546 



Mr. N. E. Campbell on Radioactivity 



in the figure ; by this means all induction effects due to 

 surface electrification on the insulation were avoided. 



Ffr. 1. 



TO ELECTROSCOPE 



SULPHUR 



-GUARD-RING TO EARTH-* 



SEALING WAX 

 -LEAD TO ELECTRODE 



BATTERY 



A 



C.1LMWSED 

 IRON 



w/?£ ivrrr/m 



/METRE 



fc WINDOW OF PAPER OR FO/L 



TRAY TD/SH CONTAINING 

 REAGENTS 



The bottom of the cylinder was covered by a window which 

 in various experiments consisted of paper coated with graphite, 

 aluminium-leaf, or tinfoil. Over the window was placed the 

 shallow dish of galvanized iron T, which prevented the de- 

 position on the window of the excited activity from the 

 atmosphere, when the vessel was charged negatively. The 

 electroscope was arranged as in the previous paper. The 

 time required for the electrode to reach a potential of 1*3 

 volts was about one minute ; individual readings could be 

 taken at the rate of four in five minutes. 



It should be noted incidentally that the variations in the 

 leak of this large vessel (see ante, § 5) were extremely small, 

 never more than 5 per cent. The effects which were now 

 observed when heat was applied by placing a bunsen-burner 

 under the tray were somewhat complicated ; their nature 

 and cause has not been fully elucidated, but only investi- 

 gated so far as to enable the question to be decided whether 

 chemical change is accompanied by radioactivity. It will be 

 seen that the heat effect is clearly " instrumental," — uncon- 

 nected with the essential processes of ray-emission from 

 ordinary material ; hence further search into its origin has 

 been postponed until more urgent problems are decided. 



§ 4. The tray was placed directly over the opening in the 

 bottom of the cylinder which was not covered by a window. 



Immediately after the bunsen had been lighted a change 

 in the leak was observed whose nature and sign varied with 



