occasionally follow the constant inspiration of air rendered foul by
human exhalations.

The more important advantages of acetylene as an illuminant have now been
indicated, and it remains to discuss the cost of acetylene lighting in
comparison with other modes of procuring artificial light. At the outset
it may be stated that a very much greater reduction in the price of
calcium carbide--from which acetylene is produced--than is likely to
ensue under the present methods and conditions of manufacture will be
required to make acetylene lighting as cheap as ordinary gas lighting in
towns in this country, provided incandescent burners are used for the
gas. On the score of cheapness (and of convenience, unless the acetylene
were delivered to the premises from some central generating station)
acetylene cannot compete as an illuminant with coal-gas where the latter
costs, say, not more than 5s. per 1000 cubic feet, if only
reasonable attention is given to the gas-burners, and at least a quarter
of them are on the incandescent system. If, on the other hand, coal-gas
is misused and wasted through the employment only of interior or worn-out
flat-flame burners, while the best types of burner are used for
acetylene, the latter gas may prove as cheap for lighting as coal-gas at,
say, 2s. 6d. per 1000 cubic feet (and be far better hygienically);
whereas, contrariwise, if coal-gas is used only with good and properly
maintained incandescent burners, it may cost over 10s. per 1000 cubic
feet, and be cheaper than acetylene burned in good burners (and as good
from the hygienic standpoint). More precise figures on the relative costs
of coal-gas lighting and acetylene lighting are given in the tabular
statement at the close of this chapter.

With regard to electric lighting it is somewhat difficult to lay down a
fair basis of comparison, owing to the wide variations in the cost of