The gas producer is a combustion chamber filled with coal in which the coal in the upper layer is burnt. Generators may have horizontal or sloping grate (see figs. 15 and 16). The _Siemens’s_ heaters or regenerators are chambers built of, and filled loosely with, fireclay bricks and arranged in couples. Should the gas producers become too hot, instead of the chambers subdivided air heaters are used, whereby the hot furnace gases are brought into contact with a system of thin-walled, gastight fireclay pipes, to which they give up their heat, while the secondary air supply for the furnace is led beside these pipes and so becomes heated indirectly. Previous heating of the producer gas is here not necessary; no valves are needed because the three streams of gas all pass in the same direction. [Illustration: FIG. 17A.—Siemens’s Regenerative Furnace L Air; G Gas FIG. 17B.—Siemens’s Regenerative Furnace] Such air heating arrangements are used for heating the retorts in gas works, for melting the ‘metal’ in glass works, and very generally in other industries, as they offer many technical and hygienic advantages. Generator gas from coke contains 34 per cent. carbonic oxide, 0·1 per cent. hydrogen, 1·9 per cent. carbon dioxide, and 64 per cent. nitrogen. _Blast furnace gas._—Blast furnace gas is formed under the same conditions as have been described for generator gas; it contains more carbon dioxide (about 10 per cent.). (Further details are given in the section on Iron—Blast Furnaces.) _Water gas._—Water gas is made by the passage of steam through incandescent coal, according to the equation: C + H₂O = CO + 2H. The iron gas producer, lined with firebrick, is filled with anthracite or coke and heated by blowing hot air through it. This causes producer gas to escape, after which steam is blown through, causing water gas to escape—containing hydrogen and carbonic oxide to the extent of 45-50 per cent., carbon dioxide and nitrogen 2-6 per cent., and a little methane. The blowing of hot air and steam is done alternately, and both kinds of gas are led away and collected separately, the water gas being previously purified in scrubbers, condensers, and purifiers. It serves for the production of high temperatures (in smelting of metals). Further, when carburetted and also when carefully purified in an uncarburetted state, it serves as an illuminant. The producer gas generated at the same time is used for heating purposes (generally for heating boilers). _Dowson gas._—Dowson gas is obtained by collecting and storing together the gases produced in the manner described for water gas. Under the grating of the wrought-iron gas producer (lined with firebrick and similarly filled with coke or anthracite) a mixture of air and steam, produced in a special small boiler, is blown through by means of a Körting’s injector.