed to settle, and the clear liquor being decanted the vessel is filled with water, and when the precipitate settles the liquor is again decanted, and continue to repeat these washings until the sulphate of potash is washed quite out; this is known by adding a little chloride of barium to a small quantity of the washings, and when there is no white precipitate formed by the test, the precipitate is sufficiently washed. A solution of cyanide of potassium is now added to this precipitate until it is dissolved, during which process the solution becomes warm by the chemical re-action which takes place. The solution is filtered, and allowed to repose all night. If the solution of cyanide of potassium that is used is strong, the greater portion of the ferrocyanide of potassium crystalises in the 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.