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. The black mark on a ceiling
over a source of light is caused by a rising current of hot air and
combustion products set up by the heat accompanying the light, which
current of hot gas carries with it the dust and dirt always present in
the atmosphere of an inhabited room. As this current of air and burnt gas
travels in a fairly concentrated vertical stream, and as the ceiling is
comparatively cool and exhibits a rough surface, that dust and dirt are
deposited on the ceiling above the flame, but the stain is seldom or
never composed of soot from the illuminant itself. Proof of this
statement may be found in the circumstance that a black mark is
eventually produced over an electric glow-lamp and above a pipe
delivering hot water. Clearly, therefore, the depth and extent of the
mark will depend on the volume and temperature of the hot gaseous
current; and since per unit of light acetylene emits a far smaller
quantity of combustion products and a far smaller amount of heat than any
other flame illuminant except incandescent coal-gas, the inevitable black