Volume for volume, acetylene
yields on combustion about three and a half times as much heat as coal-
gas, yet, owing to its superior efficiency as an illuminant, any required
light may be obtained through it with no greater evolution of heat than
the best practicable (incandescent) burners for coal-gas produce. The
heat evolved by acetylene burners adequate to yield a certain light is
very much less than that evolved by ordinary flat-flame coal-gas burners
or by oil-lamps giving the same light, and is not more than about three
times as much as that from ordinary electric lamps used in numbers
sufficient to give the same light. More exact figures for the ratio
between the heat developed in acetylene lighting and that in other modes
of lighting are given in the table already referred to.
In connexion with the smaller amount of heat developed per unit of light
when acetylene is the illuminant, the frequently exaggerated claim that
acetylene does not blacken ceilings at all may be studied. Except it be a
carelessly manipulated petroleum-lamp, no form of artificial illuminant
employed nowadays ever emits black smoke, soot, or carbon, in spite of
the fact that all luminous flames commercially capable of utilisation do
contain free carbon in the elemental state.
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