304 II. A. Bumstead — Eontgen Rays in lead and Zvnc. 



(without being removed) was exposed for ten minutes to strong 

 Eontgen rays from the large bulb with its focus 20 " 1 from 

 the plate. The ionization was then measured again several 

 times. The following is an example of the results obtained : 



Activity before exposure 7'56 ± '056 



Activity after exposure 7 - 54 ± "076 



Repetition of the experiment gave similar results. 



Conclusions. 



1. The result previously obtained by the writer, that the 

 heat generated in lead by Rontgen rays was twice that in zinc 

 is not confirmed by further experiments, which show that the 

 quantities of heat are equal, with an uncertainty of from 5 to 

 10 per cent. The source of the error was imperfect heat- 

 insulation of the metals ; this escaped the control experiment 

 on account of a difference in the coefficients of absorption of 

 the surfaces for light, which, by an unfortunate accident, was 

 just sufficient to compensate the other inequality. 



2. No rays capable of producing scintillations on a zinc- 

 blende screen are present among the secondary radiations from 

 lead when exposed to Rontgen rays. 



3. The disintegration of the active deposit from thorium 

 emanation is not hastened by exposure to Rontgen rays. 



New Haven, Conn., Dec. 6, 1907. 



