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. What is the lowest proportion of such vapours in
admixture with air which will serve on combustion to maintain a mantle in
a state of incandescence, or even to afford a flame at all, does not
appear to have been precisely determined, but it cannot be much below 1-
1/2 per cent. Hence the apparatus for producing air-gas of this first
class must be provided with controlling or governing devices of such
nicety that the proportion of hydrocarbon vapour in the air-gas is
maintained between about 1-1/2 and 2 per cent. It is fair to say that in
normal working conditions a number of devices appear to fulfil this
requirement satisfactorily. The second of the two mixtures referred to
above, viz., air with more hydrocarbon vapour than constitutes an
explosive mixture, is primarily suitable for combustion in self-luminous
burners, but may also be consumed in properly designed incandescent
burners. But the generating apparatus for such air-gas must be equipped
with some governing or controlling device which will ensure the
proportion of hydrocarbon vapour in the mixture never falling below, say,