Acetylene lighting presents also certain important hygienic advantages
over other forms of flame lighting, in that it exhausts, vitiates, and
heats the air of a room to a less degree, for a given yield of light,
than do either coal-gas, oils, or candles. This point in favour of
acetylene is referred to here only in general terms; the evidence on
which the foregoing statement is based will be recorded in a tabular
comparison of the cost and qualities of different illuminants. Exhaustion
of the air means, in this connexion, depletion of the oxygen normally
present in it. One volume of acetylene requires 2-1/2 volumes of oxygen
for its complete combustion, and since 21 volumes of oxygen are
associated in atmospheric air with 79 volumes of inert gases--chiefly
nitrogen--which do not actively participate in combustion, it follows
that about 11.90 volumes of air are wholly exhausted, or deprived of
oxygen, in the course of the combustion of one volume of acetylene. If
the light which may be developed by the acetylene is brought into
consideration, it will be found that, relatively to other illuminants,
acetylene causes less exhaustion of the air than any other illuminating
agent except electricity. For instance, coal-gas exhausts only about 6-
1/2 times its volume of air when it is burnt; but since, volume for
volume, acetylene ordinarily yields from three to fifteen times as much
light as coal-gas, it follows that the same illuminative value is
obtainable from acetylene by considerably less exhaustion of the air than
from coal-gas. The exact ratio depends on the degree of efficiency of the
burners, or of the methods by which light is obtained from the gases, as
will be realised by reference to the table which follows. Broadly
speaking, however, no illuminant which evolves light by combustion
(oxidation), and which therefore requires a supply of oxygen or air for
its maintenance, affords light with so little exhaustion of the air as
acetylene. Hence in confined, ill-ventilated, or crowded rooms, the air
will suffer less exhaustion, and accordingly be better for breathing, if