Nevertheless the atmospheric flame given by a certain
fuel may be appreciably hotter than its luminous flame, because the
former is usually smaller than the latter. Unless the luminous flame of a
rich fuel is made to expose a wide surface to the air, part of its carbon
may escape ultimate combustion; soot or smoke may be produced, and some
of the most valuable heat-giving substance will be wasted. But if the
flame is made to expose a large surface to the air, it becomes flat or
hollow in shape instead of being cylindrical and solid, and therefore in
proportion to its cubical capacity it presents to the cold air a larger
superficies, from which loss of heat by radiation, &c., occurs. Being
larger, too, the heat produced is less concentrated.
It does not fall within the province of the present book to discuss the
relative merits of luminous and incandescent lighting; but it may be
remarked that acetylene ranks with petroleum against coal-gas,
carburetted or non-carburetted water-gas, and semi-water-gas, in showing
a comparatively small degree of increased efficiency when burnt under the
mantle. Any gas which is essentially composed of carbon monoxide or
hydrogen alone (or both together) burns with a non-luminous flame, and
can therefore only be used for illuminating purposes on the incandescent
system; but, broadly speaking, the higher is the latent illuminating
power of the gas itself when burnt in a non-atmospheric burner, the less
marked is the superiority, both from the economical and the hygienic
aspect, of its incandescent flame.
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