It was during the period of his connection with the Glasgow Gas-works that Mr. Neilson directed his attention to the smelting of iron. His views in regard to the subject were at first somewhat crude, as appears from a paper read by him before the Glasgow Philosophical Society early in 1825. It appears that in the course of the preceding year his attention had been called to the subject by an iron-maker, who asked him if he thought it possible to purify the air blown into the blast furnaces, in like manner as carburetted hydrogen gas was purified. The ironmaster supposed that it was the presence of sulphur in the air that caused blast-furnaces to work irregularly, and to make bad iron in the summer months. Mr. Neilson was of opinion that this was not the true cause, and he was rather disposed to think it attributable to the want of a due proportion of oxygen in summer, when the air was more rarefied, besides containing more aqueous vapour than in winter. He therefore thought the true remedy was in some way or other to throw in a greater proportion of oxygen; and he suggested that, in order to dry the air, it should be passed, on its way to the furnace, through two long tunnels containing calcined lime. But further inquiry served to correct his views, and eventually led him to the true theory of blasting. Shortly after, his attention was directed by Mr. James Ewing to a defect in one of the Muirkirk blast-furnaces, situated about half a mile distant from the blowing-engine, which was found not to work so well as others which were situated close to it. The circumstances of the case led Mr. Neilson to form the opinion that, as air increases in volume according to temperature, if he were to heat it by passing it through a red-hot vessel, its volume would be increased, according to the well-known law, and the blast might thus be enabled to do more duty in the distant furnace. He proceeded to make a series of experiments at the Gas-works, trying the effect of heated air on the illuminating power of gas, by bringing up a stream of it in a tube so as to surround the gas-burner. He found that by this means the combustion of the gas was rendered more intense, and its illuminating power greatly increased. He proceeded to try a similar experiment on a common smith's fire, by blowing the fire with heated air, and the effect was the same; the fire was much more brilliant, and accompanied by an unusually intense degree of heat.