Ionization of Hydrogen by X-Rays. 655 



occasions one radiator alone was used for the series, while 

 on others both the radiators were used, and thus the ioniza- 

 tions were measured for the two types of rays for the same 

 sample of gas. For these cases the ratio of the two is given 

 in the third column of the table. 



Discussion of Results. 



It will be seen that the ionization of hydrogen relative to 

 that of air is very small. It is also evident that the several 

 determinations vary considerably among themselves, much 

 more than can be accounted for by observational errors, or 

 by slight variations in the ionizing radiation. This indicates 

 that the gas itself is the cause of the greater part of the 

 variation, and this is hardly to be wondered at when we 

 remember that the effect of even a very small trace of im- 

 purity is very important, and also that it is very unlikely 

 that the same impurity would be present to the same extent 

 in two different samples of gas. In support of this is the 

 fact that when the gas was prepared electrolytically the 

 ionizations observed were lower. Also the preparation oi: 

 hydrogen in any quantity free from impurities is a task of 

 such difficulty that we cannot perhaps expect to settle ex- 

 perimentally whether this residual eif'eet is really due to the 

 gas itself or merely to the presence of foreign elements. 

 The important part played by impurities is shown by the 

 fact that about one part in 40,000 of a heavy substance such 

 as AsH 3 would be sufficient to account for the observed 

 ionization. In view of these facts, we are led to the conclusion 

 that the values given in the above table are upper limits to 

 the value of the ionization of hydrogen relative to that 

 of air. 



Even if we take the mean value in the above experiments, 

 we obtain an estimate which is smaller than that given by 

 Beattv, while in the light of the above considerations it is 

 clear that the mean value is an overestimate of the relative 

 ionization. In the case of copper rays several values have 

 been obtained which average about '001, while one measure- 

 ment was also made which gave '0003 as the result ; on this 

 determination, however, too much reliance is not to be placed. 

 In the case of tin rays, the lowest reliable estimate was 

 •0009, a value which agrees with the lowest determination 

 for copper rays. If, in order to get a measure of the ioniza- 

 tion of hydrogen relative to air for copper and tin rays, we 

 take the mean of the three lowest valuos obtained (excepting 

 the value "0003 in the case of copper as not sufficiently 



