The Romance of Modern Chemistry

CHEMISTRY AND ELECTRICITY result of the electrolysis is that copper is transferred from the anode to the cathode, the latter increasing in weight exactly as fast as the former becomes lighter. This simple operation is really of very great technical importance, for the greater part of the world’s supply of copper is refined on the same principle. Plates of the impure copper which comes from the smelter are used as anodes in baths of acidified copper sulphate, while sheets of pure copper act as the cathodes. When a current is passed through such a bath, the anode is gradually dissolved, as already described, and pure copper is de- posited on the cathode. The impurities in the anode either pass into the solution and remain there, or else settle down to the bottom of the bath as a sort of sludge. The small quantities of gold and silver which are present in crude copper are thus deposited in the sludge, which is worked up for the sake of these valuable metals after the electrolysis is over. It is estimated that in the United States alone about 250,000 tons of copper are refined every year by this electrolytic process, 27,000,000 ounces of silver, and 346,000 ounces of gold being obtained as by-products from the sludge. Electrolysis, however, is applied, not only in the purification of metals which have been produced by smelting, but in obtaining the metals themselves from their compounds. Aluminium furnishes the best example of this operation, for nowadays it is obtained exclusively by the electrolysis of alumina, the oxide of the metal. This material is found in various forms and in great abundance on the surface of the earth, but if it is to be employed in the electrolytic production of aluminium, it must first be purified and separated from the dross which 299