712 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



it has been generally found that if one radiation is more 

 easily absorbed than another, in aluminium for example, it is 

 also more easily absorbed in air. Since the excited radiation 

 from the air is slightly less absorbed in aluminium than that 

 due to thorium, we can thus conclude that it is slightly less 

 absorbed in air. 



Fig. 4. 



/- 2 3 4 S 6 



Now it is known that the intensity of the excited radiation 

 from thorium falls to half value after passing through 1*6 cms. 

 of air. It thus follows that the intensity of the excited 

 radiation from air falls to half value after passing through 

 about 2 cms. of air, and is almost completely absorbed in a 

 distance of 10 or 12 cms. 



From the differences observed for the penetrating power 

 and ratio of decay w T e can conclude that the excited radiation 

 from air cannot be ascribed to the presence of any known 

 radioactive substance in the atmosphere. 



Transmission of Excited Activity. 



We have seen that the excited radiation from the air is 

 similar in all respects to the known types of excited activity 

 by thorium and radium. In both cases the activity is 

 confined to the cathode in an electric field, and can be partly 

 removed by rubbing with a cloth or by solution in acid. 



The differences observed in the rate of decay and pene- 

 trating power of the radiations show that the effects obtained 



