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.