From the strictly chemical point
of view the light-giving substance in the incandescent flame lasts
indefinitely, for it experiences no change except in temperature; whereas
the light-giving substance in a luminous flame lasts but for an instant,
for it only evolves light during the act of its combination with the
oxygen of the atmosphere. Any fluid combustible which burns with a flame
can be made to give light on the incandescent system, for all such
materials either burn naturally, or can be made to burn with a non-
luminous flame, which can be employed to raise the temperature of some
mantle; but only those fuels can be burnt on the self-luminous system
which contain some ingredient that is liberated in the elemental state in
the flame, the said ingredient being one which combines energetically
with oxygen so as to liberate much local heat. In practice, just as there
are only two or three substances which are suitable for the construction
of an incandescent mantle, so there is only one which renders a flame
usefully self-luminous, viz., carbon; and therefore only such fuels as
contain carbon among their constituents can be burnt so as to produce
light without the assistance of the mantle.
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