the Electrical Conduction produced by it, 



Thickness of Metal Leaf -00008 cm. 

 Layer of Uranium Oxide on plate. 



115 



Number of 



Leak per nrin. in 



Ratio for each 



Layers. 



scale-divisions. 



layer. 







91 





1 



77 



•85 



2 



60 



•78 



a 



49 



•82 



4 



42 



•86 



5 



33 



•79 



6 



24-7 



•75 



8 



15-4 



•79 



10 



91 



•77 



13 



5-8 



•86 



It will be shown later (§ 8) that the rate of leak between 

 two plates for a saturating voltage is proportional to the 

 intensity of the radiation after passing through the metal. 

 The voltage of 50 employed was not sufficient to saturate the 

 gas, but it was found that the comparative rates of leak under 

 similar conditions for 50 and 200 volts between the plates 

 were nearly the same. When we are dealing with very 

 small rates of leak, it is advisable to employ as small a voltage 

 as possible, in order that any small changes in the voltage 

 of the battery should not appreciably affect the result. For 

 this reason the voltage of 50 was used, and the comparative 

 rates of leak obtained are very approximately the same as 

 for saturating electromotive forces. 



Since the rate of leak diminishes in a geometrical pro- 

 gression with the thickness of metal, we see from the above 

 statement that the intensity of the radiation falls off in a 

 geometrical progression, i. e. according to an ordinary absorp- 

 tion law. This shows that the part of the radiation considered 

 is approximately homogeneous. 



With increase of the number of layers the absorption 

 commences to diminish. This is shown more clearly by using 

 uranium oxide with layers of thin aluminium leaf (see table 

 p. 116). 



It will be observed that for the first three layers of aluminium 

 foil, the intensity of the radiation falls on according to the 

 ordinary absorption law, and that, after the fourth thickness, 

 the intensity of the radiation is only slightly diminished by 

 adding another eight layers. 



12 



