Mr Lushy, Some Experiments on lonisation in Dried Air. 459 



Some Experiments on lonisation in Dried Air. By S. G. 

 LusBY, Emmanuel College. (Communicated by Professor Sir 

 J. J, Thomson.) 



[Read 14 March 1910.] 



(1) Introduction. In examining the physical properties of 

 ions, one cannot help being struck by the important part which 

 moisture plays in the determination of the coefficieKts which 

 define these properties. 



For example, in moist air, the coefficient of diffusion of positive 

 ions is -032, and that of negative ions is "035. If, however, the 

 gas be dried, the coefficient for positive ions drops to '028, whilst 

 in the case of negative ions there is a large increase to '043. 

 Under certain conditions one can take advantage of this difference 

 in the rates of diffusion between the two kinds of ions and com- 

 municate a decided positive charge to the gas. 



Again, the velocities of ions in any gas are affected if the gas 

 be allowed to retain any moisture ; for example, in moist air, the 

 velocity of the positive ion under a potential gradient of 1 volt 

 per centimetre is 1"37 centimetres per second, for the negative 

 ion I'ol ; if the air be dried, the velocity of the positive ion is 

 changed very slightly to 1*3 6, whilst that of the negative ion rises 

 to 1-87. 



In both sets of measurements it is seen that the action of 

 drying the gas is more pronounced in its effect on the negative 

 ion, but it should be remembered that the positive ion is also 

 affected, though in a lesser degree. Air does not show this latter 

 modification very well — probably on account of its complex struc- 

 ture — but in other gases, e.g. oxygen, hydrogen, and carbon dioxide, 

 the effect is very marked, as is seen by studying any standard 

 tables of coefficients. 



In condensation experiments this difference in the action of 

 positive and negative ions is again noticeable ; each kind of ion has 

 its own condensation point. The experiments of C. T. R. Wilson* 

 have shown that when the ratio of expansion of a gas is 1'25, 

 condensation occurs round negative ions, but not till the ratio is 

 increased to 1*31 do the positive ions produce a condensation 

 cloud. This suggests that the negative ions act more readily 

 than the positive ions as nuclei for the condensation of water 

 vapour. 



All these facts point to the conclusion that the negative ion is 

 much more intimately connected with water vapour than the 

 positive, and it was thought that if by some means an ionised gas 



* C. T. E. WUson, Phil. Trans. A, cxciii. 1899, p. 289. 



