854 Prof. Barkla and Mr. Philpot : Ionization in Gases and 



Similarly, the direct ionization in air + the ionization due 

 to its corpuscles is 



(l-/)-373 +||/x '373 + ^g/x '373 x 1-5, 



when mixed with air. 



Summing these two and equating to the observed ionization 

 •989, we find 



/=-63. 



Similar experiments with C 2 H 5 Br at pressures 3'8, 4*7, 

 5*15 cm. of mercury were made, and the results were all 

 of the same order of magnitude, the average value of / being 

 •60. Thus again, though these experiments on the ionization 

 in gaseous mixtures all indicate that a large proportion of 

 the ionization is due to corpuscular radiation, they would not 

 of themselves lead us to conclude that all the ionization is 

 attributable to this secondary radiation. It is true that 

 a small error in some of our experimental determinations 

 would considerably alter the result of the calculation. But 

 these calculations have all agreed in indicating a departure 

 in the same direction from what might have been expected, 

 on assumptions which have, under slightly different con- 

 ditions, been justified with a certain degree of accuracy, 

 It must be admitted that but for the experiments of C. T. ft. 

 Wilson and perhaps those of Beatty to which we have 

 referred, we should regard these results as strong evidence 

 in favour of a considerable direct ionization by X-rays in 

 addition to that due to the swiftly moving corpuscles ejected 

 from the atoms of the gas traversed. They, however, lead 

 us to examine the assumptions and the data upon which the 

 calculations are based. We have assumed that (J) the 

 energies of the corpuscular radiation absorbed by the con- 

 stituents of a gaseous mixture are proportional to their 

 densities ; (2) the number of corpuscles ejected by a con- 

 stituent gas for a given energy of X-radiation absorbed, and 

 the ionization produced in a constituent gas for a given 

 energy of corpuscular radiation absorbed, are precisely the 

 same as when the gas is alone ; (3) the relative ionizations 

 produced in air, SH 2 , and C 2 H 5 Br by the complete absorption 

 of a corpuscular radiation are as indicated in Table II. Our 

 results would, however, require a large deviation from the 

 density law of absorption of corpuscular energy to afford an 

 explanation. Yet we know of no evidence of such a deviation ; 

 indeed, there is much evidence that there is little deviation 

 even for comparatively slowly moving electrons. We have 



