or a small rod of magnesium, as in the Nernst lamp.
Careful measurements effected by M. Fery have furnished, in
particular, important information on the radiation of the white
oxides; but the phenomena noticed have not yet found a satisfactory
interpretation. Moreover, the radiation of calorific origin is here
accompanied by a more or less important luminescence, and the problem
becomes very complex.

In the same way that, for the purpose of knowing the constitution of
matter, it first occurred to us to investigate gases, which appear to
be molecular edifices built on a more simple and uniform plan than
solids, we ought naturally to think that an examination of the
conditions in which emission and absorption are produced by gaseous
bodies might be eminently profitable, and might perhaps reveal the
mechanism by which the relations between the molecule of the ether and
the molecule of matter might be established.

Unfortunately, if a gas is not absolutely incapable of emitting some
sort of rays by simple heat, the radiation thus produced, no doubt by
reason of the slightness of the mass in play, always remains of
moderate intensity. 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.

If we are not yet very far advanced in the study of the mechanism of
the production of the spectrum,[47] we are, on the other hand, well
acquainted with its constitution