tirely opposed to the practice which an erroneous theory had caused to be universally adopted. Finding that iron, in greater quantity and of better quality, was turned out by the blast furnace in winter than in summer, the ironmasters had come to the conclusion that this was due to the greater coldness of the blast in winter than in summer. So strongly were they convinced of the truth of this theory that they had recourse to various devices for the artificial refrigeration of the blast. It is one of the chief merits of Neilson as an inventor that he discovered the baselessness of this theory, and convinced himself that the superior yield of the blast furnaces in winter was to be accounted for, partly at least, by the increased moisture of the air in summer. It was, however, the comparative inefficiency of the blast in a particular case, in which the blowing-engine, instead of being near the furnace, was half a mile distant from it, that drew Neilson's attention immediately to the experiments which led ultimately to his great invention. Neilson concluded that the effects of distance between the furnace and blowing-engine would be overcome if the blast were heated by passing it through a red-hot vessel, by which its volume, and therefore the work done by it, would be increased. Experimenting on gas and on an ordinary smith's fire, he found in the one case that heated air in a tube surrounding the gas-burner increased the illuminating power of the gas, and in the other that by blowing heated air instead of air at its ordinary temperature into the fire its heat was much more intense. Of course, the cause of the increase was that the fire had not to expend a portion of its caloric to heat the cold air poured into it in the ordinary way. Neilson was now on the verge of the fruitful discovery that the blast was to be made more efficient by heating it, not by refrigerating it. Owing to a deep-seated belief in the erroneous theory that cold benefited the blast, the ironmasters were reluctant to allow Neilson to try in their furnaces the effects of a substitution of the hot for the cold blast; and even those who were disposed to permit it strongly objected to the alterations in the arrangements of their furnaces which Neilson thought necessary for a fair trial of his invention. A trial under anything like adequate conditions was consequently long deferred. Its effects were first fairly tested at the Clyde ironworks, and with such success that Charles Macintosh [q. v.], the inventor of the well-known waterproof, Colin Dunlop, and John Wilson of Dundyvan entered into a partnership with Neilson for patenting the invention. Ultimately the partnership appears to have consisted of Neilson, Macintosh, and Wilson; Neilson being entitled to six-tenths of the profits, Macintosh to three-tenths, and Wilson to one-tenth (Neilson and Harford, p. 2). Separate patents were taken out in 1828 for England, Scotland, and Ireland, that for England being dated 11 Sept., those for Scotland and Ireland 1 Oct. The specification was dated 28 Feb. 1829. To encourage the employment of the hot blast by the trade, the charge for a license to smelt iron with the hot blast was fixed at a shilling a ton on all iron produced by the new process. In 1832 Neilson joined the Institution of Civil Engineers in London.
Neilson and others soon improved the apparatus. After five years' trial at the Clyde ironworks it was found that with the hot blast the same amount of fuel produced three times as much iron, and that the same amount of blast did twice as much work as the cold blast formerly. A subsidiary benefit was that, whereas with the cold blast coke—at least in Scotland—had to be used, with the hot blast raw coal could be, and was, substituted, with a great saving of expenditure. To Scotland the invention was an inestimable benefit. It made available the black band ironstone which, since its discovery by David Mushet [q. v.], had been almost useless in the iron manufacture. In 1839 the proprietor of one estate in Scotland derived a royalty of 16,500l. from the black band, although before the invention of the hot blast it had yielded him nothing (Smiles, p. 161). In the course of time the anthracite coal of England, which could not be used in smelting iron with the cold blast, was made available for that purpose by the invention of the hot blast. By 1835 the hot blast was in operation in every ironwork in Scotland save one, and there it was in course of introduction. Except in the case of a few special bands of iron, it is now in general use in Great Britain and out of it. It has been justly said that Neilson did for the iron manufacture what Arkwright did for the cotton manufacture.
Like Arkwright, Neilson was not allowed to enjoy undisturbed the fruits of his invention. He and his partners, by beginning legal proceedings, had compelled at least one firm to give up infringing their patent and to take out a license for using it, when towards 1840 an association of Scottish ironmasters was formed, each member of which bound himself, under a penalty of 1,000l., to resist, by every method which a majority should recommend, any practical acknowledgment of the validity of Neilson's patent.