656 



Mr. G. Shearer on the 



reliable) we get, as values for the upper limit of the ionization 

 of hydrogen by these rays, 



Ionization of H 

 Ionization of Air 



r- (Upper limit). 



Cu Rays. Sn Rays. 



•0010 -0016 



These values are much lower than any hitherto published, 

 and there is, in addition, a very strong probability that they 

 are overestimates. 



The lowest reliable values for copper and tin rays are 

 practically equal, although the average for all the determi- 

 nations for copper is smaller than that for tin. Also, 

 although on each occasion except one when the same sample 

 of gas was used, the value obtained for tin rays was larger 

 than that for copper rays, we are not entitled to assume that 

 this is a true effect in hydrogen. On one occasion the 

 opposite was the case, while the two values were within 

 7 per cent, of each other. This apparent increase of relative 

 ionization with decrease of wave-length is much more pro- 

 bably due to impurities in the gas, these impurities being of 

 greater importance in the case of tin rays than in the case 

 of the softer copper rays. Beatty obtained in the interval 

 between Cu rays and Sn rays a sevenfold increase in the 

 ionization, while Crowther also obtained a sudden rise for 

 rays whose wave-length was probably between that of copper 

 and that of tin. In view of the above experiments, it seems 

 that these effects were spurious, and there is no conclusive 

 evidence that the relative ionization of hydrogen increases with 

 a decrease in icave-length. All the evidence goes to suggest 

 that if such an increase exists it is small. 



In an attempt to seek out the various sources of impurity, 

 various chemicals were used, but on the whole the results 

 obtained were always of the same order. Also at a certain 

 stage all the rubber tubing connecting the various parts of 

 the apparatus was removed, and metal or glass substituted 

 without producing any change in the magnitude of the 

 quantities obtained. 



Theoretical Considerations. 



The fact that hydrogen seems practically incapable of 

 ionization by the action of X-rays raises interesting questions 



