new illuminant with that of the candles and oil, to bear in mind the
superior degree of illumination which is secured in all rooms, at least
where candles were formerly used.

In regard to exhaustion and vitiation of the air, and to heat evolved,
self-luminous petroleum lamps stand on much the same footing as coal-gas
when the latter is burned in flat-flame burners, if the comparison is
based on a given yield of light. A large lamp, owing to its higher
illuminating efficiency, is better in this respect than a small one--
light for light, it is more hygienic than ordinary flat-flame coal-gas
burners, while a small lamp is less hygienic. It will therefore be
understood at once, from what has already been said about the superiority
on hygienic grounds of acetylene to flat-flame coal-gas lighting, that
acetylene is in this respect far superior to petroleum lamps. The degree
of its superiority is indicated more precisely by the figures quoted in
the tabular statement which concludes this chapter.

Before giving the tabular statement, however, it is necessary to say a
few words in regard to one method of lighting which, may possibly develop
into a more serious competitor with acetylene for the lighting of the
better class of country house than any of the illuminating agents and
modes of lighting so far referred to. The method in question is lighting
by so-called air-gas used for raising mantles to incandescence in
upturned or inverted burners of the Welsbach-Kern type. "Air-gas" is
ordinary atmospheric air, more or less completely saturated with the
vapour of some highly volatile hydrocarbon. The hydrocarbons practically
applied have so far been only "petroleum spirit" or "carburine," and
"benzol." "Petroleum spirit" or "carburine" consists of the more highly
volatile portion of petroleum, which is removed by distillation before
the kerosene or burning oil is recovered from the crude oil. Several