in 1893 Mr Barus
established that the approach of a stick of phosphorus brings about
the condensation of water vapour, and we really have before us,
therefore, in this instance, an ionisation. M. Bloch has succeeded in
disentangling the phenomena, which are here very complex, and in
showing that the ions produced are of considerable dimensions; for
their speed in the same conditions is on the average a thousand times
less than that of ions due to the X rays. M. Bloch has established
also that the conductivity of recently-prepared gases, already studied
by several authors, was analogous to that which is produced by
phosphorus, and that it is intimately connected with the presence of
the very tenuous solid or liquid dust which these gases carry with
them, while the ions are of the same order of magnitude. These large
ions exist, moreover, in small quantities in the atmosphere; and M.
Langevin lately succeeded in revealing their presence.

It may happen, and this not without singularly complicating matters,
that the ions which were in the midst of material molecules produce,
as the result of collisions, new divisions in these last. Other ions
are thus born, and this production is in part compensated for by
recombinations between ions of opposite signs. The impacts will be
more active in the event of the gas being placed in a field of force
and of the pressure being slight, the speed attained being then
greater and allowing the active force to reach a high value. The
energy necessary for the production of an ion is, in fact, according
to Professor Rutherford and Professor Stark, something considerable,
and it much exceeds the analogous force in electrolytic decomposition.

It is therefore in tubes of rarefied gas that this ionisation by
impact will be particularly felt. This gives us the reason for the
aspect presented by Geissler tubes. Generally, in the case of
discharges, new ions produced by the molecules struck come to add
themselves to the electrons produced, as will be seen, by the cathode.
A full discussion has led to the interpretation of all the known
facts, and to our understanding, for instance, why there exist bright
or dark spaces in certain regions of the tube. M. Pellat, in
particular, has given some very fine examples of this concordance
between the theory and the facts he has skilfully observed.

In all the circumstances, then, in which ions appear, their formation
has doubtless been provoked by a mecha