Large quantities of coke are, however, consumed in the feeding of furnace fires, and in the heating of boilers of locomotives, as well as in metallurgical operations; and in order to supply the demand, large quantities of coal are "coked," a process by which the volatile products are completely combusted, pure coke remaining behind. This process is therefore the direct opposite to that of "distillation," by which the volatile products are carefully collected and re-distilled. The sulphurous impurities which are always present in the coal, and which are, to a certain extent, retained in coke made at the gas-works, themselves have a value, which in these utilitarian days is not long likely to escape the attention of capitalists. In coal, bands of bright shining iron pyrites are constantly seen, even in the homely scuttle, and when coal is washed, as it is in some places, the removal of the pyrites increases the value of the coal, whilst it has a value of its own. The conversion of the sulphur which escapes from our chimneys into sulphuretted hydrogen, and then into sulphuric acid, or oil of vitriol, has already been referred to, and we can only hope that in these days when every available source of wealth is being looked up, and when there threatens to remain nothing which shall in the future be known as "waste," that the atmosphere will be spared being longer the receptacle for the unowned and execrated brimstone of millions of fires and furnaces. CHAPTER VII. THE COAL SUPPLIES OF THE WORLD. As compared with some of the American coal-fields, those of Britain are but small, both in extent and thickness. They can be regarded as falling naturally into three principal areas. The northern coal-field, including those of Fife, Stirling, and Ayr in Scotland; Cumberland, Newcastle, and Durham in England; Tyrone in Ireland. The middle coal-field, all geologically in union, including those of Yorkshire, Derbyshire, Shropshire, Staffordshire, Flint, and Denbigh. The southern coal-field, including South Wales, Forest of Dean, Bristol, Dover, with an offshoot at Leinster, &c., and Millstreet, Cork. Thus it will be seen that while England and Scotland are, in comparison with their extent of surface, bountifully supplied with coal-areas, in the sister island of Ireland coal-producing areas are almost absent. The isolated beds in Cork and Tipperary, in Tyrone and Antrim, are but the remnants left of what were formerly beds of coal extending the whole breadth and length of Ireland. Such beds as there remain undoubtedly belong to the base of the coal-measures, and observations all go to show that the surface suffered such extreme denudation subsequent to the growth of the coal-forests, that the wealth which once lay there, has been swept away from the surface which formerly boasted of it.