produced in gases by Röntgen rays or by the rays from active substances has been made by Townsend[1]. 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[2],
R = 4(·195e^{-3·66KZ/(a^2V)} + ·0243e^{-22·3KZ/(a^2V)} + &c.),
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. 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. Coefficients of diffusion of ions into gases.
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| |K for |K for |Mean value| Ratio of |
| Gas | + ions | - ions | of K |values of K|
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| 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 |
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