The condensation vessel was similar to that employed by C. T. R. Wilson. Two parallel horizontal plates were fitted in the vessel and the radiation from an X ray tube or radio-active substance ionized the gas between them. A difference of potential _V_, small compared with that required to saturate the gas, was applied between the parallel plates distant _l_ cms. from each other. The small current _i_ through the gas is given (section 28) by _NuVe_ _i_ = ------ _l_ where _N_ = number of ions present in the gas, _e_ = charge on each ion, _u_ = sum of the velocities of the positive and negative ions. Since the value of _N_ is the same as the number of drops, and the velocity _u_ is known, the value of _e_ can be determined. In his last determination J. J. Thomson found that _e_ = 3·4 × 10⁻¹⁰ electrostatic units. A very concordant value, namely, 3·1 × 10⁻¹⁰, has been obtained by H. A. Wilson[70], by using a modified method of counting the drops. A check on the size of the drops, determined by their rate of fall, was made by observing the rate at which the drops moved in a strong electric field, arranged so as to act with or against gravity. J. J. Thomson found that the charge on the ions produced in hydrogen and oxygen is the same. This shows that the nature of the ionization in gases is distinct from that occurring in the electrolysis of solutions where the oxygen ion always carries twice the charge of the hydrogen ion. =37. Diffusion of the ions.= Early experiments with ionized gases showed that the conductivity was removed from the gas by passage through a finely divided substance like cotton-wool, or by bubbling through water. This loss of conductivity is due to the fact that the ions in passing through narrow spaces diffuse to the sides of the boundary, to which they either adhere or give up their charge. A direct determination of the coefficient of diffusion of the ions produced in gases by Röntgen rays or by the rays from active substances has been made by Townsend[71]. The general method employed was to pass a stream of ionized gas through a diffusion vessel made up of a number of fine metal tubes arranged in parallel. Some of the ions in their passage through the tubes diffuse to the sides, the proportion being greater the slower the motion of the gas and the narrower the tube. Observations were made of the conductivity of the gas before and after passage through the tubes. In this way, correcting if necessary for the recombination during the time taken to pass through the tubes, the proportion _R_ of either positive or negative ions which are abstracted can be deduced. The value of _R_ can be expressed mathematically by the following equation in terms of _K_, the coefficient of diffusion of the ions into the gas with which they are mixed[72],