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
mark over its flame takes very much longer to appear. Quite roughly
speaking, as may be deduced from what has already been said on this
subject, the luminous flame of acetylene "blackens" a ceiling at about
the same rate as a coal-gas burner of the best Welsbach type.
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