The invention of the hot blast gave a new and mighty impulse to the production of iron. Though Mr. Scrivenor[58] mentions the remarkable fact that in the furnaces of Peru a contrivance has been noticed for letting the air pass over hot coals, and thus become heated in its passage to the fire, yet it was personal observation, and not archæological research, that, in 1829, suggested to Mr. Neilson, of the Clyde Iron Works, the possibility of economising fuel by substituting hot for cold air in blowing his furnaces. Before this important discovery more than eight tons of coke had been required to produce one ton of pig-iron; but on heating the blast, previously to its entering the smelting-oven, to a temperature of 300° F., it was found that a saving of two and a half tons of coal could be made on every ton of iron, and on raising it to the temperature of 600° F.—a heat somewhat more than sufficient to melt lead—a still more considerable saving of fuel was effected, while at the same time the important discovery was made that at this high temperature bituminous and even anthracitic coal might be used instead of coke. Another advantage was that the same steam power now sufficed for applying the blast to four furnaces which had formerly been required for three; and the total result of the improvement was a saving of 72 per cent. of fuel. Thus we have here another instance of the important results that may be gained from a single good idea when worked out by clever practical men, for the hot blast has most assuredly increased the wealth of England by many millions a year! Footnote 58: ‘History of the Iron Trade.’ Another circumstance likewise tended considerably to increase the production of pig-iron. It was found that the hot blast not only had power sufficient to produce in the raw coal the requisite intensity of heat, but also to expel from it, to a certain extent, the sulphur, which injured the quality of the iron, and thus a great economy in labour as well as in the quantity of fuel was effected. Since then the black-band, an iron-stone found in great quantities in Scotland, and also, to a less extent, in Wales, but not readily convertible into iron by the old methods, and also the Northamptonshire and the Cleveland ores, discoveries of a later date and of an incalculable extent, have been made by the hot blast to yield their iron in great abundance. The power of using the black-band alone in the furnace, and not, as before the introduction of the hot blast, in small quantities only, and combined with other ores, constituted a new era in the manufacture of iron, and gave to Scotland, till then an iron-making district of little importance,[59] the pre-eminence over all others for the production of soft fluid iron, best suited to ordinary founding purposes. Footnote 59: