In nearly all the experiments, new energies of
chemical or electrical origin come into force. On incandescence,
luminescence is superposed; and the advantage which might have been
expected from the simplicity of the medium vanishes through the
complication of the circumstances in which the phenomenon is produced.
Professor Pringsheim has succeeded, in certain cases, in finding the
dividing line between the phenomena of luminescence and that of
incandescence. Thus the former takes a predominating importance when
the gas is rendered luminous by electrical discharges, and chemical
transformations, especially, play a preponderant role in the emission
of the spectrum of flames which contain a saline vapour. In all the
ordinary experiments of spectrum analysis the laws of Kirchhoff cannot
therefore be considered as established, and yet the relation between
emission and absorption is generally tolerably well verified. No doubt
we are here in presence of a kind of resonance phenomenon, the gaseous
atoms entering into vibration when solicited by the ether by a motion
identical with the one they are capable of communicating to it.
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