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.
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