658 On the Ionization oj Hydrogen by X-Hays. 



atom is just ionized is re-emitted as a characteristic radiation 

 when the displaced electron returns*, we can calculate what 

 the characteristic radiation of hydrogen is from the observed 

 value of its ionization potential. For, there being only one 

 electron in the hydrogen atom, the electron which is dis- 

 placed in the experiments which measure the ionization 

 potential must be the same as that which takes part in the 

 emission of any characteristic radiation. Thus if n is its 

 frequency, then 



Jin = Ye, 



where V is the ionization potential and Ji Planck's constant. 

 Substituting the values of h, V, e for hydrogen, we obtain 



a value which agrees with that obtained from other lines of 

 reasoning. There is thus no other evidence in favour of a 

 characteristic radiation near to that of tin, and the natural 

 conclusion, especially in view of the above experiments, would 

 be that it is really non-existent. Looked at from this stand- 

 point, the position of hydrogen is not so anomalous as at first 

 sight it appears; for it is an experimentally observed fact 

 that as the difference between the wave-length of the exciting 

 radiation and that of the characteristic radiation which is 

 being excited increases, the smaller is the observed ionization. 

 The characteristic radiation of copper has a wave-length 

 6 Jq and of tin ^oW °f tue value of that of hydrogen. 

 The characteristic X-radiations of air, on the other hand, 

 will probably have wave-lengths about 50 times that of tin, 

 and are thus much nearer the radiations of copper and tin 

 than that of hydrogen. This, together with the fact that 

 hydrogen contains only one electron per atom, seems capable 

 of explaining, in terms of discovered empirical laws, the 

 small observed value of the ionization of hydrogen relative 

 to that of air. 



In conclusion, I have much pleasure in thanking Professor 

 C. G. Barkla, F.R.S., who suggested the research, for his 

 kind interest and help throughout the work. 



(Part of the expense of the apparatus used in the above 

 experiments was met by a grant from the Carnegie Trust.) 



The Physical Laboratory, 

 University of Edinburgh. 



* Barkla, 'Nature/ March 4, 1915. 



