Acetylene, the Principles of Its Generation and Use | Page 9

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 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. These air-gas producers, or at least the reservoir of
volatile hydrocarbon, may be placed in an outbuilding, so that the risk
of fire in the house itself is minimised. They require, however, as much
attention as an acetylene generator, usually more. It is difficult to give
reliable data as to the cost of air-gas, inclusive of the expenses of
production. It varies considerably with the description of hydrocarbon
employed, and its market price. Air-gas is only slightly inferior
hygienically to acetylene, and the colour of its light is that of the
incandescent light as produced by coal-gas or acetylene. Air-gas of a
certain grade may be used for lighting by flat-flame burners, but it has
been available thus for very many years, and has failed to achieve even
moderate success. But the advent of the incandescent burner has
completely changed its position relatively to most other illuminants,
and under certain conditions it seems likely to be the most formidable
competitor with acetylene. Since air-gas, and the numerous chemically
identical products offered under different proprietary names, is simply
atmospheric air more or less loaded with the vapour of a volatile
hydrocarbon which is normally liquid, it possesses no definite chemical
constitution, but varies in composition according to the design of the
generating plant, the atmospheric temperature at the time of preparation,
the original degree of volatility of the hydrocarbon, the remaining
degree of volatility after the more volatile portions have been vaporised,
and the speed at which the air is passed through the carburettor. The
illuminating power and the calorific value of air-gas, unless the

manufacture is very precisely controlled, are apt to be variable, and the
amount of light, emitted, either in self-luminous or in incandescent
burners, is somewhat 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