_Longsden process._—We have mentioned that water gas generated by various processes has been used for lighting towns in America on the incandescent system. The drawback of the system is the exceedingly poisonous nature of carbon monoxide even when very dilute, and it is all the more dangerous because it is odourless. Many accidents have in consequence occurred, some of them terminating fatally. Mr. Longsden has attempted to produce a gas entirely free from carbon monoxide, so as to avoid this difficulty. He first attempted to rid water gas of the monoxide, but not being able to find a cheap enough solvent for it he tried other means. His present process consists of adding a sodium salt to the carbon in the generator. The gas produced has then the following composition:— Hydrogen 62·2 volumes Carbonic acid gas 26·4 ” Carbon monoxide 1·2 ” Hydrocarbons 6·5 ” Nitrogen 2·2 ” Oxygen 1·5 ” ————— 100 ” This very rich gas, which might easily be ridded of its carbonic acid by passing it through lime, is very suitable for supplying motive power through the medium of the gas engine. _Gayon and Métais process._—This process, which, as far as we know, has not been put into practice, was intended by its inventors to diminish the price of coal gas. Instead of allowing coke to be deposited in the gas retorts, as it usually is, and selling it as a bye-product, their intention was to use it there and then for the production of water gas, which was to be mixed with the coal gas and increase its heat of combustion, while diminishing its cost of production. One ton of coal would produce 450 cubic metres of gas instead of 300; the idea has, however, never been put into practical shape. CHAPTER VII ENGINES FOR USE WITH POOR GASES _Simplex motor_ (Fig. 45).—Although this motor works equally well with coal gas and oil, we have put off the description of it until now because it has become associated particularly with the production of poor gases, and by their help forms one of the most formidable rivals of the steam engine.