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
carbide at the present day including about 31 grains of the former and
25 grains of the latter per 100 cubic feet. But crude acetylene is also
accompanied by a third impurity, viz., phosphoretted hydrogen or
phosphine, which in unknown in coal-gas, and which is considerably more
objectionable than either ammonia or sulphur. The formation, behaviour,
and removal of those various impurities will be discussed in Chapter V.;
but here it may be said that there is no reason why, if calcium carbide
of a fair degree of purity has been used, and if the gas has been
generated from it in a properly designed and smoothly working apparatus--
this being quite as important as, or even more important than, the purity
of the original carbide--the gas should not be freed from phosphorus,
sulphur, and ammonia to the utmost necessary or desirable extent, by
processes which are neither complicated nor expensive.
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