rom the flame was raised by
external means, the velocity of the ions increased.

We can derive some information as to the constitution of the ions by
calculating the velocity with which a molecule of the gas would move
in the electric field if it carried the same charge as the ion. From
the theory of the diffusion of gases, as developed by Maxwell, we know
that if the particles of a gas A are surrounded by a gas B, then, if
the partial pressure of A is small, the velocity u with which its
particles will move when acted upon by a force Xe is given by the
equation

Xe
u = ------- D,
(p1/N1)

where D represents the coefficient of inter-diffusion of A into B, and
N1 the number of particles of A per cubic centimetre when the pressure
due to A is p1. Let us calculate by this equation the velocity with
which a molecule of hydrogen would move through hydrogen if it carried
the charge carried by an ion, which we shall prove shortly to be equal
to the charge carried by an atom of hydrogen in the electrolysis of
solutions. Since p1/N1 is independent of the pressure, it is equal to
[Pi]/N, where [Pi] is the atmospheric pressure and N the number of
molecules in a cubic centimetre of gas at atmospheric pressure. Now Ne
= 1.22 X 10^10, if e is measured in electrostatic units; [Pi] = 10^6
and D in this case is the coefficient of diffusion of hydrogen into
itself, and is equal to 1.7. Substituting these values we find

u = 1.97 X 10^4X.

If the potential gradient is 1 volt per centimetre, X = 1/300.
Substituting this value for X, we find u = 66 cm./sec, for the
velocity of a hydrogen molecule. We have seen that the velocity of the
ion in hydrogen is only about 5 cm./sec, so that the ion moves more
slowly than it would if it were a single molecule. One way of
explaining this is to suppose that the ion is bigger than the
molecule, and is in fact an aggregation of molecules, the charged ion
acting as a nucleus around which molecules collect like dust round a
charged body. This view is supported by the effect produced by
moisture in diminishing the velocity of the negative ion, for, as C.
T. R. Wilson (_Phil. Trans._ 193, p. 289) has shown, moisture tends to
collect round the ions, and condenses more easily on the negative than
on the positive ion. In connexion with the velocities of ions in the
gases drawn from flames, we find other inst