84(5 FURNACE engine by means of a central belt of spur gearing. Furnaces of this kind were first used in alkali works for the conversion uf sulphate into carbonate of sodium in the process known as black ash fusion, but have since been applied to puddling in America and elsewhere by Danks and others ; but for the latter purpose they are still to some extent in the experi mental stage. As calciuers they are used in tin mines and for the chlorination of silver ores. Use of Heated Air. The calorific intensity of fuelis found to be very considerably enhanced, if the combustion be ef fected with air previously heated to any temperature between that of boiling water and a dull red heat, the same effect being observed both with solid and gaseous fuel. The latter, especially when brought to the burning point at a high temperature, produces a heat that can be resisted by the most refractory substances only, such as silica, alumina, and magnesia. This is attained in the regenerative furnace of Siemens, detailed consideration of which belongs more properly to the subject of iron. Economy of Waste Heat and Gas. In every system of artificial heating, the amount of heat usefully applied is but a small proportion of that developed by combustion. Even under the most advantageous application, that of evaporation of water in a steam boiler where the gases of the fire have to travel through a great length of flues bounded by thin iron surfaces of great heat-absorbing capacity, the tempe rature of the current at the chimney is generally much above that required to maintain an active draught in the fire-place ; and other tubes containing water, often in considerable numbers, forming the so-called fuel economizers, may often be interposed between the boiler and the chimney with marked advantage as regards saving of fuel. In re- verberatory and air furnaces used in the different operations of iron manufacture, where an extremely high temperature has to be maintained in spaces of comparatively small extent, such as the beds of puddling, welding, and steel-melting furnaces, the temperature of the exhaust gases is exceedingly high, and if allowed to pass directly into the chimney they appear as a great body of flame at the top, It is now general to save a portion of this heat by passing the flame through flues of steam boilers, air-heating apparatus, or both so that the steam required for the necessary operations of the forge and heated blast for the furnace itself may be obtained without further expenditure of fuel. The most perfect method of utilizing the waste heat hitherto applied is that of the Siemens regenerator, in which the spent gases are made to travel through chambers, known as regenerators or recuperators of heat, containing a quantity of thin fire-bricks piled into a cellular mass so as to offer a very large heat-absorbing surface, whereby their tem perature is very considerably reduced, and they arrive at the chimney at a heat not exceeding 300 or 400 degrees. As soon as the bricks have become red hot, the current is diverted to an adjacent chamber or pair of chambers, and the acquired heat is removed by a current of cool gas or air passing towards the furnace, where it arrives at atemperature sufficiently high to ensure the greatest possible heating effect in combustion. This system being alternative, four regene rators, two for air and two for gas, are required for each furnace ; but in some of the newer French patterns of so- called recuperative furnaces, a system of tubular bricks is adopted in the chambers and only the air is heated, the gas being brought hot from the producer to the furnace instead of cuoling it first by atmospheric exposure in a long tube in the manner adopted by Siemens. This allows a consider able simplification in the apparatus; only a single regene rator is required working continuously, the flame travelling outwards though one set of passages in the bricks and the uir inwards through another ; and as the former consists only of burnt gases, no explosion can take place if a communi cation be established between the two currents through a leaking joint or broken brick. In iron-smelting blast furnaces the waste gases, though not escaping at as high sensible temperatures as those of the fur naces previously considered, are of considerable fuel value, and may render important services if properly applied. Owing to the conditions of the work, which require the maintenance of a sensibly reducing atmosphere, they contain a very notable proportion of carbonic oxide, and are drawn off by large wrought-iron tubes near the top of the furnace and conveyed by branch pipes to the different boilers and air-heating apparatus, which are now as a rule entirely heated by the combustion of such gases. Formerly they were allowed to burn to waste at the mouth of a short chimney place above the furnace top, forming a huge body of flame, which was one of the most striking features of the Black Country landscape at night, but is now less commonly seen than formerly. Perhaps the greatest number of flaming fur naces to be seen at present are those of the Scotch founding iron district about Qartsherrie, Coatbridge, &c. Figs. 11, 12, and 13 represent a modern furnace heated by gas burnt with hot air as applied to heating FIG. 11. Gas-heated Retort Furnace (transverse section on A B fig. 13). gas retorts. The retorts of fire clay r r r, seven in number, are mounted upon supports at both ends in Fid. 12. Gas-heated Retort Furnace (longitudinal section on C D fig. 11). an arched chamber having a long flue running along the centre line at the bottom, covered with bricks pierced at
