Townsend has used the following method to compare the charge carried by a gaseous ion with that carried by an atom of hydrogen in the electrolysis of solution. We have u/D = Ne/[Pi], where D is the coefficient of diffusion of the ions through the gas, u the velocity of the ion in the same gas when acted on by unit electric force, N the number of molecules in a cubic centimetre of the gas when the pressure is [Pi] dynes per square centimetre, and e the charge in electrostatic units. This relation is obtained on the hypothesis that N ions in a cubic centimetre produce the same pressure as N uncharged molecules. We know the value of D from Townsend's experiments and the values of u from those of Zeleny. We get the following values for Ne × 10^-10:-- +---------------+---------------------+---------------------+ | | Moist Gas. | Moist Gas. | +---------------+----------+----------+----------+----------+ | Gas. | Positive | Negative | Positive | Negative | | | Ions. | Ions. | Ions. | Ions. | +---------------+----------+----------+----------+----------+ | Air | 1.28 | 1.29 | 1.46 | 1.31 | | Oxygen | 1.34 | 1.27 | 1.63 | 1.36 | | Carbonic acid | 1.01 | .87 | .99 | .93 | | Hydrogen | 1.24 | 1.18 | 1.63 | 1.25 | +---------------+----------+----------+----------+----------+ | Mean | 1.22 | 1.15 | 1.43 | 1.21 | +---------------+----------+----------+----------+----------+ Since 1.22 cubic centimetres of hydrogen at the temperature 15° C. and pressure 760 mm. of mercury are liberated by the passage through acidulated water of one electromagnetic unit of electricity or 3 × 10^10 electrostatic units, and since in one cubic centimetre of the gas there are 2.46 N atoms of hydrogen, we have, if E is the charge in electrostatic units, on the atom of hydrogen in the electrolysis of solutions 2.46NE = 3 × 10^10, or NE = 1.22 × 10^10.