14 THE THEORY OF IONIZATION OF GASES 



initial ionization was produced by ultra-violet light, are 

 air, hydrogen, carbon dioxide, water vapour, hydrochloric 

 acid, 1 nitrogen, 2 argon, 3 and helium. 3 A high degree 

 of accuracy can be obtained by this method, and the 

 experiments are simpler than those in which Kontgen 

 rays are used to generate the initial ionization. 



Again experiments with Kontgen rays have been made 

 with air, 4 hydrogen, and carbon dioxide, and it is found 

 that the values of a are the same over large ranges of 

 pressure as those obtained for these gases with ultra- 

 violet light. Thus under all variations of pressure and 

 electric force the negative ions produced in gases, either 

 by Kontgen rays or by collisions, follow the same changes 

 in ionizing power as the negative ions set free from a 

 metal plate by ultra-violet light. It may be concluded, 

 therefore, that they are all identical, and this is sup- 

 ported by independent experiments, which show that 

 the charges on negative ions produced by various methods 

 are all equal to a fixed atomic charge. 5 



It is interesting to remark that the positive ions, 

 although they have the same charge 6 as the negative, 



1 See papers by the author, Philosophical Magazine, June, 1902, 

 and April, 1903. 



2 H. E. Hurst, Philosophical Magazine, April, 1906. 



8 E. W. B. Gill and F. B. Pidduck, Philosophical Magazine, 

 August, 1908. 



4 See papers by the author, Philosophical Magazine, February, 

 1901, and by the author and Mr. Kirkby, Philosophical Magazine, 

 June, 1901. 



5 See papers on Diffusion of Ions, Philosophical Transactions, 

 Vol. cxciii., 1899, and Vol. cxcv., 1900, also papers in Vols. Ixxx., 

 Ixxxi., Ixxxii. of the Proceedings of the Royal Society. 



6 Kecent experiments show that in some cases positive ions have 

 twice the atomic charge, but as the charges on positive ions 

 generated by collisions have not yet been specially investigated, 



