mark over its flame takes very much longer to appear. Quite roughly
speaking, as may be deduced from what has already been said on this
subject, the luminous flame of acetylene "blackens" a ceiling at about
the same rate as a coal-gas burner of the best Welsbach type.

There is one respect in which acetylene and other flame illuminants are
superior to electric lighting, viz., that they sterilise a larger volume
of air. All the air which is needed to support combustion, as well as the
excess of air which actually passes through the burner tube and flame in
incandescent burners, is obviously sterilised; but so also is the much
larger volume of air which, by virtue of the up-current due to the heat
of the flame, is brought into anything like close proximity with the
light. The electric glow-lamp, and the most popular and economical modern
enclosed electric arc-lamp, sterilise only the much smaller volume of air
which is brought into direct contact with their glass bulbs. Moreover,
when large numbers of persons are congregated in insufficiently
ventilated buildings--and many public rooms are insufficiently
ventilated--the air becomes nauseous to inspire and positively
detrimental to the health of delicate people, by reason of the human
effluvia which arise from soiled raiment and uncleansed or unhealthy
bodies, long before the proportion of carbonic acid by itself is high
enough to be objectionable. Thus a certain proportion of carbonic acid
coming from human lungs and skin is more harmful than the same proportion
of carbonic acid derived from the combustion of gas or oil. Hence
acetylene and flame illuminants generally have the valuable hygienic
advantages over electric lighting, not only of killing a far larger
number of the micro-organisms that may be present in the air, but, by
virtue of their naked flames, of burning up and destroying a considerable
quantity of the aforesaid odoriferous matter, thus relieving the nose and
materially assisting in the prevention of that lassitude and anaemia