It must be conceded to be possible (the author says) that the light
radiation of hot gases, as also the heat radiation, is only exceedingly
weak, and therefore may escape observation. It is, therefore, much to
be desired that the experiments should be repeated at still higher
temperatures and with more exact instruments, in order to determine
the limit of temperature at which heated gases undoubtedly become
self-incandescent. The fact, however, that gases, at a temperature of
more than 1,500 deg. C, are not yet luminous, proves that the incandescence
of the flame is not to be explained as a self-incandescence of the
products of combustion. This is confirmed by the circumstance that, with
rapid mixture of the burning gases, the flame becomes shorter because
the combustion process goes on more quickly, and hotter because less
cold air has access. Further, the flame also becomes shorter and hotter
if the gases are strongly heated previous to combustion. As the rising
products of combustion still retain for a time the temperature of the
flame, the reverse must occur if the gases were self-luminous. The
luminosity of the flame, however, ceases at a sharp line of demarkation,
and evidently coincides with completion of the chemical action.
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