Induced-Radioactivity in Air. 353 



cellar becomes radio-active, i. <?., it increases the electrical 

 conductivity of the air in its neighbourhood. The most 

 natural explanation of this phenomenon is that, as Elster and 

 Geitel suppose, the atmosphere contains some radio-active 

 constituent which is attracted to the negatively electrified 

 surface ; this constituent of the atmosphere behaving like 

 the " emanation " from thorium which has been shown by 

 Rutherford * to induce radio-activity in bodies with which it 

 comes in contact and to be attracted to negatively electrified 

 surfaces. I have, however, as the result of the experiments 

 described below come to the conclusion that though the exist- 

 ence of this radio-active substance in the air is possible it is 

 not necessary for the explanation of the effect observed by 

 Elster and Geitel. and that negatively electrified surfaces may 

 become radio-active without the deposition upon them of 

 substances having specific radio-active properties. 



As long as we have to experiment either in the open air 

 or in large rooms, it is exceedingly difficult to alter the con- 

 ditions sufficiently to afford an adequate test of any proposed 

 explanation ; I have therefore been experimenting with air 

 contained in a closed vessel of moderate size, and although 

 under normal conditions L have not been able to get any 

 appreciable amount of induced radio-activity, I have found that 

 a negatively electrified wire placed in the vessel acquired, 

 when the gas in the vessel was exposed to Rontgen rays or 

 had been bubbled through water, properties analogous to 

 those found by Elster and Geitel in wires placed in the open 

 air. The effects with the gas which had bubbled through 

 water were very large. 



The method was as follows : — a large cvlindrical zinc gas- 

 holder 102 cm. long and 75 cm. in diameter, was supported 

 on insulating feet, and closed by a lid made of millboard ; 

 the outer portions of the top and bottom of the lid were put in 

 metallic connexion with the gas-holder by rings of tinfoil 

 which overlapped the lid and were fastened to the cylinder ; 

 circular guard-rings of tinfoil connected with the earth were 

 pasted on the upper and under surfaces of the lid, these pre- 

 vented any leakage of electricity across the lid from the 

 cylinder to a metal rod placed along its axis; this rod, which 

 was connected with the electrometer, passed through a short 

 metal tube in an ebonite disk which occupied the central 

 portion of the lid, a flange on the rod resting on the top of 

 the tube. The current between the rod and the cylinder was 

 measured by an electrometer which was connected with the 



* Phil. Mag. [6]i. pp. 1 & 161. 



