acetylene is chosen rather than any other illuminant, except electricity.

Next, in regard to vitiation of the air, by which is meant the alteration
in its composition resulting from the admixture of products of combustion
with it. Electric lighting is as superior to other modes of lighting in
respect of direct vitiation as of exhaustion of the air, because it does
not depend on combustion. Putting it aside, however, light is obtainable
by means of acetylene with less attendant vitiation of the air than by
means of any other gas or of oil or candles. The principal vitiating
factor in all cases is the carbonic acid produced by the combustion. Now
one volume of acetylene on combustion yields two volumes of carbonic
acid, whereas one volume of coal-gas yields about 0.6 volume of carbonic
acid. But even assuming that the incandescent system of lighting is
applied in the case of coal-gas and not of acetylene, the ratio of the
consumption of the two gases for the development of a given illuminative
effect will be such that no more carbonic acid will be produced by the
acetylene; and if the incandescent system is applied either in both cases
or in neither, the ratio will be greatly in favour of acetylene. The
other factors which determine the vitiation of the air of a room in which
the gas is burning are likewise under ordinary conditions more in favour
of acetylene. They are not, however, constant, since the so-called
"impurities," which on combustion cause vitiation of the air, vary
greatly in amount according to the extent to which the gases have been
purified. London coal-gas, which was formerly purified to the highest
degree practically attainable, used to contain on the average only 10 to
12 grains of sulphur per 100 cubic feet, and virtually no other impurity.
But now coal-gas, in London and most provincial towns, contains 40 to 50
grains of sulphur per 100 cubic foot. At least 5 grains of ammonia per
100 cubic foot in also present in coal-gas in some towns. Crude acetylene
also contains sulphur and ammonia, that coming from good quality calcium