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. And if this is
done, as it always should be whenever the acetylene is required for
domestic lighting, the vitiation of the air of a room due to the
"impurities" in the gas will become much less in the case of acetylene
than in that of even well-purified coal-gas; taking equal illuminating
effect as the basis for comparison.

Acetylene is similarly superior, speaking generally, to petroleum in
respect of impurities, though the sulphur present in petroleum oils, such
as are sold in this country for household use, though very variable, is
often quite small in amount, and seldom is responsible for serious
vitiation of the atmosphere.

Regarding somewhat more closely the relative convenience and safety of
acetylene and paraffin for the illumination of country residences, it may
be remarked that an extraordinarily great amount of care must he bestowed
upon each separate lamp if the whole house is to be kept free from an
odour which is very offensive to the nostrils; and the time occupied in