7 per cent. On the other hand, if saturation of the air with the vapour
is practically attained, should the temperature of the gas fall before it
arrives at the point of combustion, part of the spirit will condense out,
and the product will thus lose part of its illuminating or calorific
intensity, besides partially filling the pipes with liquid products of
condensation. The loss of intensity in the gas during cold weather may or
may not be inconvenient according to circumstances; but the removal of
part of the combustible material brings the residual air-gas nearer to
its limit of explosibility--for it is simply a mixture of combustible
vapour with air, which, normally, is not explosive because the proportion
of spirit is too high--and thus, when led into an atmospheric burner, the
extra amount of air introduced at the injector jets may cause the mixture
to be an explosive mixture of air and spirit, so that it will take fire
within the burner tube instead of burning quietly at the proper orifice.
This matter will be made clearer on studying what is said about explosive
limits in Chapter VI., and what is stated about incandescent acetylene
(carburetted or not) in Chapters IX. and X. Clearly, however, high-grade
air-gas is only suitable for preparation at the immediate spot where it
is to be consumed; it cannot be supplied to a complete district unless it
is intentionally made of such lower intensity that the proportion of
spirit is too small ever to allow of partial deposition in the mains
during the winter.

It is perhaps necessary to refer to the more extended use of candles for
lighting in some few houses in which lamps are disliked on aesthetic, or,
in some cases, ostensibly on hygienic grounds. Candle lighting, speaking
broadly, is either very inadequate so far as ordinary living-rooms are
concerned, or, if adequate, is very costly. Tests specially carried out
by one of the authors to determine some of the figures required in the
ensuing table show that ordinary paraffin or "wax" candles usually emit