Hence, if the
apparatus for saturating air with the vapour of the light petroleum is
placed well above all the points at which the air-gas is to be burnt--
_e.g._, on the roof of the house--the production of the air-gas may
by simple devices become automatic, and the only attention the apparatus
will require will be the replenishing of its reservoir from time to time
with light petroleum. But a number of precautions are required to make
this simple process operate without interruption or difficulty. For
instance, the evaporation of the spirit must not be so rapid relatively
to its total bulk as to lower its temperature, and thereby that of the
overflowing air, too much; the reservoir must be protected from extreme
cold and extreme heat; and the risk of fire from the presence of a highly
volatile and highly inflammable liquid on or near the roof of the house
must be met. This risk is one to which fire insurance companies take
exception.
More commonly, however, air-gas is made non-automatically, or more or
less automatically by the employment of some mechanical means. The light
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.
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