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. 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