Radiation from Gases subject to X-Rays. 693 



were the thin window through which the primary beam 

 entered the box, and the end C through which it left. To 

 protect against any effect from these, a leaden screen S 3 with 

 rectangular aperture D was placed at a distance of 5 cms. 

 from the side window F, and with the aperture opposite this 

 window. In that position no rays from the end window, or 

 from that portion of the opposite end of the chamber which 

 was subjected to the primary rays, passed through the side 

 window and through the aperture in the screen, so that the 

 electroscope, which was placed immediately behind this screen, 

 did not receive any secondary radiation from solids. 



But it was obvious that it would receive tertiary radiation 

 from at least one source. To properly guard against this 

 would have necessitated a considerable diminution in the 

 energy of secondary radiation from the gas which it was 

 desired to measure. Instead of this, then, the tertiary radia- 

 tion falling on the thin face of the electroscope was made 

 as small as possible, and was afterwards proved to be 



negligible. 



This tertiary radiation arose from the secondary rays from 

 the first window E falling on the side window F of the gas 

 chamber and on leaden screens surrounding the thin face of 

 the electroscope, also from the secondary rays from the other 

 end of the gas chamber falling on the side window and leaden 

 screens. 



To ascertain the magnitude of this effect, experiments 

 were made with air. In the first case the parchment side 

 window F was removed and placed immediately in front of 

 the thin face of the electroscope, so that no secondary radia- 

 tion fell upon it ; also the leaden screens protecting the thin 

 face from other radiations were placed a considerable dis- 

 tance from the electroscope so that the tertiary radiation 

 received by this must have been excessively minute. In 

 the other case these were brought as near as possible and 

 the parchment window was placed in its normal position 

 in the gas chamber, so that it was a source of tertiary 

 radiation. 



In the first case the electroscope measured the ionization 

 arising from the secondary radiation from the air and from 

 the very weak tertiary radiation from the leaden screens, hi 

 the other case the ionization arose from the secondary radiation 

 from the gas and a maximum tertiary radiation from screens and 

 windows. The difference between the intensities of the total 

 radiation received in the two cases was too small to be 

 observed. 



Phil. .Mag. S. 6. Vol. 5. No. 30. June 1903. 3 A 



