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ping gas does not light, lights must be extinguished, and the pipes again blown through with acetylene into the open air. The burner must invariably be in position when a light is applied, because, in the event of the pipes still containing an explosive mixture, ignition would not be communicated through the small orifices of the burner to the mixture in the pipes, and the application of the light would not entail any danger of an explosion.
Gasfitters familiar with coal-gas should remember, when putting a system of acetylene pipes into use for the first time, that the range over which mixtures of acetylene and air are explosive is wider than that over which mixtures of coal-gas and air are explosive, and that greater care is therefore necessary in getting the pipes and rooms free from a dangerous mixture.
The mains for very large installations of acetylene--_e.g._, for lighting a small town--may advisedly be freed from air by some other plan than simple expulsion of the air by acetylene, both from the point of view of economy and of safety. If the chimney gases from a neighbouring furnace are found on examination to contain not more than about 8 per cent of oxygen, they may be drawn into the gasholder and forced through the pipes before acetylene is admitted to them. The high proportion of carbon dioxide and the low proportion of oxygen in chimney gases makes a mixture of acetylene and chimney gases non-explosive in any proportions, and hence if the air is first wholly or to a large extent expelled from a pipe, main, or apparatus, by means of chimney gases, acetylene may be admitted, and a much shorter time allowed for the expulsion by it of the contents of the pipe, before a light is applied at the burners, &c. This plan, however, will usually only be adopted in the case of very large pipes, &c.; but on a smaller scale the air may be swept out of a distributing system by bringing it into connexion with a cylinder of compressed or liquefied carbon dioxide, the pressure in which will drive t