IS Bumstead — Heating Effects produced by Rbntgen Bays. 



the rays and an aluminium screen of the same thickness as the 

 window was set up in front- of the window ; the lead and zinc 

 of the thickness of the strips were introduced behind this 

 screen. The following deflections in succession were obtained 

 by a three-minute exposure to the rays in each case : 



Al 8-4 



Al + Pb 0-8 



Al-f Zn 0-9 



Al + Pb 0-6 



Al 6-6 



It is plain that the rays did not remain very constant during this 

 somewhat prolonged use, but it is also plain that, whatever 

 agent it may be which affects the radiometer, it is practically 

 equally absorbed by the lead and the zinc. Taking the means of 

 the readings as they stand, we find that, of the energy which 

 gets through the aluminium, 88 per -cent is absorbed by the 

 zinc and 9i per cent by the lead. 



9. Even if one assumed the substantial correctness of the 

 energy measurements, there still remains the possibility that 

 the difference between the two metals may not be due to atomic 

 disintegration. It is known that secondary rays are generated 

 by the absorption of the primary rays and it may be supposed 

 that these carry away from the metal a considerable fraction of 

 the energy of the primary rays — a much greater fraction in 

 the case of zinc than in the case of lead, thus giving rise to 

 the observed difference. An examination of this possibility is 

 therefore necessary. 



The secondary rays from such metals as lead and zinc consist 

 of two well-defined groups, one of which is completely absorbed 

 in less than 10 mm of air, the other being much more penetra- 

 ting. Let us consider first the less penetrating rays. It is 

 stated in the introductory portion of this paper that these rays 

 must form a large fraction of all the secondary rays generated ; 

 this statement appears at first sight to be in contradiction with 

 the experimental result obtained by H. S. Allen,* who found 

 that the ionization produced by these easily absorbed second- 

 ary rays from brass is 1/1900 of that which would be pro- 

 duced by complete absorption of the primary rays in the gas. 

 A little consideration, however, shows that, with rays so easily 

 absorbed, a large number must be generated in the metal to 

 permit even so small a fraction to escape. 



Let the intensity of the primary rays which have pene- 

 trated a distance x into the metal be l x = \ x e-^ x . The 

 absorption in an element of thickness dx will be A, Ij dx 

 and we may assume that this is proportional to the intensity 



- * Phil. Mag. iii, p. 126 (1902). 



