Cyanogen 1·86 1·94 1·86 1·71 1·05 Sulphur dioxide 2·19 2·04 2·31 2·13 7·97 Chloroform 4·32 4·44 4·89 4·88 31·9 Methyl iodide 5·05 3·51 5·18 4·80 72·0 Carbon 5·31 5·34 5·83 5·67 45·3 tetrachloride With the exception of hydrogen, it will be seen that the ionization of gases is approximately proportional to their density for the α, β, γ rays of radium. The results obtained by Strutt for Röntgen rays are quite different; for example, the relative conductivity produced by them in methyl iodide was more than 14 times as great as that due to the rays of radium. The relative conductivities of gases exposed to X rays has been recently re-examined by McClung[77] and Eve[78], who have found that the conductivity depends upon the penetrating power of the X rays employed. The results obtained by them will be discussed later (section 107). This difference of conductivity in gases is due to unequal absorptions of the radiations. The writer has shown[79] that the total number of ions produced by the α rays for uranium, when completely absorbed by different gases, is not very different. The following results were obtained: Gas Total Ionization Air 100 Hydrogen 95 Oxygen 106 Carbonic acid 96 Hydrochloric acid gas 102 Ammonia 101 The numbers, though only approximate in character, seem to show that the energy required to produce an ion is probably not very different for the various gases. Assuming that the energy required to produce an ion in different gases is about the same, it follows that the relative conductivities are proportional to the relative absorption of the radiations. A similar result has been found by McLennan for cathode rays. He proved that the ionization was directly proportional to the absorption of the rays in the gas, thus showing that the same energy is required to produce an ion in all the gases examined. =46. Potential Gradient.= The normal potential gradient between two charged electrodes is always disturbed when the gas is ionized in the space between them. If the gas is uniformly ionized between two parallel plates, Child and Zeleny have shown that there is a sudden drop of potential near the surface of both plates, and that the electric field is sensibly uniform for the intermediate space between them. The disturbance of the potential gradient depends upon the difference of potential applied, and is different at the surface of the two plates.