In order that the iron may be obtained from the ore it is necessary for the oxygen to be separated from it, an operation which requires the intervention of heat, and the heat must be obtained from a fuel which consists mainly of carbon. Wood fulfils these requirements, but there is not enough wood in the whole world to smelt all the iron which we need. It was not until "pit-cole" displaced "char-cole" (to use the spelling of the period) that the iron industry began to assume its present importance. To produce iron cheaply, therefore, ore and coal should for preference lie side by side, and in some few favoured localities that state of things exists. Generally speaking, however, the ore and the coal are not found together, with the result that one has to be taken to the other, and in practice it is usually the ore which is taken to the coal. Hence, the iron and steelworks are generally to be found on the coalfields, while the ore comes by rail or ship from, it may be, remote parts of the world. The method by which the metal is obtained from the ore is in principle very simple. Coal and ore are mixed together in a furnace, the fire being fanned by a powerful blast of air. The result is that the bonds uniting iron and oxygen are relaxed by the heat, when the oxygen, having a preference for union with carbon rather than with iron, leaves the latter to join up with some of the carbon of the coal. The furnace in which this operation is carried out is a tall, vertical cylinder of iron, lined with firebrick. The fire is at the bottom and the fresh fuel and ore are thrown in at the top. As the ore is "reduced" (the chemist's term for removing oxygen from anything) the liquid iron accumulates in the lowest part of the furnace, whence it is drawn off at intervals, being allowed to run into grooves or gutters in a bed of sand, where it solidifies into what is called "pig iron." Along with the coal and ore, there is thrown into the furnace from time to time quantities of limestone which combines with the earthy impurities with which the ore is contaminated. Together these form what is called "slag," which also exists, while in the furnace, as a liquid, but is so much lighter than the molten iron that it keeps quite separate and can periodically be drawn off through a hole higher up than that through which the iron is obtained. The slag solidifies into a hard stone which is broken up and used for making concrete and tar-paving, also for road metal.