His efforts to improve the steam-engine, however, were speedily followed by a still more important application of the mechanical theory of heat to industrial purposes. In 1857 his younger brother, and then pupil, Frederick (who, since the death of Sir William, has undertaken the sole charge of the development of this branch of his elder brother’s work), suggested to him the employment of regenerators for the purpose of saving some of the heat wasted in metallurgical operations, and for four years he labored to attain this result, constructing several different forms of furnace. His chief practical difficulties arose from the use of solid fuel—coal or coke—but when, in 1859, he hit upon the plan of converting the solid fuel into gaseous, which he did by the aid of his gas-producer, he found that the results obtained with his regenerators exceeded his most sanguine expectations. In 1861 the first practical regenerative gas furnace was erected at the glass works of Messrs. Chance Bros. in Manchester, and it was found to be very economical in its results. Early in 1862 the attention of Faraday was drawn to this matter, and on June 20 of the same year, that prince of experimentalists appeared before the Royal Institution audience for the last time to explain the wonderful simplicity, economy, and power of the Siemens regenerative gas furnace. Age and experience have not diminished the high estimation in which it is held; after nearly twenty years of continuous working and extended application, Sir Henry Bessemer described it in 1880 as an “invention which was at once the most philosophic in principle, the most powerful in action, and the most economic, of all the contrivances for producing heat by the combustion of coal.” The furnace consists essentially of three parts; (1) the gas producer, which converts the solid coal into gaseous fuel; (2) the regenerators, usually four in number, which are filled with fire-brick piled in such a way as to break up into many parts a current of air or gas passing through them; (3) the furnace proper, where the combustion is actually accomplished. In using the furnace, the gaseous fuel and air are conducted through one pair of regenerators to the combustion chamber; the heated gases from this, on their way to the chimney, pass through the other pair of regenerators, heating them in their passage. In the course of, say, one hour, the currents are reversed, so that the comparatively cold gas and air pass over these heated regenerators before entering the furnace, and rob them of their heat. While this is going on, the first pair of regenerators is being heated again, and thus, by working them in alternate pairs, nearly all the heat, which would otherwise have escaped unused into the chimney, is utilised.