Radiation from Gases subject to X.-Kays. 



689 



When the length of the aperture B was kept constant and 

 the width altered, the rate of motion of the gold-leaf was 



Fig. 2. 



Ea/?th 



£a*7 H 



approximately proportional to the width of the slit and there- 

 fore to the breadth of the beam. This proved that the effect 

 was not due to secondary radiation from the edges of the 

 lead round B, for when the slit was very narrow, doubling 

 its width added little to the length of the edge, while the 

 motion of the gold-leaf was twice as rapid. 



The same thing was also proved by screening the paper 

 face of the electroscope from direct radiation from B by a 

 sheet of lead, which was so arranged as to intercept very 

 little radiation proceeding from the air through which the 

 primary beam passed. 



When the paper face was covered with a sheet of lead, the 

 electroscope was perfectly screened. 



It was at first (when very thin paper was used for the face 

 of the electroscope) doubtful whether some of the ions formed 

 in the air by the primary radiation were not pulled through 

 pores in the paper into the electroscope. The paper was 

 moistened with acidulated water, so that it could be kept 

 perfectly at zero-potential with the other parts of the case, 

 and hence so that there was no electric field outside due 

 to the charge on the plate and gold-leaf. There was thus 

 no electric field drawing the ions into the electroscope. The 



