the Absorption of Rontgen Rays. 753 



radiation characteristic o£ the substance B. When the in- 

 cident radiation is made more penetrating than the secondary 

 radiation characteristic of the absorbing substance B, the 

 absorption first ceases to diminish as rapidly as the absorption 

 in Al, then it increases along RS. At the same time the 

 secondary radiation characteristic of B begins to be emitted, 

 then rapidly increases. As the primary radiation becomes 

 still more penetrating, the absorption in B begins to diminish 

 as along ST, and the intensity of the secondary radiation 

 diminishes at the same rate as the ionization produced by 

 the primary radiation in air, which is probably at approxi- 

 mately the same rate as the absorption in air and conse- 

 quently in Al, over the range of penetrating powers experi- 

 mented upon. 



The portion of the ordinates above PO may be regarded as 

 representing the absorption in the substance B which is con- 

 nected with the emission of the secondary radiation charac- 

 teristic of B. We may thus divide the absorption coefficient 

 into two parts, one bearing an approximately constant ratio 

 to the corresponding quantity for any absorbing substance, 

 and the other the part connected with the emission of the 

 characteristic secondary radiation. 



A few features require special mention. The greatest 

 deviations from the law of proportionality occur in those 

 cases in which the energy scattered is a considerable fraction 

 of the total energy. It has been calculated that in the light 



elements the portion of - due to scattering is of the order *2. 



It is seen here, however, that if we subtract "16 from 



Age Ag_>ig an j g Ai £ nere j g s^ti'iofc proportionality within 



the limits of experimental error, between the coefficients of 

 absorption in 0, Mg, and Al. 



Again, the apparent departure in the cases of Ag^Ag and 

 Ag X Sn is much diminished by a similar consideration. 



There appears, however, to be a slight residual change in 

 the ratios in the cases of Ag and Sn absorptions, and possibly 

 very slight in the cases of absorption by Pt and Au, but the 

 energy of the radiation scattered in these two cases has not 

 been determined. 



We may therefore conclude that the proportionality spoken 

 of is a very accurate one through a big range of penetrating 

 powers, if we subtract the energy scattered from that absorbed 

 before determining the absorption coefficient. 



Without yet entering into the discussion of any theory, let 

 Phil. Mag. S. 6. Vol. 17. No. 101. May 1909. 3 E 



