568 On Water Radioactlvitij. 



was thoroughly sand-papered so as to remove all the mduced 

 radioactivity. The following table gives the relative amounts 

 of induced radioactivity of a brass rod when charged to 

 different potentials for the same length of time : — 



Charged for 10 minutes at 



Induced Radioactivity. 



-1000 volts 



32 



- 800 



28 



~ 600 



23 



- 400 



18 



- 200 



12 



-flOOO 



5 



+ 800 



4-5 



+ 600 



3-5 



+ 400 



3-0 



+ 200 



2-0 











To compare this with the effect of radium emanation, air 

 was drawn through a weak solution of a radium compound 

 into a closed brass cylinder of about 8 litres capacity, in 

 which a brass rod could be suspended and charged to any 

 desired potential. The rod was then tested for radioactivity 

 in another similar vessel filled with room air, the rod being- 

 connected to the electrometer and the cylinder to the battery. 

 The following results give the magnitude of the effect :— 



Charged 10 minutes at 



Induced Radioactivity 



+ 200 volts 



26 



+ 400 



50 



-200 



150 







6 



It is thus seen that the induced radioactivity due to radium 

 emanation on a rod at potential is only about one-eighth of 

 that on a rod at •+ 400 volts. But in the case of the gas 

 from water, this would have been too small an amount to 

 measure. This probably accounts for the fact that with the 

 radioactive gas from water it has not been possible to observe 

 any induced radioactivity on a conductor at potential. It 

 is also seen that for the induced radioactivity due to both 

 radium emanation and the water radioactivity, the effects 

 are about six times as great for a negatively charged con- 

 ductor as for a positively charged one. 



Co7iclusion, 



The above results lead to the view that the water radio- 

 activity is due to a substance very similar to, if not identical 



