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]],
where
a = radius of the tube,
Z = length of the tube,
V = mean velocity of the gas in the tube.
Only the first two terms of the series need be taken into account when narrow tubes are used.
In this equation R, V, and a are determined experimentally, and K can thus be deduced.