Radiation from Gases subject to X-Rays. 691 



aluminium '012 cm. thick. The difference showed the amount 

 of secondary radiation previously absorbed by the air in the 

 electroscope now absorbed by the aluminium. The secondary 

 beam was, however, in this case not directed, so that the 

 effective thickness of the plate was greater than *012 cm. 

 With one bulb the absorption amounted to 47 per cent, of the 

 initial intensity — taking the amount of ionization in the 

 electroscope as proportional to the intensity of the radiation 

 passing through — while the absorption of the directed primary 

 beam (in this case falling perpendicularly on the absorbing 

 plate) was 30 per cent. Thus the absorption was of the same 

 order of magnitude as that of the primary beam. 



To test this more accurately, the electroscope was placed 

 several centimetres away from the primary beam, and received 

 the secondary radiation which passed through rectangular 

 apertures in two parallel lead plates. All the rays entering the 

 electroscope through the second aperture passed through the 

 first, so that the effective thickness of a protecting plate was 

 very little greater than the actual thickness. As the beam 

 was more perfectly directed in successive experiments, the 

 proportional absorption by similar plates of aluminium of the 

 direct and secondary beams was found to come closer and 

 closer into agreement. The absorption-coefficients for the 

 primary and secondary radiations cannot differ by more than 

 10 per cent, of their value, for the radiations experimented 

 upon. 1 conclude that the penetrability of the primary and 

 secondary rays is practically the same. 



In order to investigate the subject more fully — more par- 

 ticularly to ascertain what the intensity and the penetrability 

 of the secondary radiation were dependent upon — different 

 gases were used in place of the air through which the primary 

 beam was directed. As the gas had to be inclosed, it was of 

 course necessary to guard against secondary effects from the 

 containing chamber reaching the electroscope. The shielding 

 from this secondary radiation made it impossible to expose 

 the electroscope to radiation from a large quantity of gas, 

 and made it necessary to place the electroscope some little 

 distance from the pas the radiation from which was being 

 studied. The effects were thus considerably diminished. 

 A metal box 14 cms. x 10 cms. x 10 cms. was constructed with 

 inlet and outlet tubes for the gases, and with two parchment 

 windows, one at the end and the other at the side. In some 

 experiments the former was of thin aluminium (*011 cm.). 

 This was for the admission of primary radiation from the bulb. 

 The secondary radiation passing through the side window 

 was studied. 



