152 Mr Richardson, The Theory of the Rate, etc. 



the simplest possible constitution of the ions has been assumed. 

 Many facts however point to the conclusion that the negative ions 

 in air are more than mere corpuscles and that the positive ions 

 are more than a molecule from which a corpuscle has been re- 

 moved. Among such facts which have long been known are : 

 (1) the numerical values of the velocities and coefficients of 

 diffusion of the ions which indicate that they are of greater 

 than molecular dimensions, (2) the fact that the velocities of the 

 positive and negative ions are of the same order of magnitude, 

 and (3) the effect of moisture on the velocities of the ions. 



Recently several other results have been obtained which point 

 to the same conclusion. Langevin * has found that at low pressures 

 the velocity of the ions increases more rapidly than the inverse 

 ratio of the pressure which is required by the formula 



_ia 



before mentioned. The effect is specially marked in the case of 

 the negative ions, the positive ions showing very little increase at 

 the pressures investigated. 



As a result of some work at present being carried out in the 

 Cavendish Laboratory, Mr McClung finds that the coefficient of 

 recombination of X-ray ions in air increases very rapidly when 

 the temperature is raised, instead of varying inversely as the 

 square root of the absolute temperature, as the simple theory 

 requires. Thus Mr McClung finds that at 270° C. the coefficient 

 of recombination is about eight times as big as at 15° C. 



Both these results can only be explained by supposing that 

 X-ray ions in air at atmospheric pressure are complicated struc- 

 tures which become smaller as the pressure is lowered or the 

 temperature raised. 



It is at once evident that we cannot apply the foregoing 

 theory to the case of X-ray ions in air without making considerable 

 reservations. For example, it will not give the correct variation 

 with conditions which change the nature of the ions, such as 

 temperature and pressure (at low pressures). In addition, our 

 present ignorance of the nature of the ions prevents the absolute 

 value of the coefficient being calculated. The theory also assumes 

 that the diameter of the ions is small compared with the mean 

 free path. This condition however appears to be fulfilled by X-ray 

 ions at atmospheric pressure "f\ 



* Theses de VUniversite de Paris, a 431. Gautier Villars, 1902. 

 t See Townsend, Phil. Trans, a. vol. 193, p. 156. 



