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 the gas to any spot where an outlet is provided. As these cylinders of "carbonic acid" are in common employment for preparing aerated waters and for "lifting" beer, &c., they are easy to hire and use. TABLE (B). Giving the Sizes of Pipe which should be used in practice for Acetylene when the fall of pressure in the Pipe is not to exceed 0.1 inch. (Based on Morel's formula.)