The replacement of charcoal by coal led to a great diminution of the cost of production, and permitted the use of low-grade ores, but it was not in itself a great improvement. Charcoal is a singularly pure form of carbon, and its use as the reducing agent gives a high quality of iron. Coal, on the other hand, often contains impurities which spoil the iron, and have to be provided against in various ways. Not all coal, indeed, is suitable for iron smelting. The result is that where charcoal can still be obtained cheaply, as in the Scandinavian countries and in parts of Russia, it is still used in smelting, and the iron so produced is particularly valuable. The original demand for iron, as we have seen, was very small, and even down to the middle of the eighteenth century remained insignificant. But with the use of machinery, the spread of railways, the replacement of wood by iron in shipbuilding and for the framework of buildings, etc., the demand in all civilised communities has become enormous, has become too great to permit of any forests supplying the necessary fuel. With the far increased demand has come an elaboration of methods which means very costly installations and much skill and training among the workers. From the time of the industrial revolution till the present, then, a well-developed iron industry has demanded the following:--fuel, usually furnished by coal; an abundant supply of the ore, either furnished locally or easily obtained by water carriage, ores being so bulky that land carriage is rarely profitable; certain accessories, notedly limestone to serve as a flux, and ganister, a kind of sandstone used to form a lining in parts of the apparatus used; capital, necessary for the purchase and fitting up of the costly plant; the tradition and skill which come from the long practice of the industry. The significance of these various necessaries may perhaps be realised by a few examples. Let us consider first the Mediterranean area. We have seen that it was civilised from a very early date, that a considerable part at least of that civilisation was indigenous, and that its early smiths showed no lack of skill. But with the advent of the age of iron its natural handicap became obvious. The forests of the region never had the luxuriance of those further north, and must have been early destroyed, and coal is virtually absent. Iron ores are present and are widely distributed; some, like those of Elba and of northern Spain, which is outside the area, are even rich; but the absence of fuel is a terrible handicap, and to its absence we must ascribe the present poverty and backwardness of Mediterranean countries.