ed. We are, in fact, in presence of a natural unit, or, if you
will, of an atom of electricity.

We must, however, guard against the belief that the gaseous ion is
identical with the electrolytic ion. Sensible differences between
those are immediately apparent, and still greater ones will be
discovered on closer examination.

As M. Perrin has shown, the ionisation produced by the X-rays in no
way depends on the chemical composition of the gas; and whether we
take a volume of gaseous hydrochloric acid or a mixture of hydrogen
and chlorine in the same condition, all the results will be identical:
and chemical affinities play no part here.

We can also obtain other information regarding ions: we can ascertain,
for instance, their velocities, and also get an idea of their order of
magnitude.

By treating the speeds possessed by the liberated charges as
components of the known speed of a gaseous current, Mr Zeleny measures
the mobilities, that is to say, the speeds acquired by the positive
and negative charges in a field equal to the electrostatic unit. He
has thus found that these mobilities are different, and that they
vary, for example, between 400 and 200 centimetres per second for the
two charges in dry gases, the positive being less mobile than the
negative ions, which suggests the idea that they are of greater
mass.[30]

[Footnote 30: A full account of these experiments, which were executed
at the Cavendish Laboratory, is to be found in _Philosophical
Transactions_, A., vol. cxcv. (1901), pp. 193 et seq.--ED.]

M. Langevin, who has made himself the eloquent apostle of the new
doctrines in France, and has done much to make them understood and
admitted, has personally undertaken experiments analogous to those of
M. Zeleny, but much more complete. He has studied in a very ingenious
manner, not only the mobilities, but also the law of recombination
which regulates the spontaneous return of the gas to its normal state.
He has determined experimentally the relation of the number of
recombinations to the number of collisions between two ions of
contrary sign, by studying the variation produced by a change in the
value of the field, in the quantity of electricity which can be
collected in the gas separating two parallel metallic plates, after
the passage through it for a very short time of the Roentgen rays
emitted during one discharge of a Crookes tube. If the image of the
ions is indeed conformable to reality, this