not appear, under certain conditions, to be the same
for the two modes of electrification.

It is not devoid of interest to note that Erman, a German scholar,
once very celebrated and now generally forgotten, drew attention as
early as 1815 to the unipolar conductivity of a flame. His
contemporaries, as may be gathered from the perusal of the treatises
on physics of that period, attached great importance to this
discovery; but, as it was somewhat inconvenient and did not readily
fit in with ordinary studies, it was in due course neglected, then
considered as insufficiently established, and finally wholly
forgotten.

All these somewhat obscure facts, and some others--such as the
different action of ultra-violet radiations on positively and
negatively charged bodies--are now, on the contrary, about to be
co-ordinated, thanks to the modern ideas on the mechanism of conduction;
while these ideas will also allow us to interpret the most striking
dissymmetry of all, i.e. that revealed by electrolysis itself, a
dissymmetry which certainly can not be denied, but to which sufficient
attention has not been given.

It is to a German physicist, Giese, that we owe the first notions on
the mechanism of the conductivity of gases, as we now conceive it. In
two memoirs published in 1882 and 1889, he plainly arrives at the
conception that conduction in gases is not due to their molecules, but
to certain fragments of them or to ions. Giese was a forerunner, but
his ideas could not triumph so long as there were no means of
observing conduction in simple circumstances. But this means has now
been supplied in the discovery of the X rays. Suppose we pass through
some gas at ordinary pressure, such as hydrogen, a pencil of X rays.
The gas, which till then has behaved as a perfect insulator,[29]
suddenly acquires a remarkable conductivity. If into this hydrogen two
metallic electrodes in communication with the two poles of a battery
are introduced, a current is set up in very special conditions which
remind us, when they are checked by experiments, of the mechanism
which allows the passage of electricity in electrolysis, and which is
so well represented to us when we picture to ourselves this passage as
due to the migration towards the electrodes, under the action of the
field, of the two sets of ions produced by the spontaneous division of
the molecule within the solution.

[Footnote 29: At least, so long as it is not introduced between t