frow, air and ethyl bromide. 105 



solid, and to assume that the ratio of the absorptions was the same 

 for solid and gas. The phenomena of the selective transmission 

 of the primary rays by air, which Bragg has pointed out in the 

 paper already quoted, were at the time far from clear, and unfor- 

 tunately tinfoil, a substance which does not shew this selective 

 transmission of hard rays, was employed as the absorbing substance, 

 and the values obtained for it were used in calculating the final 

 result. The ratio of the coefficients of absorption of the secondary 

 rays from ethyl bromide, and air, is for tinfoil approximately 2"8. 

 The ratio for air itself is undoubtedly much higher. As mentioned 

 above it is hardly possible to measure the absorption of air itself 

 for these different rays, the absorption in any reasonable distance 

 being so small ; but filter paper suggests itself as being composed 

 of elements approximating very nearly in atomic weight to air. 

 The absorption of filter paper, therefore, has been measured for 

 the secondary rays from ethyl bromide and air ; and the ratio of 

 the two coefficients of absorption for filter paper has been found to 

 be 5"4. Using this corrected value for the coefficients of absorp- 

 tion, and assuming again that the ionization is proportional to the 

 energy absorbed (that is to say in the case of air, where the 

 percentage absorbed is very small, to the coefficients of absorption), 

 we deduce from our original measurements of secondary radiation 

 made in air the corrected value 101, for the relative amount of 

 secondary radiation from ethyl bromide. This value agrees re- 

 markably well with the value 105 obtained for the same quantity 

 from the present measurements, using ethyl bromide as the 

 absorbing gas in the secondary ionization chamber. 



Summary. 



The relative amounts of ionization produced in ethyl bromide 

 vapour by the secondary Rontgen radiation from ethyl bromide 

 and air have been measured. From this, knowing the absorption 

 of ethyl bromide for the two kinds of secondary rays, the relative 

 intensity of the secondary radiation from ethyl bromide has been 

 calculated ; and has been found to be 105 times that from air. 



The relative absorption of the two kinds of secondary rays has 

 been measured for a substance of low atomic weight ; and the 

 value obtained has been used to correct the original results for the 

 secondary radiation from ethyl bromide, which were obtained from 

 measurements made in air. With this correction, the two values 

 for the secondary radiation from ethyl bromide, obtained by 

 measurements made in ethyl bromide, and in air, have been 

 shewn to be in agreement. 



In conclusion I wish to express my best thanks to Prof. Sir J. J. 

 Thomson for his kindly interest in these experiments. 



8—2 



