Air, dry 1·36 1·87 1·375 13°·5 C. „ moist 1·37 1·51 1·10 14° Oxygen, dry 1·36 1·80 1·32 17° „ moist 1·29 1·52 1·18 16° Carbon dioxide, 0·76 0·81 1·07 17°·5 dry „ „ 0·81 0·75 0·915 17° moist Hydrogen, dry 6·70 7·95 1·15 20° „ moist 5·30 5·60 1·05 20° Langevin determined the velocity of the ions by a direct method in which the time taken for the ion to travel over a known distance was observed. The following table shows the comparative values obtained for air and carbon dioxide. Air _K₁_ Air _K₂_ Air CO₂ _K₁_ CO₂ _K₂_ CO₂ K₂/K₁ K₂/K₁ Direct 1·40 1·70 1·22 0·86 0·90 1·05 method (Langevin) Current of 1·36 1·87 1·375 0·76 0·81 1·07 gas (Zeleny) These results show that for all gases except CO₂, there is a marked increase in the velocity of the negative ion with the dryness of the gas, and that, even in moist gases, the velocity of the negative ions is always greater than that of the positive ions. The velocity of the positive ion is not much affected by the presence of moisture in the gas. The velocity of the ions varies inversely as the pressure of the gas. This has been shown by Rutherford[63] for the negative ions produced by ultra-violet light falling on a negatively charged surface, and later by Langevin[64] for both the positive and negative ions produced by Röntgen rays. Langevin has shown that the velocity of the positive ion increases more slowly with the diminution of pressure than that of the negative ion. It appears as if the negative ion, especially at pressures of about 10 mm. of mercury, begins to diminish in size. =34. Condensation experiments.= Some experiments will now be described which have verified in a direct way the theory that the conductivity produced in gases by the various types of radiation is due to the production of charged ions throughout the volume of the gas. Under certain conditions, the ions form nuclei for the condensation of water, and this property allows us to show the presence of the individual ions in the gas, and also to count the number present.