By proceeding for ten minutes with this air process, and then stopping it and generating water gas, two different mixtures are obtained, which can be combined together forming a gaseous product containing 10 parts of hydrogen, 20 of carbon monoxide, and 50 of nitrogen. One kilogramme of coal produces 4·5 cubic metres of this mixture. Instead of performing the operations of producing Siemens gas, or producer gas, as it is sometimes termed, and water gas alternately and separately, it is possible to so arrange the furnace and apparatus, that both are generated at the same time, and continuously instead of intermittently. The combination of the two is called poor gas. The invention of gas-producing plant is due to two Frenchmen, Thomas and Laurens, who studied deeply the question of the economic generation of poor gas, and constructed the first working plant. These two inventors stood, however, in the same position relative to the production of poor gas that Beau de Rochas had held relative to the gas engine, and it was not till Siemens came forward, and showed how they might most economically be generated, that poor gases were generally adopted. Siemens adapted them especially to metallurgy and the manufacture of glass. We shall now describe the most interesting processes which have been brought out since the time of the appearance of the Siemens plant. _Dowson gas-producer_ (Fig. 43).—This process was the first to appear after the Siemens process, and the gas produced by it is used in a large number of manufacturing operations. It consists of a generator, a boiler for producing superheated steam, an hydraulic box, the scrubbers and the gasometer. The generator is simply a gas retort lined internally with fire-bricks, and placed vertically in position. It will be seen in front on the left-hand side of the illustration. The fuel is usually anthracite coal, and is supported on a grate. It is fed in through a hopper placed at the top of the generator. By an arrangement of valves the anthracite enters without direct communication being ever established between the interior of the generator and the exterior atmosphere, which would result in explosions. The steam which is to be decomposed by the heated coal is generated in the small boiler, seen in front on the right, at a pressure of 50 lbs. per square inch, and superheated in a spiral coil inside. The steam passes into the retort through an injector, drawing a quantity of air along with it whilst passing from a nozzle across an air space. The air enters the generator along with the steam and causes the coal to burn, and the steam is decomposed, forming a mixture of producer gas and water gas. The quantity of gas produced is therefore regulated by the injector. The gases generated by the combustion of the anthracite are conveyed by a pipe into a flat hydraulic box seen behind the generator, and divided into two parts and half filled with water. The gases are washed