produced in gases by Röntgen rays or by the rays from active substances has been made by Townsend28. 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 mixed29,
where
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.
The following table shows the results obtained by Townsend when X rays were used. Almost identical results were obtained later, when the radiations from active substances replaced the X rays.
| Gas | K for + ions | K for – ions | Mean value of K | Ratio of values of K |
|---|---|---|---|---|
| Air, dry | ·028 | ·043 | ·0347 | 1·54 |
| „ moist | ·032 | ·035 | ·0335 | 1·09 |
| Oxygen, dry | ·025 | ·0396 | ·0323 | 1·58 |
| „ moist | ·0288 | ·0358 | ·0323 | 1·24 |
| Carbonic acid, dry | ·023 | ·026 | ·0245 | 1·13 |
| „ „ moist | ·0245 | ·0255 | ·025 | 1·04 |
| Hydrogen, dry | ·123 | ·190 | ·156 | 1·54 |
| „ moist | ·128 | ·142 | ·135 | 1·11 |
The moist gases were saturated with water vapour at a temperature of 15° C.
It is seen that the negative ion in all cases diffuses faster than the positive. Theory shows that the coefficients of diffusion should be directly proportional to the velocities of the ions, so that this result is in agreement with the observations on the greater velocity of the negative ion.
This difference in the rate of diffusion of the ions at once explains an interesting experimental result. If ionized gases are blown through a metal tube, the tube gains a negative charge while the gas itself retains a positive charge. The number of positive and negative ions present in the gas is originally the same, but, in consequence of the more rapid diffusion of the negative ions, more of the negative ions than of the positive give up their charges to the tube. The tube consequently gains a negative and the gas a positive charge.
38. A very important result can be deduced