Unfortunately, however, copper is found most plentifully in combination with sulphur with which it forms what is termed sulphide. This copper sulphide is called by miners "copper pyrites." Another trouble is that mixed with the copper pyrites there is usually more or less of iron pyrites, or sulphate of iron, so that to obtain the copper not only has the sulphur to be got rid of but also the iron. This complicates the operations very much, the ore having to be subjected to repeated roastings and meltings during which the sulphur passes off in the form of sulphur dioxide (a material from which sulphuric acid can be obtained), leaving oxygen in its place. Thus the copper sulphide becomes copper oxide, after which the oxygen is carried away by carbon, leaving the relatively pure metal. Moreover, at each operation various substances are thrown into the furnace called fluxes, which do not mingle with the metal but float on the top in the form of slag, and into the slag the iron passes, so that finally the copper is obtained alone. Zinc is another important material for shell-making. Its ores used to be found in great plenty in Silesia, but the chief source of supply is now Australia. It is what is called "zinc blende," and consists of zinc sulphide, or zinc and sulphur in combination. In all these names, it may be interesting to mention, at this point, the termination "ide" indicates a compound of two substances, so that we can safely conclude that the "ides" consist of the two elements named in their titles and no others. Thus zinc sulphide is zinc and sulphur and nothing else, iron sulphide is iron and sulphur, copper oxide is copper and oxygen, and so on. The blende is first roasted in huge furnaces specially built for the purpose. To ensure its being thoroughly treated it has to be "rabbled" or turned over and over, since otherwise all of it might not be brought into contact with the necessary oxygen. At one time done by men with rakes, it is now generally accomplished by mechanical means. A description of one such furnace will be of interest. It consists of a long rectangular building of brickwork bound together with steel framework. Inside it is divided up into low chambers, the roof of each forming the floor of the one above. At intervals along its length mighty shafts of iron pass up from underneath right through all the floors, emerging finally above the topmost, while along underneath the furnace there runs a shaft the action of which turns the vertical shafts slowly round and round. Attached to the vertical shafts are long strong arms of iron, one arm to each floor, and upon the arms are placed rabbles, as they are termed, pieces of iron shod sometimes with fireclay, resembling most of any familiar objects a small ploughshare. As the arms slowly revolve, at the rate of once or twice per minute, the arms are carried round and round and the rabbles plough up and turn over and over the layer of ore lying upon the floor.