11] IONIZATION THEORY OF GASES 43 
Langevin determined the velocity of the ions by a direct method 
in which the time taken for the ion to travel over a known distance 
was observed. 
The followmg table shows the comparative values obtained for 
air and carbon dioxide. 

Air CO, 
Svein eal ge oT yopnne me 
€ E eed € ¢ ea} 
Ig Ky K, Ky Ky K, 
Direct method (Langevin) 1:40 170 1°22 0:86 090 1:05 
Current of gas (Zeleny)... 136 187 1°375 OPS. CORSE TSO 
These results show that for all gases except CO,, there is a 
marked increase in the velocity of the negative ion with the dry- 
ness of the gas, and that, even in moist gases, the velocity of the 
negative ions is always greater than that of the positive ions. The 
velocity of the positive ion is not much affected by the presence 
of moisture in the gas. 
The velocity of the ions varies inversely as the pressure of the 
gas. This has been shown by Rutherford! for the negative ions 
produced by ultra-violet light fallmg on a negatively charged sur- 
face, and later by Langevin? for both the positive and negative ions 
produced by Réntgen rays. Langevin has shown that the velocity 
of the positive ion increases more slowly with the diminution of 
pressure than that of the negative ion. It appears as if the nega- 
tive ion, especially at pressures of about 10 mm. of mercury, 
begins to diminish in size. 
34. Condensation experiments. Some experiments will 
now be described which have verified in a direct way the theory 
that the conductivity produced in gases by the various types 
of radiation is due to the production of charged ions throughout 
the volume of the gas. Under certain conditions, the ions form 
nuclei for the condensation of water, and this property allows us 
to show the presence of the individual ions in the gas, and also to 
count the number present. 
It has long been known that if air saturated with water-vapour 
is suddenly expanded, a cloud of small globules of water is formed. 
These drops are formed round the dust particles present in the gas, 
1 Proc. Camb. Phil. Soc. 9, p. 410, 1898. 2 Thesis, p. 190, 1902. 
