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,
7 per cent. On the other hand, if saturation of the air with the vapour
is practically attained, should the temperature of the gas fall before it
arrives at the point of combustion, part of the spirit will condense out,
and the product will thus lose part of its illuminating or calorific
intensity, besides partially filling the pipes with liquid products of
condensation.
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