There are arms on the top of the furnace, too, sometimes, where the ore is first laid so that it may be dried by the heat escaping from the furnace beneath, an interesting example of economy effected by utilizing heat which would otherwise be wasted. The whole of the furnace, from end to end and on every floor, is thus swept continually by the rotating arms with their dependent rabbles, and the latter are cunningly shaped so that they not only turn the ore over and over, but gradually pass it along the different floors or hearths. It is fed automatically by a mechanical feeder which pushes it on, a small quantity at a time, to the drying hearth on the top. Then the rabbles take charge of it and gradually pass it from the area swept by one shaft to that of the next until it has passed right along the top and has become thoroughly dried. Arrived there it falls through a hole on to the topmost hearth or floor, along which it travels by the same means but in the contrary direction until it again falls through a hole on to the top floor but one. And so it goes on until at last, fully roasted, it falls from the bottom floor of the furnace into trucks or other provision for carrying it away. Some kinds of ore require to be heated by means of gas which is generated in a "gas-producer" near by. In others, however, the sulphur in the ore acts as the fuel, and so the furnace, having been once started, can be kept up for long periods without the expenditure of any coal at all. Very little attention is needed by furnaces such as these, so that with no fuel to pay for and very little labour, they are extremely economical. Owing to the great heat, too, the arms would stand a very good chance of getting melted were they not kept cool by a continual stream of water flowing through the shafts and arms. This furnishes a continual supply of hot water which is sometimes used for other purposes in the works. The process of roasting, whether carried on in furnaces such as these or not, results in the formation of oxide instead of sulphide; in other words, the sulphur is turned out and oxygen takes its place. The dislodged sulphur then joins up with some more oxygen and forms sulphur dioxide, which can be led away to the sulphuric acid plant and there, by union with water, turned into that extremely valuable substance, sulphuric acid. We cannot, however, treat zinc oxide as we would iron oxide or copper oxide, for zinc is volatile, and so, instead of accumulating in the bottom of a blast furnace as the iron and copper do, would pass off up the chimney. The oxide is therefore mixed with coal or some other form of carbon and placed in retorts made of fireclay. These retorts are fixed in rows one above the other like the retorts at a gasworks, and hot gases from a gas-producer down below pass around and among them. To the mouth of each retort is fitted a condenser, also made of fireclay.