This is due to the fact that the
added oxygen increases the temperature of the flame by doing the work
of the air, but without the cooling and diluting action of the
nitrogen; when, however, a certain proportion is added, it begins to
burn up the heavy hydrocarbons, and although the temperature goes on
increasing, the light-giving power is rapidly diminished by the
diminution of the amount of free carbon in the flame.
It has been proposed to carburet and enrich poor coal gas by
admixture with it of an oxy-oil gas made under Tatham's patents, in
which crude oils are cracked at a comparatively low temperature, and
are there mixed with from 12 to 24 per cent. of oxygen gas. Oil gas
made at low temperatures, _per se_, is of little use as an illuminant,
as it burns with a smoky flame, and does not travel well, but when
mixed with a certain amount of oxygen, it gives a very brilliant white
light, and no smoke, while as far as experiments have at present gone,
its traveling powers are much improved.
At first sight it seems a dangerous experiment to mix a heavy
hydrocarbon gas with oxygen, but it must be remembered that although
hydrogen and carbon monoxide only need to be mixed with half their own
volume of oxygen to give a most explosive mixture, yet as the number
of carbon and hydrogen atoms in the combustible gas increase, so does
the amount of oxygen needed to give explosion. Thus coal gas needs
rather more than its own volume, and ethylene three times its volume,
to give the maximum explosive results, while these mixtures begin to
be explosive when 10 per cent.
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