234 



Mr. D. 0. H. Florance on 



Table IV. 



Radiators. 



Absorption coefficients measured in 1*14 cm. lead. 



E i- 



E 2 . 



E, 



E 4 . 



2 "26 cm. carbon ... 

 1*3 cm. aluminium . 



•77 

 •77 

 "77 

 •66 



•77 



•77 

 - 73 

 •61 



•71 



•72 

 •70 

 •63 



•74 

 •75 

 •71 

 •64 



0-326 cm. lead 



The lead side of the electroscope has now cut off a con- 

 siderable portion of the secondary radiation due to the 7 rays 

 of radium B, and in consequence the absorption of the 

 primary beam has apparently been decreased. The lead 

 screen reduces the emergent and the returned radiation in 

 the same proportion for the lead radiator. 



The Distribution of Secondary Radiation 

 round the Radiator. 



Some experiments were made to examine the distribution 

 of secondary radiation under varying conditions. 



The emanation was screened with lead 1 cm. thick and the 

 side of the electroscope was an aluminium plate '202 cm. 



In figs. 4, 5, 6, 7 are shown the relations between the 

 secondary radiation and thickness of radiator for the four 

 standard positions E 1? E 2 , E 3 , E 4 respectively of the electro- 

 scope. In the same figures the relations between the secon- 

 dary radiations for equal weights of carbon, aluminium, zinc, 

 and lead and their atomic weights are also given. None of 

 these readings have been corrected for the absorption of the 

 secondary radiation in the material of the radiator or for the 

 absorption in the side of the electroscope. 



These corrections are uncertain and, moreover, considering 

 the difficulty of always reproducing exactly the same con- 

 ditions, it has been useless for the present to aim at anything 

 but general conclusions. 



At first sight it would appear that substances of low atomic 

 weight produce more secondary radiation than substances of 

 high atomic weight. It is to be noticed from the figures 

 that only with a thickness of lead less than 0'05 cm. is the 

 secondary radiation as great as from the corresponding mass 

 of zinc. Yet if a suitable correction be made for the absorp- 

 tion of the secondary radiation, it seems that lead produces 



