104 Mr Growther, On the secondary Rontgen radiation 



ionization produced in ethyl bromide by the secondary rays from 

 ethyl bromide was 162 times that produced in ethyl bromide by 

 the secondary rays from air under similar conditions. The corre- 

 sponding value when air was used in the secondary ionization 

 chamber was 543. It will thus be at once apparent that the 

 ethyl bromide is much more favourable to the secondary rays from 

 air than is air itself. 



But although the coefficients of absorption for different types 

 of rays are more nearly equal for ethyl bromide than for air, they 

 are still not independent of the hardness of the rays. The coeffi- 

 cient of absorption of ethyl bromide for the primary rays used in 

 these experiments, and therefore, also, for the secondary rays from 

 air, was 'IS ; while its coefficient of absorption for its own secondary 

 rays was "SS. Knowing the dimensions of the secondary ionization 

 chamber B, and the pressure of the ethyl bromide in it, we can 

 easily calculate the quantity of each type of ray absorbed by the 

 gas in B. The width of the chamber was 3'5 cms. ; the pressure 

 of the ethyl bromide in it 210 mm. of mercury. The percentage 

 energy absorbed in the gas was, therefore, for the soft rays from 

 ethyl bromide 19'2 °/^, and for the hard rays from air 12"4 %. If 

 we assume that the amount of ionization produced is proportional 

 to the amount of energy absorbed, we thus obtain for the relative 

 amount of secondary radiation from ethyl bromide, compared 

 with that from air, the final value 105 ; a result which is just 

 about half that given in the original paper. Though thus con- 

 siderably reduced, the secondary radiation from ethyl bromide 

 still remains abnormally large. If we divide the relative secondary 

 radiation by the relative density (o"78) we obtain the value 28 ; 

 as against TO for the lighter elements; 8"0 for stannic chloride; 

 and 8'4 for methyl iodide. 



It may be noted that, as the penetrating power of the 

 secondary rays from stannic chloride, methyl iodide, and sub- 

 stances of low atomic weight is the same as that of the secondary 

 rays from air, the values obtained for the relative amounts of 

 secondary radiation from these substances compared with air are 

 not subject to any correction for selective absorption by the 

 medium in the secondary ionization chamber, and therefore need 

 no modification as the result of the present experiments. 



It has already been mentioned that in my original paper the 

 relative ionizations produced in the air of the secondary ionization 

 chamber by the different types of secondary rays were corrected 

 on the assumption that the amount of ionization was proportional 

 to the absorption. It was not possible, however, to measure the 

 absorption of the different types of rays in air itself, the absorption 

 by air being too small to be readily appreciable. What was done, 

 therefore, was to measure the absorption of the rays by some 



