the Electrical Conduction produced by it. 



121 



simple and that the intensity falls off according to the above 

 law : — 



Substance. 



\ 



for the a radiation. 



X 



for the /3 radiation. 



Dutch metal 



Aluminium 



Tinfoil 



2700 

 1600 

 2650 



15 



108 



49 



97 

 240 

 5-6 



Copper 



Silver 





Glass 





The above results show what a great difference there is in 

 the power of penetration of the two types of radiation. The 

 transparency of aluminium for the (3 radiation is over 100 

 times as great as for the a radiation. The opacity of the 

 metals aluminium, copper, silver, platinum for the /3 radiation 

 follows the same order as their atomic weights. Aluminium 

 is the most transparent of the metals used, but glass is more 

 transparent than aluminium for the (3 radiation. Platinum 

 has an opacity 16 times as great as aluminium. For the 

 a radiation, aluminium is more transparent than Dutch metal 

 or tinfoil. 



For a thickness of aluminium "09 cm. the intensity of the 

 ft radiation was reduced to '25 of its value ; for a thickness 

 of copper '03 cm. the intensity was reduced to *23 of its 

 value. These results are not in agreement with some given 

 by Becquerel * 9 who found copper was more transparent than 

 aluminium for uranium radiation. 



The /3 radiation has a penetrating power of about the same 

 order as the radiation given out by an average #-ray bulb. 

 Its power of penetration is, however, much less than for the 

 rays from a " hard " bulb. The a radiation, on the other 

 hand, is far more easily absorbed than rays from an ordinary 

 bulb, but is very similar in its penetrating power to the 

 secondary radiation | sent out when .z-rays fall upon a metal 

 surface. 



It is possible that the a radiation is a secondary radiation 

 set up at the surface of the uranium by the passage of the 

 /3 radiation through the uranium, in exactly the same way 



* C. R. 1896, p. 763. 



f Perrin, C. R. cxxiv. p. 455 ; Sagnac, C. R. 1898. 



