At length, however, Mr. Neilson succeeded in inducing Mr. Charles Macintosh of Crossbasket, and Mr. Colin Dunlop of the Clyde Iron Works, to allow him to make a trial of the hot air process. In the first imperfect attempts the air was heated to little more than 80 degrees Fahrenheit, yet the results were satisfactory, and the scoriae from the furnace evidently contained less iron. He was therefore desirous of trying his plan upon a more extensive scale, with the object, if possible, of thoroughly establishing the soundness of his principle. In this he was a good deal hampered even by those ironmasters who were his friends, and had promised him the requisite opportunities for making a fair trial of the new process. They strongly objected to his making the necessary alterations in the furnaces, and he seemed to be as far from a satisfactory experiment as ever. In one instance, where he had so far succeeded as to be allowed to heat the blast-main, he asked permission to introduce deflecting plates in the main or to put a bend in the pipe, so as to bring the blast more closely against the heated sides of the pipe, and also increase the area of heating surface, in order to raise the temperature to a higher point; but this was refused, and it was said that if even a bend were put in the pipe the furnace would stop working. These prejudices proved a serious difficulty in the way of our inventor, and several more years passed before he was allowed to put a bend in the blast-main. After many years of perseverance, he was, however, at length enabled to work out his plan into a definite shape at the Clyde Iron Works, and its practical value was at once admitted. At the meeting of the Mechanical Engineers' Society held in May, 1859, Mr. Neilson explained that his invention consisted solely in the principle of heating the blast between the engine and the furnace, and was not associated with any particular construction of the intermediate heating apparatus. This, he said, was the cause of its success; and in some respects it resembled the invention of his countryman, James Watt, who, in connection with the steam-engine, invented the plan of condensing the steam in a separate vessel, and was successful in maintaining his invention by not limiting it to any particular construction of the condenser. On the same occasion he took the opportunity of acknowledging the firmness with which the English ironmasters had stood by him when attempts were made to deprive him of the benefits of his invention; and to them he acknowledged he was mainly indebted for the successful issue of the severe contests he had to undergo. For there were, of course, certain of the ironmasters, both English and Scotch, supporters of the cause of free trade in others' inventions, who sought to resist the patent, after it had come into general use, and had been recognised as one of the most valuable improvements of modern times.[3]
The patent was secured in 1828 for a term of fourteen years; but, as Mr. Neilson did not himself possess the requisite capital to enable him to perfect the invention, or to defend it if attacked, he found it necessary to invite other gentlemen, able to support him in these respects, to share its profits; retaining for himself only three-tenths of the whole. His partners were Mr. Charles Macintosh, Mr. Colin Dunlop, and Mr. John Wilson of Dundyvan. The charge made by them was only a shilling a ton for all iron produced by the new process; this low rate being fixed in order to ensure the introduction of the patent into general use, as well as to reduce to a minimum the temptations of the ironmasters to infringe it.
The first trials of the process were made at the blast-furnaces of Clyde and Calder; from whence the use of the hot blast gradually extended to the other iron-mining districts. In the course of a few years every furnace in Scotland, with one exception (that at Carron), had adopted the improvement; while it was also employed in half the furnaces of England and Wales, and in many of the furnaces on the Continent and in America. In course of time, and with increasing experience, various improvements were introduced in the process, more particularly in the shape of the air-heating vessels; the last form adopted being that of a congeries of tubes, similar to the tubular arrangement in the boiler of the locomotive, by which the greatest extent of heating surface was provided for the thorough heating of the air. By these modifications the temperature of the air introduced into the furnace has been raised from 240 degrees to 600 degrees, or the temperature of melting lead. To protect the nozzle of the air-pipe as it entered the furnace against the action of the intense heat to which it was subjected, a spiral pipe for a stream of cold water constantly to play in has been introduced within the sides of the iron tuyere through which the nozzle passes; by which means the tuyere is kept comparatively cool, while the nozzle of the air-pipe is effectually protected.[4]
This valuable invention did not escape the usual fate of successful patents, and it was on several occasions the subject of protracted litigation. The first action occurred in 1832; but the objectors shortly gave in, and renewed their licence. In 1839, when the process had become generally adopted throughout Scotland, and, indeed, was found absolutely essential for smelting the peculiar ores of that country—more especially Mushet's Black Band—a powerful combination was formed amongst the ironmasters to resist the patent. The litigation which ensued extended over five years, during which period some twenty actions were proceeding in Scotland, and several in England. Three juries sat upon the subject at different times, and on three occasions appeals were carried to the House of Lords. One jury trial occupied ten days, during which a hundred and two witnesses were examined; the law costs on both sides amounting, it is supposed, to at least 40,000L. The result was, that the novelty and merit of Mr. Neilson's invention were finally established, and he was secured in the enjoyment of the patent right.
We are gratified to add, that, though Mr. Neilson had to part with two-thirds of the profits of the invention to secure the capital and influence necessary to bring it into general use, he realized sufficient to enable him to enjoy the evening of his life in peace and comfort. He retired from active business to an estate which he purchased in 1851 in the Stewartry of Kirkcudbright, where he is found ready to lend a hand in every good work—whether in agricultural improvement, railway extension, or the moral and social good of those about him. Mindful of the success of his Workmen's Institution at the Glasgow Gas-Works, he has, almost at his own door, erected a similar Institution for the use of the parish in which his property is situated, the beneficial effects of which have been very marked in the district. We may add that Mr. Neilson's merits have been recognised by many eminent bodies—by the Institution of Civil Engineers, the Chemical Society, and others—the last honour conferred on him being his election as a Member of the Royal Society in 1846.
The invention of the hot blast, in conjunction with the discovery of the Black Band ironstone, has had an extra ordinary effect upon the development of the iron-manufacture of Scotland. The coals of that country are generally unfit for coking, and lose as much as 55 per cent. in the process. But by using the hot blast, the coal could be sent to the blast-furnace in its raw state, by which a large saving of fuel was effected.[5] Even coals of an inferior quality were by its means made available for the manufacture of iron. But one of the peculiar qualities of the Black Band ironstone is that in many cases it contains sufficient coaly matter for purposes of calcination, without any admixture of coal whatever. Before its discovery, all the iron manufactured in Scotland was made from clay-band; but the use of the latter has in a great measure been discontinued wherever a sufficient supply of Black Band can be obtained. And it is found to exist very extensively in most of the midland Scotch counties,—the coal and iron measures stretching in a broad belt from the Firth of Forth to the Irish Channel at the Firth of Clyde. At the time when the hot blast was invented, the fortunes of many of the older works were at a low ebb, and several of them had been discontinued; but they were speedily brought to life again wherever Black Band could be found. In 1829, the year after Neilson's patent was taken out, the total make of Scotland was 29,000 tons. As fresh discoveries of the mineral were made, in Ayrshire and Lanarkshire, new works were erected, until, in 1845, we find the production of Scotch pig-iron had increased to 475,000 tons. It has since increased to upwards of a million of tons, nineteen-twentieths of which are made from Black Band ironstone.[6]
Employment has thus been given to vast numbers of our industrial population, and the wealth and resources of the Scotch iron districts have been increased to an extraordinary extent. During the last year there were 125 furnaces in blast throughout Scotland, each employing about 400 men in making an average of 200 tons a week; and the money distributed amongst the workmen may readily be computed from the fact that, under the most favourable circumstances, the cost of making iron in wages alone amounts to 36s. a-ton.[7]
An immense additional value was given to all land in which the Black Band was found. Mr. Mushet mentions that in 1839 the proprietor of the Airdrie estate derived a royalty of 16,500L. from the mineral, which had not before its discovery yielded him one farthing. At the same time, many fortunes have been made by pushing and energetic men who have of late years entered upon this new branch of industry. Amongst these may be mentioned the Bairds of Gartsherrie, who vie with the Guests and Crawshays of South Wales, and have advanced themselves in the course of a very few years from the station of small farmers to that of great capitalists owning estates in many counties, holding the highest character commercial men, and ranking among the largest employers of labour in the kingdom.
[1] Article by Dugald Bannatyne in Glasgow Mechanic's Magazine, No. 53, Dec. 1824.
[2] Glasgow Mechanic's Magazine, vol. iii. p. 159.