690 Mr. 0. 6r. Barkla on Secondary 



moisture, however, affected the normal fall of the gold-leaf * ; 

 but the great increase produced in the rate of motion by 

 what appeared to be a secondary radiation was not materially 

 affected. 



A very thin leaf of aluminium was put over the paper 

 face and was connected to earth with the brass case, and it 

 was found that the ionization produced inside was not appre- 

 ciably reduced. 



It was therefore evident that the ionization inside [was 

 produced by a radiation proceeding from the gas which 

 was subjected to primary X-radiation, or that there was 

 still a diffusion of ions through the paper and aluminium 

 leaf. 



An aluminium plate was then placed on the side of the 

 primary beam opposite to the face of the electroscope but 

 outside the beam. This was connected first to the positive, then 

 to the negative terminal of a battery of 100 cells, whose other 

 terminal in each case was earthed. As the electroscope-face 

 was at zero-potential ? there was in each case an electric field 

 of about 40 volts per centimetre across the beam in opposite 

 directions in the two cases. The ions which would discharge 

 the electroscope were in one case attracted away from the 

 electroscope to the plate, and in the other case attracted to 

 the face of the electroscope. But it was found that there was 

 no appreciable difference between the two leaks in the two 

 cases, showing : — 



(1 ) That there was not a diffusion of ions through the paper 

 and aluminium face ; and 



(2) That the radiation which came from the gas was not 

 in any way the result of recombination of the ions, for 

 the ions were withdrawn before they had sufficient 

 time to recombine. 



The ionization of the air in the electroscope was therefore 

 the result of radiation from the gas upon which primary 

 radiation fell, and this secondary radiation was independent 

 of the subsequent behaviour of the ions in the primary 

 beam. 



The absorbability of the secondary radiation was then in- 

 vestigated. To ascertain the order of magnitude of this 

 absorption, the electroscope was placed with aluminium and 

 paper face just outside the primary beam. When this face 

 was covered with lead no ionization was detected. The rate 

 of ionization was measured when there was no protect- 

 ing plate and when the face was covered with a sheet of 



* Probably due to a radioactive substance iu the water. 



