The metal copper, as obtained from copper ore, contains many impurities of various kinds. For most purposes these impurities greatly affect the value of the copper, and before the metal can be of much commercial use they must be got rid of in some way. In the previous chapter, in describing how to copperplate an old spoon, we saw that the anode need not consist of pure copper, because in any case nothing but the pure metal would be deposited upon the spoon. This fact forms the basis of the important industry of electrolytic copper refining. The process is exactly the same as ordinary copperplating, except that the cathode always consists of absolutely pure copper. This is generally in the form of a sheet no thicker than thin paper, but sometimes a number of suspended wires are used instead. A solution of copper sulphate is used as usual for the electrolyte, and the anode is a thick cast plate of the impure copper. The result of passing a current through the solution is that copper is taken from the anode and carried to the cathode, the impurities falling to the bottom of the vat and accumulating as a sort of slime. In this way thick slabs of pure copper are obtained, ready to be melted down and cast into ingots. The impurities in the raw copper vary according to the ore from which it is obtained, and sometimes gold and silver are found amongst them. When the copper is known to contain these metals the deposit at the bottom of the refining vats is carefully collected, and from it a considerable quantity of gold and silver is recovered. It is estimated that about half a million tons of copper are refined every year. An immense amount of this pure copper is used for electrical purposes, for making conducting wires and cables, and innumerable parts of electric appliances and machinery of all kinds; in fact it is calculated that more than half of the copper produced all over the world is used in this way. A similar method is employed to obtain the precious metals in a pure state, from the substance known as “bullion”; which consists usually of an intermingling of gold, silver, and copper, with perhaps also lead. Just as in copper refining, the raw material is used as the anode, and a strip of pure gold or silver, according to which metal is required, as the cathode. A silver solution is used if silver is wanted, and a gold solution if gold is to be deposited.