ances which suggest that
condensation takes place round the ions. An increase in the size of
the system is not, however, the only way by which the velocity might
fall below that calculated for the hydrogen molecule, for we must
remember that the hydrogen molecule, whose coefficient of diffusion is
1.7, is not charged, while the ion is. The forces exerted by the ion
on the other molecules of hydrogen are not the same as those which
would be exerted by a molecule of hydrogen, and as the coefficient of
diffusion depends on the forces between the molecules, the coefficient
of diffusion of a charged molecule into hydrogen might be very
different from that of an uncharged one.

Wellisch (_loc. cit._) has shown that the effect of the charge on the
ion is sufficient in many cases to explain the small velocity of the
ions, even if there were no aggregation.

_Mixture of Gases._--The ionization of a mixture of gases raises some
very interesting questions. If we ionize a mixture of two very
different gases, say hydrogen and carbonic acid, and investigate the
nature of the ions by measuring their velocities, the question arises,
shall we find two kinds of positive and two kinds of negative ions
moving with different velocities, as we should do if some of the
positive ions were positively charged hydrogen molecules, while others
were positively charged molecules of carbonic acid; or shall we find
only one velocity for the positive ions and one for the negative? Many
experiments have been made on the velocity of ions in mixtures of two
gases, but as yet no evidence has been found of the existence of two
different kinds of either positive or negative ions in such mixtures,
although some of the methods for determining the velocities of the
ions, especially Langevin's, ought to give evidence of this effect, if
it existed. The experiments seem to show that the positive (and the
same is true for the negative) ions in a mixture of gases are all of
the same kind. This conclusion is one of considerable importance, as
it would not be true if the ions consisted of single molecules of the
gas from which they are produced.

_Recombination._--Several methods enable us to deduce the coefficient
of recombination of the ions when we know their velocities. Perhaps
the simplest of these consists in determining the relation between the
current passing between two parallel plates