11] IONIZATION THEORY OF GASES 49 
produced in gases by Réntgen rays or by the rays from active 
substances has been made by Townsend. 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 A of 
either positive or negative ions which are abstracted can be 
deduced. The value of R can be mathematically expressed by 
the following equation in terms of K, the coefficient of diffusion 
of the ions into the gas with which they are mixed”, 


_3:06KZ _223KZ 
R=4(195e “’” 4:0243e “VY + &c.), 
where a =radius of the tube, 
Z=\ength 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 tons into gases. 










| 
Gas K for+ions | K for —ions ee arehodt a 
| | 
Atmdry osc.) st. ‘028 O430 Ones 034700 ml enam lcs 4 
Mmoists 224.) 032 035. | 20335. Ur 71509 
| Oxygen, dry Pee 025 ‘0396 ‘0323 1:58 
| mi moist Me 0288 0358 =, 0323 1:24 
| Carbonic acid, dry... | 023 026 | 0245 113 
| 5 y moist) | -0245 0255 | 025 1:04 
Hydrogen,dry  ... | ‘123 1 STO) | °156 | 154 
Fe Moist 24. |) 28 Lz) eeeraltcae) | Teil 
| | | 
1 Phil. Trans. p. 129, 1899. 2 Townsend, loc. cit. p. 139. 
THe eho 4 
