o 



Ionization of Hydrogen by X-Rays. 657 



from the theoretical point of view. The most natural sug- 

 gestion might be that since the hydrogen atom consists of 

 only one electron and a nucleus, these might be so tightly 

 joined together that the separation might be difficult, and 

 X-rays might not have the necessary energy. This is, 

 however, negatived by observations on the ionization of 

 hydrogen by electrons, for it has been shown that the 

 electrons emitted by X-rays can ionize hydrogen, so that it 

 cannot be any lack of energy in the X-rays which makes 

 the ionization difficult. Also observations on the ionization 

 potential of the various gases do not place hydrogen in the 

 place which the above results might suggest, for the value 

 of 11 volts obtained by Thomson from observations on posi- 

 tive rays is smaller than the ionization potential of many 

 gases which are readily ionized. 



The only data as to the absorption of hydrogen by X-rays 

 are contained in a paper by Growdy*, who measured the 

 absorption of Fe X-rays by hydrogen, and obtained the value 

 '0003 for X. This value is about ^ of that of air, so that 

 if ionization is a measure of absorption the ionization of 

 hydrogen for Fe rays should only be about { ) {) that of air. 

 This result is certainly not supported by the work described 

 in this paper, but Gowdy states that his determination in the 

 case of h}'drogen is only approximate. He was unable to 

 determine the absorption coefficient for Cu rays, and it 

 would seem that the value for Fe rays is a very large over- 

 estimate. 



The work of Bohrf on the hydrogen spectrum suggests 

 that the greatest frequency possible is 3*26 xlO 15 , which 

 corresponds to the extreme ultra-violet, and is about -^\ {) 

 of the frequency of copper X-rays (which lias a value 

 1'9-ixlO 18 ) and about . 2[) \ )n of the frequency of tin rays. 

 On Bohr's theory we should not expect to find a radiation 

 for hydrogen of shorter wave-length than this line in the 

 extreme ultra-violet, so that it is unlikely that there would 

 be a characteristic radiation of wave-length intermediate 

 between that of arsenic and that of tin. Again, the impro- 

 bability of such a radiation is strengthened by the fact that 

 if we extrapolate from experimental results on the K-radia- 

 tions of the elements, we find that the K-radiation of hydrogen 

 would have a wave-length of the order of magnitude of that 

 which Bohr's theory su££ests. 



©r« 



Also, if we assume that the energy absorbed when an 



* Journal dc Physique, nei. 5, p. 622 (1013). 

 t Phil. Mag. [6] xxvi. p. 1 (1913). 



Phil May. S. G. Vol. 30. No. 178. Oct. 1915. 2 U 



