12 Prof. Rutherford and Miss Brooks : Comparison of 



with the distance. The thickness of air through which each 

 radiation passes before the intensity of the radiation falls to> 

 half its original value, is given in the following table. 



Radiations. 



Distance in cms. I 



Excited radiation from radium and thorium. 

 Thorium 



1-65 

 1-0 





•75 





•45 







The penetrating power of the different radiations in air 

 thus follows the same order as metals and solid substances. 



Connexion between Absorption and Density. 



From the curves of absorption of the radiations in metals 

 and air, the coefficient of absorption X can be readily deter- 

 mined. The following table gives the value of X for 

 aluminium and air for the different radiations. 



Eadiation. 



X for aluminium. 



X for air. 





830 

 1250 



1600 

 2750 



•42 

 •69 

 •90 

 1-6 



Thorium 





Uranium 





Taking the density of air at 20° C. and 760 mm. as '00120 

 compared with water, the following table shows the value 

 of X divided by density for the different radiations. 



Eadiation. 



Aluminium. 



Air. 



Excited 



320 



480 



620 



1060 



350 



550 



740 



1300 





Badium 







