_dp_ _nXeK_ ---- = 0 and _u_ = ----, _dx_ _p_ Let _N_ be the number of molecules in a cubic centimetre of gas at the pressure _P_ and at the temperature 15° C., for which the values of _u_ and _K_ have been determined. Then _N_/_P_ may be substituted for _n_/_p_, and, since _P_ at atmospheric pressure is 10⁶, 3 × 10⁸_u₁_ _Ne_ = ---------- electrostatic units, _K_ where _u₁_ is the velocity for 1 volt (_i.e._ ¹⁄₃₀₀ E. S. unit) per cm. It is known that one absolute electromagnetic unit of electricity in passing through water liberates 1·23 c.c. of hydrogen at a temperature of 15° C. and standard pressure. The number of atoms in this volume is 2·46_N_, and, if _e´_ is the charge on the hydrogen atom in the electrolysis of water, 2·46 _Ne´_ = 3 × 10¹⁰ E. S. units, _Ne´_ = 1·22 × 10¹⁰ E. S. units. _e_ _u₁_ Thus --- = 2·46 × 10⁻² ---- _e´_ _K_ For example, substituting the values of _u₁_ and _K_ determined in moist air for the positive ion, _e_ 2·46 1·37 --- = ----- × ----- = 1·04. _e´_ 100 ·032 Values of this ratio, not very different from unity, are obtained for the positive and negative ions of the gases hydrogen, oxygen, and carbon dioxide. Taking into consideration the uncertainty in the experimental values of _u₁_ and _K_, these results indicate that the _charge carried by an ion in all gases is the same and is equal to that carried by the hydrogen ion in the electrolysis of liquids_. =39. Number of the ions.= We have seen that, from experimental data, Townsend has found that _N_, the number of molecules present in 1 c.c. of gas at 15° C. and standard pressure, is given by _Ne_ = 1·22 × 10¹⁰. Now _e_, the charge on an ion, is equal to 3·4 × 10⁻¹⁰ E. S. units; thus _N_ = 3·6 × 10⁻¹⁹. If _I_ is the saturation current through a gas, and _q_ the total rate of production of ions in the gas, _I_ _q_ = --- . _e_