Ionization of Hydrogen by X-Rays. 



647 



in the gas itself. Thus, by measuring the slopes of the 

 two curves after they have become straight lines, Beatty 

 obtained a result which gave the relative ionization of 

 hydrogen and air. His results are given in Table III. 



Table III. 



Kays used. 



Ionization of H., 

 Ionization of Air 



Fe (Series K) 



Cu 



•00571 

 •00573 

 •00570 

 •00573 

 •0400 



Zn 



As 



Sn , 



For the rays from Fe to As the value of the relative ioni- 

 zation is constant, and is much lower than that obtained by 

 previous workers. There is, however, a sudden increase as 

 we pass from arsenic rays to tin rays, when the value of the 

 ionization increases sevenfold. The most natural interpre- 

 tation of this rise would be that there exists a radiation 

 characteristic of hydrogen, and of such a wave-length that 

 rays of the wave-length of arsenic rays or greater are unable 

 to excite it, while rays of shorter wave-length such as tin 

 rays can. If this were true, then it would be a result of 

 extreme importance, as no evidence of characteristic X-radia* 

 tion from the lightest gases has been otherwise obtained. 

 On the other hand, the rise occurs at a very significant place, 

 for if there were present in the gas a small amount of 

 arseniuretted hydrogen, it would produce just such an effect 

 as was observed. We have no data as to the ionization of 

 AsH 3 by X-rays, but Beatty* has measured the ionization 

 of SeH 2 , and from an X-ray point of view the behaviour of 

 the two gases should be very similar. If we plot the mass 

 absorption coefficient, X/p in Al, for the rays used, against 

 the ionization observed for both H 2 and SeH 2 , making the 

 ionization scales comparable, we get two curves which prac- 

 tically coincide, as is shown in fig. 1. The rise observed 

 occurs in very nearly the same region of the spectrum. 



* Proe. Roy. Soc. lxx.w. v . 578 (1911). 



