The amount of heat developed by the combustion of acetylene also is less for a given yield of light than that developed by most other illuminants. The gas, indeed, is a powerful heating gas, but owing to the amount consumed being so small in proportion to the light developed, the heat arising from acetylene lighting in a room is less than that from most other illuminating agents, if the latter are employed to the extent required to afford equally good illumination. The ratio of the heat developed in acetylene lighting to that developed in, _e.g._, lighting by ordinary coal-gas, varies considerably according to the degree of efficiency of the burners, or, in other words, of the methods by which light is obtained from the gases. Volume for volume, acetylene yields on combustion about three and a half times as much heat as coal- gas, yet, owing to its superior efficiency as an illuminant, any required light may be obtained through it with no greater evolution of heat than the best practicable (incandescent) burners for coal-gas produce. The heat evolved by acetylene burners adequate to yield a certain light is very much less than that evolved by ordinary flat-flame coal-gas burners or by oil-lamps giving the same light, and is not more than about three times as much as that from ordinary electric lamps used in numbers sufficient to give the same light. More exact figures for the ratio between the heat developed in acetylene lighting and that in other modes of lighting are given in the table already referred to.