O N 295 temperatures of coke-fired furnaces, even when the least fusible mildest steels are being prepared, can be easily melted when the furnace is pushed ; these bricks are made from a silicions clay (con taining 98 31 per cent, of silica, 072 of alurnina.O lS of ferrous oxide, 2 2 of lime, 14 of potash and soda, and 35 of combined water), mixed with 1 per cent, of lime, and are usually considered the most refractory in ordinary use. A specially prepared brick made from a mixture of crushed pure quartz and 2 per cent, of lime answers much better. Bauxite bricks arc somewhat less refractory, and have the further objectionable quality of shrinking much when highly heated, whilst fresh bauxite introduced for repairing cavities caused by wear and tear will not adhere properly to them ; where FIG. 6. Open-Hearth Furnace Cross Section through Regenerators, Air and Gas Flues. ores requiring lime as flux are employed, however, they are less readily corroded than silica bricks. The hot air and gas currents and the waste gases are reversed through the regenerators at con venient intervals by means of a cast iron valve on the principle of a four- way cock ; when the regenerators are placed vertically and heated from the top, their action is more uniform than when the draught is in any other direction ; they should be at a lower level than the heating chamber, and may be worked either with a gas pressure just about atmospheric, or preferably with a slightly increased pressure so as to avoid possible chilling of the furnace by the drawing in of cold air, the pressure being regulated by the chimney damper and the valves governing the gas and air supplies. Since the composition of the gas from a Siemens gas producer is, roughly speaking, somewhat less than one-third carbon oxide or gases equivalent thereto, and somewhat more than two-thirds nitrogen and carbon dioxide, and as carbon oxide requires half its volume of oxygen and hence about two and a half times its volume of air for complete combustion, the volumes of gas and air equivalent to one another are roughly equal ; but, since an excess of air is usually requisite, and is indispensable when an oxidizing atmosphere is desired, the regenerators by which the air is heated are made some what larger than those used for heating the gas ; by suitably adjusting the speed of the air current by the valve, the atmosphere can then be rendered neutral, reducing, or oxidizing at will. This point is of less importance for other applications of the regenerative furnace such as glass making or steel melting in crucibles than it is for puddling and reheating furnaces. For every pound of coal burnt per hour about 6 square feet of surface is requisite in the regenerators to take up the heat ; whilst about 60 lb weight of brickwork is requisite to expose the surface to the best advantage, i.e., between three or four times the weight of brickwork which would have the same capacity for heat as the waste gases (equal to about 17 lb). Lundin s furnace (or gas producer}, employed in Sweden for the production of gas from moist sawdust, is constructed on much the same principles as Siemens s gas producer, saving that the air is driven in by a blast ; as the sawdust contains upwards of 40 per cent, of moisture, the steam and hot gases passing off from the furnace are cooled down, and the former condensed, by jets of water-spray and a kind of scrubber consisting of piles of iron pigs over which water flows. Peat and turf can be used with the same arrangement, if not too wet. The gas evolved from sawdust has about the following composition after condensation of steam, ex clusive of about 3 volumes per cent, of aqueous vapour : Ry Volume. By Weight. Carbon dioxide .. 11 8 19 6 Carbon oxide I9-9 20-8 Hydrogen 11-3 9 4 y-4 Nitrogen.. 53-1 5G 3 100-0 .. 100-0 Brook & Wilson s Producer (fig. 7) consists of a solid hearth with no firebars : the coal is fed in at the top by means of a hopper-shaped conical tube closed by a "bell and cone" arrangement; the air requisite for combustion is supplied by means of a steam jet, and blows into a bell-mouthed pipe outside, communicating with a box- shaped cast iron chamber in the middle of the base of the producer ; this chamber, being perforated, distributes air and steam uniformly throughout the mass of fuel, and so prevents unchanged steam and excess of air from passing away in the gases, which are led away by a tube communicating with the annular upper part of the producer between the hopper and the outer wall. Siemens has recently in troduced a modification of his gas producer differing chiefly from FIG. 7. Brook and Wilson s Gas Producer Sectional elevation, this one in details of construction, being more simple. The Tcsstt du Motay generator is in form like a small close-topped blast furnace fed by means of a cup and cone with coal dust or other low-class fuel ; the hearth is cylindrical, with a brick bottom, on which are formed four channels, each communicating at its ends by passages with cast iron mouthpieces or windboxes, connected with an annular blast main through which blast is supplied at a pressure of about 8 inches of water. Doors are provided at the mouthpieces for the removal of ashes from time to time (see Engineering, April 23, 1880). Several other gas producers have been introduced by various in ventors, and are employed to a greater or lesser extent; the limits of the present article forbid these being discussed. Peat and peat charcoal have been proposed by Kidd as sources of gaseous fuel. Steam at a pressure of 20 R> being injected, together with a considerable volume of air carried along with it, into amass of incandescent peat charcoal in a suitable chamber produces a fuel of much the same composition as that obtained from a Sie mens gas producer, but absolutely free from sulphur dioxide. Keates gives the following analysis of gas thus produced : Carbon oxide 28-6 Hvdrogen 14-6 Nitrogen 53-0 Carbon dioxide 4-0 100-2 the figures representing the volume in cubic feet of gas formed from 1 lb of peat charcoal, so that upwards of 200,000 cubic feet of gas are yielded by a ton of charcoal. 11. Fluxes and Cinder. When a very pure iron ore is smelted, such as Cumberland haematite or Swedish mag netite, the amount of silicious and earthy matter present relatively to the iron oxide is but small, and in conse quence the amount of flux requisite to be added is also small. By proper combination of ores of different kinds the necessity for the addition of flux may be almost or altogether avoided ; thus a highly aluminous ferric oxido known as bauxite (valuable as a source of aluminium and its compounds, as well as serviceable as a source of iron and flux in the blast furnace) and silicious haematite smelted
