
52 IONIZATION THEORY OF GASES Kes 
The saturation current through air was found to be 1:2 x 10-8 
amperes, 1.e. 36 E.S. units, for parallel plates, 45 cms. apart, when °45 
gramme of radium of activity 1000 times that of uranium was spread 
over an area of 33 sq. cms. of the lower plate. This corresponds to a 
production of about 10" ions per second. Assuming, for the purpose 
of illustration, that the ionization was uniform between the plates, 
the volume of air acted on by the rays was about 148 c.c., and the 
number of ions produced per ¢.c. per second about 7 x 10% Since 
N =3°6 x 10", it is thus seen that, 1f one molecule produces two 
ions, the proportion of the gas ionized per second is about 10—" of the 
whole. For uranium the fraction is about 10“, and for pure radium, 
of activity one million times that of uranium, about 10~* Thus 
even in the case of pure radium, only about one molecule of gas is 
acted on per second in every 100 millions. 
The electrical methods are so delicate that the production of 
one ion per cubic centimetre per second can readily be detected. 
This corresponds to the ionization of about one molecule in every 
10° present in the gas. 
40. Size and nature of the ions. An approximate estimate 
of the mass of an ion, compared with the mass of the molecule of 
the gas in which it is produced, can be made from the known data 
of the coefficient K of inter-diffusion of the ions into gases. The 
value of K for the positive ions in moist carbon dioxide has been 
shown to be ‘0245, while the value of AK for the mter-diffusion of 
carbon dioxide with air is ‘14. The value of K for different gases © 
has been found to be approximately inversely proportional to the 
square root of the products of the masses of the molecules of the 
two inter-diffusing gases; thus, the positive ion in carbon dioxide 
behaves as if its mass were large compared with that of the 
molecule. Similar results hold for the negative as well as for the 
positive ion, and for other gases besides carbon dioxide. 
This has led to the view that the ion consists of a charged 
centre surrounded by a cluster of molecules travellmg with it, 
which are kept in position round the charged nucleus by electrical 
forces. A rough estimate shows that this cluster consists of about 
30 molecules of the gas. This idea is supported by the variation 
in velocity, 7.e. the variation of the size of the negative ion, in the 
