The method in question is lighting
by so-called air-gas used for raising mantles to incandescence in
upturned or inverted burners of the Welsbach-Kern type. "Air-gas" is
ordinary atmospheric air, more or less completely saturated with the
vapour of some highly volatile hydrocarbon. The hydrocarbons practically
applied have so far been only "petroleum spirit" or "carburine," and
"benzol." "Petroleum spirit" or "carburine" consists of the more highly
volatile portion of petroleum, which is removed by distillation before
the kerosene or burning oil is recovered from the crude oil. Several
grades of this highly volatile petroleum distillate are distinguished in
commerce; they differ in the temperature at which they begin to distil
and the range of temperature covered by their distillation, and, speaking
more generally, in their degree of volatility, uniformity, and density.
If the petroleum distillate is sufficiently volatile and fairly uniform
in character, good air-gas may be produced merely by allowing air to pass
over an extended surface of the liquid. The vapour of the petroleum
spirit is of greater density than air, and hence, if the course of the
air-gas is downward from the apparatus at which it is produced, the flow
of air into the apparatus and over the surface of the spirit will be
automatically maintained by the "pull" of the descending air-gas when
once the flow has been started until the outlet for the air-gas is
stopped or the spirit in the apparatus is exhausted.
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