170 Prof. E. Rutherford on Radioactivity 



substances, and that the amount of radioactivity depends 

 upon the amount of the " emanation." A radio-active sub- 

 stance like uranium, which gives out no emanation, produces 

 no trace of excited radioactivity, 



An experiment now to be described throws a further light 

 on the question. The general arrangement of the experiment 

 is shown in fig. 4. 



Fig. 4. 



aa T'i°J*iSniJ)_li Q£ ■ 



Mi 



A slow current of air from a gas-bag, after bubbling through 

 sulphuric acid, passed down through a rectangular wooden 

 vessel, 60 cms. in length. In order to remove spray and dust 

 and to equalize the current of air over the cross-section, the 

 air was passed through cotton-wool at W. A metal plate 

 covered the bottom of the vessel and was charged + . Four 

 insulated metal plates, A, B, C, D, placed at equal distances, 

 w T ere attached to a top metal plate connected to earth. 

 Thorium oxide covered with paper was placed under the 

 electrode A. 



The current of air was passed through the vessel at the 

 steady rate of about *2 cm. per second for a period of 7 hours, 

 w r ith 300 volts between the lower and upper plates. The 

 following results were obtained for the current due to the 

 emanation which reached A, B, C, D and the corresponding 

 radioactivity produced : — 





Relative current due 



Relative excited 





to emanation. 



radioactivity. 



ate A . 



1 



1 



„ B . 



•55 



•43 



„ C . 



•18 



•16 



„ D . 



•072 



•061 



The current due to the emanation which reaches A, and the 

 radioactivity produced in A, is in each case taken as unity for 



