The mean of the values of Ne in the preceding table is 1.24 × 10^10. Hence we may conclude that the charge of electricity carried by a gaseous ion is equal to the charge carried by the hydrogen atom in the electrolysis of solutions. The values of Ne for the different gases differ more than we should have expected from the probable accuracy of the determination of D and the velocity of the ions: Townsend (_Proc. Roy. Soc._ 80, p. 207) has shown that when the ionization is produced by Röntgen rays some of the positive ions carry a double charge and that this accounts for the values of Ne being greater for the positive than for the negative ions. Since we know the value of e, viz. 3.5 × 10^-10, and, also Ne, = 1.24 × 10^10, we find N the number of molecules in a cubic centimetre of gas at standard temperature and pressure to be equal to 3.5 × 10^19. This method of obtaining N is the only one which does not involve any assumption as to the shape of the molecules and the forces acting between them. Another method of determining the charge carried by an ion has been employed by Rutherford (_Proc. Roy. Soc._ 81, pp. 141, 162), in which the positively electrified particles emitted by radium are made use of. The method consists of: (1) Counting the number of [alpha] particles emitted by a given quantity of radium in a known time. (2) Measuring the electric charge emitted by this quantity in the same time. To count the number of the [alpha] particles the radium was so arranged that it shot into an ionization chamber a small number of [alpha] particles per minute; the interval between the emission of individual particles was several seconds. When an [alpha] particle passed into the vessel it ionized the gas inside and so greatly increased its conductivity; thus, if the gas were kept exposed to an electric field, the current through the gas would suddenly increase when an [alpha] particle passed into the vessel. Although each [alpha] particle produces about thirty thousand ions, this is hardly large enough to produce the conductivity appreciable without the use of very delicate apparatus; to increase the conductivity Rutherford took advantage of the fact that ions, especially negative ones, when exposed to a strong electric field, produce other ions by collision against the molecules of the gas through which they are moving. By suitably choosing the electric field and the pressure in the ionization chamber, the 30,000 ions produced by each [alpha] particle can be multiplied to such an extent that an appreciable current passes through the ionization chamber on the arrival of each [alpha] particle. An electrometer placed in series with this vessel will show by its deflection when an [alpha] particle enters the chamber, and by counting the number of deflections per minute we can determine the number of [alpha] particles given out by the radium in that time.