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
indeterminate. The generating plant must be so constructed that the air
cannot at any time be mixed with as much hydrocarbon vapour as
constitutes an explosive mixture with it, otherwise the pipes and
apparatus will contain a gas which will forthwith explode if it is
ignited, _i.e._, if an attempt is made to consume it otherwise than
in burners with specially small orifices. The safely permissible mixtures
are (1) air with less hydrocarbon vapour than constitutes an explosive
mixture, and (2) air with more hydrocarbon vapour than constitutes an
explosive mixture. The first of these two mixtures is available for
illuminating purposes only with incandescent mantles, and to ensure a
reasonable margin of safety the mixing apparatus must be so devised that
the proportion of hydrocarbon vapour in the air-gas can never exceed 2
per cent. From Chapter VI. it will be evident that a little more than 2
per cent. of benzene, pentane or benzoline vapour in air forms an
explosive mixture.
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