vary
inversely as the pressure. Langevin (loc. cit.) was the first to show
that at very low pressures the velocity of the negative ions increases
more rapidly as the pressure is diminished than this law indicates. If
the nature of the ion did not change with the pressure, the kinetic
theory of gases indicates that the velocity would vary inversely as
the pressure, so that Langevin's results indicate a change in the
nature of the negative ion when the pressure is diminished below a
certain value. Langevin's results are given in the following table,
where p represents the pressure measured in centimetres of mercury, V+
and V- the velocities of the positive and negative ions in air under
unit electrostatic force, i.e. 300 volts per centimetre:--

+----------------------+----------------------+
| Negative Ions. | Positive Ions. |
+-------+------+-------+-------+------+-------+
| p. | V-. |pV-/76.| p. | V+. |pV+/76.|
+-------+------+-------+-------+------+-------+
| 7.5 | 6560 | 647 | 7.5 | 4430 | 437 |
| 20.0 | 2204 | 580 | 20.0 | 1634 | 430 |
| 41.5 | 994 | 530 | 41.5 | 782 | 427 |
| 76.0 | 510 | 510 | 76.0 | 480 | 420 |
| 142.0 | 270 | 505 | 142.0 | 225 | 425 |
+-------+------+-------+-------+------+-------+

The increase in the case of pV- indicates that the structure of the
negative ion gets simpler as the pressure is reduced. Wallisch in some
experiments made at the Cavendish Laboratory found that the diminution
in the value of pV- at low pressures is much more marked in some gases
than in others, and in some gases he failed to detect it; but it must
be remembered that it is difficult to get measurements at pressures of
only a few millimetres, as the amount of ionization is so exceedingly
small at such pressures that the quantities to be observed are hardly
large enough to admit of accurate measurements by the methods
available at higher pressures.

_Effect of Temperature on the Velocity of the Ions._--Phillips (_Proc.
Roy. Soc._, 1906, 78, p. 167) investigated, using Langevin's method,
the velocities of the + and - ions through air at atmospheric pressure
at temperatures ranging from that of boiling liquid air to 411 deg.
C.; R1 and R2 are the velocities of the + and - ions respectively when
the

force is a volt per centimetre.

+-------+-------+-------