from Ordinary Materials. 



539 



materials is secondary radiation excited by external pene- 

 trating rays. In the case of platinum all the absorbable 

 radiation appears to be secondary : the conclusion is remark- 

 able, but I have no reason to think that the experiments with 

 platinum are le^s accurate than any of the others. It 

 will be noted that the value of v 2 is not the same for all 

 metals either in column 3 or 5, as might have been expected 

 if the radiation corresponding to r, was external to the 

 apparatus. But the variations mentioned in § 5 must be 

 borne in mind. In addition, it should be pointed out that 

 the value of v 2 , being- calculated as the difference of two 

 comparatively large (piantities, is liable to considerable error, 

 and that the introduction of a correction for the effect of the 

 corners of the box would cause a notable alteration. 



§ 9. It was now considered that sufficient evidence had 

 been obtained of the existence of easily absorbable rays from 

 ordinary materials : experiments were therefore devised by 



TO ELECTROSCOPE 



TO EARTH 



/ILUM//V/UAf 

 r///VDOW 



TO B/fTTS/r-r 



/}£ UM//V/UM 

 W/A/DOtT 



which it was hoped to establish more directly the existence 



of the rays and to arrive at some knowledge of the value of \. 



A rectangular vessel of zin^ sheet (fig. 3) was constructed, 



2 X 2 



