748 Dr. C. G. Barkla and Mr. C. A. Sadler on 



radiation ; Xj was determined approximately without the 

 application of the correction term. 



Thus as an example — 



When As radiation was transmitted through Cu, 



«*)-• 



128-9 



3b8x w 



X 2 or as^Cu* = 169 x /o C u 

 X 2 or cu^cu= 53 x pcu 



(\ 2 -A, 1 )= -116 X/o C u= -1038. 



Secondary Iomzation 128^ _1__ r _ (10 3 8)( .ooo9S>] 



Primary Ionization * v x U1 x ^ D0 x 60'7 - 1038 [_ 

 = '06 approximately. 



The ionization in the electroscope due to the primary beam 

 when the absorbing plate of Cu was in position was thus found 

 to be about 6 per cent, less than the observed ionization. 

 Similar corrections were made in the other cases of trans- 

 mission in which a secondary radiation characteristic of the 

 absorbing substance was set up. The maximum correction 

 affected the absorption coefficient by about 4 per cent. 



A correction was also necessary in a few cases for thu 

 presence in the radiation from Cr of the weak heterogeneous 

 scattered radiatioD. Though in most experiments the effect 

 of this was inappreciable, when the absorbing plates used 

 absorbed over 90 per cent, of radiation, the much more 

 penetrating scattered rays having been only slightly absorbed 

 appeared in much higher proportion than normally, and 

 introduced an error. 



By absorbing the radiation from Or by various thicknesses 

 of Al and comparing the absorption coefficients calculated 

 from these absorptions, the error introduced in the case of 

 the higher absorptions was obtained. A curve was plotted 

 giving the error corresponding to any particular absorption. 

 This correction was then applied to the calculated values of 

 the absorptions by C, Ag, and Sn — substances which absorb 

 similarly to Al. In no other case could the error, due to the 

 same cause, have been more than 1 or 2 per cent, f, and that 

 in one or two cases only. The values finally obtained for 

 the coefficients are given in the following table. As the 



* AsXcu means the coefficient of absorption of the radiation from As 

 by Cu. 



* | The absorptions by Pt and Au of the radiation from Cr are excepted. 

 These were too high to be relied upon. The value given in the table 

 for Pt was obtained not by experiment, but by calculation from the 

 observed ratio between the absorption coefficients. 



