Induced -Radioactivity in Air. 36o 



the vessel A they were usually tested iu pairs, which were 

 connected together and placed in symmetrical positions with 

 respect to the nxis of A. 



The amount o£ induced radio-activity on negatively electrified 

 surfaces exposed to the emanation from thorium, or placed in 

 the open air, seems also to be independent o£ the nature o£ 

 the surface. 



The ionizing power of the wire is only produced by 

 negative electrification. If the wire when placed in the 

 modified conducting gas in A is positively electrified, or if 

 it is not electrified at all, then no change in the current 

 through B with this wire as electrode is produced by the 

 immersion of the wire in the vessel A. 



To show the ionization produced by a wire after negative 

 electrification in A, it is not necessary to use the wire itself 

 as the electrode in measuring the current through B, an 

 independent wire may be used as electrode, and the ionization 

 due to the wire can be detected by the increase in the satura- 

 tion-current which takes place when the wire is put into B 

 after its negative electrification in A : the effect, although 

 very distinct, is not so large as when the electrified body is 

 itself used as the electrode ; it is desirable to use a piece of 

 metal of considerable area for the body which is negatively 

 electrified. 



The ionizing power possessed by the active metal is very 

 easily cut off by thin layers of solids. I have, however, been 

 able to detect that an appreciable effect is produced by the 

 negatively electrified metal even when surrounded by thin 

 aluminium foil or paper. In this connexion it may be 

 mentioned that if the conductivity of air when in the modified 

 state is clue to rays given out from centres of ionization, these 

 rays must have very little penetrating power, as I have drawn 

 modified air possessing high conductivity over a photographic 

 plate in the dark for more than four hours without producing 

 an impression on the plate. 



The active state in which a metal rod is put after being 

 negatively electrified in the modified air is not a permanent 

 one. As soon as the negative electrification stops, the activity 

 of the metal begins to diminish, and after a few hours it 

 entirely disappears : measurements of the rate at which this 

 activity disappeared showed that it fell off rapidly at first 

 and then much more slowly : the time taken for the activity 

 to fall to half its initial value was about 45 minutes. It 

 varied a little in the different experiments. 



When once a wire has been put into the active state it can 

 stand very rough treatment without losing its ionizing power. 



2 B 2 



