When the ores are pure, cast iron manufactured by charcoal, is not liable to such a fault. Hence the cast iron of Sweden and the United States, manufactured from the magnetic iron, or, in some cases in this country, from rich hæmatites, has very superior tenacity, insomuch that these two nations have alone been able to use this material in the construction of field pieces. When white iron is obtained from a furnace, it may have two different qualities. The first arises from a mere defect of heat, where all other circumstances are favourable, and the ore is completely reduced. The second arises when the reduction is not complete, and the separation of the earths and other oxides has not been fully effected. Of all the varieties of cast iron, this latter is by far the worst. It is indeed more easily converted into wrought iron than the other species, but the product is always of very inferior quality; it is rarely or never produced by furnaces fed with charcoal, but may be obtained by accident or design in those where coke is used, by a surcharge of ore, or by too great a proportion of flux, and sometimes cannot be avoided in warm and moist weather, where the air is rarefied and charged with vapour. The grey iron obtained by the use of each of the different kinds of fuel, has its own peculiar advantages; that made with coke possessing, as a general rule, when melted, a higher degree of fluidity which adapts it for more delicate castings; being softer and better suited for fitting; while that manufactured with charcoal, possesses a greater degree of strength. One solitary instance has been quoted, in which a manufacturer of great intelligence has obtained by the use of charcoal, from a very pure ore, a union of both these valuable properties, and another, in which iron as soft as that made with coke, has been produced by means of charcoal. In spite of this apparent balance in the properties of the two fuels, the introduction of coke into the art of reducing iron has been attended with the most important advantages. These lie in the superior economy of the process, and in the enormous quantity of the product. The manufacture of iron by charcoal is limited, by the growth of the forests, which replace themselves only at distant periods, by the large space they occupy, and the consequent labour of transportation; by the cost of cutting the wood and preparing the coal; and finally, even when the fuel can be obtained in abundance, and at small cost, the burden of the furnace, and the heat obtained in a given space are less than when coke is used, and the quantity of metal yielded is in consequence comparatively small. The coke furnaces of Great Britain, have therefore supplied cast iron in such abundance and at such diminished prices as to have brought it into use for a great variety of purposes, to which, until recently, it was hardly considered applicable.