petroleum, benzol, or other suitable volatile hydrocarbon is volatilised,
where necessary, by the application of gentle heat, while air is driven
over or through it by means of a small motor, which in some cases is a
hot-air engine operated by heat supplied by a flame of the air-gas
produced. These air-gas producers, or at least the reservoir of volatile
hydrocarbon, may be placed in an outbuilding, so that the risk of fire in
the house itself is minimised. They require, however, as much attention
as an acetylene generator, usually more. It is difficult to give reliable
data as to the cost of air-gas, inclusive of the expenses of production.
It varies considerably with the description of hydrocarbon employed, and
its market price. Air-gas is only slightly inferior hygienically to
acetylene, and the colour of its light is that of the incandescent light
as produced by coal-gas or acetylene. Air-gas of a certain grade may be
used for lighting by flat-flame burners, but it has been available thus
for very many years, and has failed to achieve even moderate success. But
the advent of the incandescent burner has completely changed its position
relatively to most other illuminants, and under certain conditions it
seems likely to be the most formidable competitor with acetylene. Since
air-gas, and the numerous chemically identical products offered under
different proprietary names, is simply atmospheric air more or less
loaded with the vapour of a volatile hydrocarbon which is normally
liquid, it possesses no definite chemical constitution, but varies in
composition according to the design of the generating plant, the
atmospheric temperature at the time of preparation, the original degree
of volatility of the hydrocarbon, the remaining degree of volatility
after the more volatile portions have been vaporised, and the speed at
which the air is passed through the carburettor. The illuminating power
and the calorific value of air-gas, unless the manufacture is very
precisely controlled, are apt to be variable, and the amount of light,
emitted, either in self-luminous or in incandescent burners, is somewhat