Negative ions 12.9 cm./sec. Gold +ions for salts of Li, Na, K, Rb, Cs 62 H. A. Wilson " 200 Marx " 80 Moreau _Ions liberated by Chemical Action._ Velocities of the order of 0.0005 cm./sec. Bloch _Ions from Point Discharge._ +---------------+------+-------+------+----------+ | Hydrogen | 5.4 | 7.43 | 6.41 | Chattock | | Carbonic acid | 0.83 | 0.925 | 0.88 | Chattock | | Air | 1.32 | 1.80 | 1.55 | Chattock | | Oxygen | 1.30 | 1.85 | 1.57 | Chattock | +---------------+------+-------+------+----------+ It will be seen from this table that the greater mobility of the negative ions is very much more marked in the case of the lighter and simpler gases than in that of the heavier and more complicated ones; with the vapours of organic substances there seems but little difference between the mobilities of the positive and negative ions, indeed in one or two cases the positive one seems slightly but very slightly the more mobile of the two. In the case of the simple gases the difference is much greater when the gases are dry than when they are moist. It has been shown by direct experiment that the velocities are directly proportional to the electric force. _Variation of Velocities with Pressure._--Until the pressure gets low the velocities of the ions, negative as well as positive, 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 | +-------+------+-------+-------+------+-------+