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° C.; R1 and R2 are the velocities of the + and - ions respectively when the force is a volt per centimetre. +-------+-------+---------------------+ | R1. | R2. |Temperature Absolute.| +-------+-------+---------------------+ | 2.00 | 2.495 | 411° | | 1.95 | 2.40 | 399° | | 1.85 | 2.30 | 383° | | 1.81 | 2.21 | 373° | | 1.67 | 2.125 | 348° | | 1.60 | 2.00 | 333° | | 1.39 | 1.785 | 285° | | 0.945 | 1.23 | 209° | | 0.235 | 0.235 | 94° | +-------+-------+---------------------+