BY THE ACTION OF A RADIO-ACTIVE SUBSTANCE, ETC. 
275 
Section V .—Effect of Pressure. 
The theory of the interdiffusion of gases shows that the coefficient of diffusion is 
inversely proportional to the total pressure of the two diffusing gases. This law has 
been confirmed by the experiments of Loschmidt and others. * The results given in 
Section II. show that the law can be extended to the case where one of the gases 
consists of ions. The pressure that the ions exert is so small that it does not 
contribute to the total pressure by an amount which could be measured. The total 
pressure in this case is the pressure of the gas into which the ions are diffusing, and 
we see that between the pressures 772 and 200 (millims. of mercury) the rate of 
diffusion is inversely proportional to the pressure. 
We conclude from this that the size of an ion does not change when the pressure 
varies between these limits. 
Ions produced by Various Methods. 
The experiments on diffusion show that the ions produced by Itdntgen rays, radio¬ 
active substances, and ultra-violet light are nearly of the same size, and subject to 
the same changes arising from the presence of moisture. The following table of 
coefficients of diffusion of ions into air shows that there are differences in the various 
cases which are greater than what might arise from experimental errors. 
The ions produced by the point discharge are larger than those produced by the 
other methods, since their rate of diffusion is much slower, except in the case of 
negative ions in moist air. 
Coefficients of Diffusion of Ions produced in Air by different Methods. 
Method. 
Dry 
air. 
Moist Air. 
Positive ions. 
Negative ions. 
Positive ions. 
Negative ions. 
Rontgen rays. 
•028 
•043 
•032 
•035 
Radio-active substance .... 
•032 
•043 
•036 
■041 
Ultra-violet light. 
_ 
•043 
— 
•037 
Point discharge. 
/ -0247 
b -0216 
•037 
•032 
•028 
•027 
•039 
•037 
* Meyer, ‘The Kinetic Theory of Gases,’ Chap. VIII, 
2 N 2 
