262 Dr Barkla, Phenomena of X-Ray Transmission. 



principal features brought to light by the investigation are 

 exhibited in curve IV in fig. 4, in which the absorptions by a 

 given element are plotted as ordinates, and the absorptions by Al 

 as abscissae. 



In all the absorbing substances examined — C, Mg, Fe, Cu, Zn, 

 Ag, Sn, Pt, Au — there is approximate proportionality between the 

 coefficients of absorption in a substance E, say and in Al through 

 a wide range of penetrating power. This approximate pro- 

 portionality holds until the primary radiation is made more 

 penetrating than the homogeneous radiation characteristic of 

 the absorbing substance R. Then the absorption begins to 

 increase more and more rapidly until, after reaching a maximum, 

 it begins to approximate again to proportionality with the absorp- 

 tion in Al, the ratio of absorption coefficients being now much 

 higher than before. The increase in the absorption was thus 

 shown to commence at the same point as that at which the 

 secondary homogeneous radiation began to be emitted, the con- 

 nection between absorption and intensity of secondary radiation 

 being shown by the curves IV" and II. As the range of penetrating 

 powers experimented upon was that from the radiation characteristic 



of Cr f- in Al = 136 j to that characteristic of Ag (- in Al = 2-5 j, 



it will be seen by comparison with fig. 2* that all elements, 

 one of whose characteristic radiations is between these limits, 

 showed the same feature in the absorption curve. All those 

 which did not emit a characteristic radiation of penetrating power 

 between these limits gave absorption curves without any special 

 deviation from the approximate proportionality. It is important, 

 however, to notice several features of the curves. The shape of 

 the curve for primary radiations is considerably more penetrating 

 than that necessary to produce the characteristic secondary radia- 

 tion in any substance approximated to the shape just previous to 

 the sudden rise in the absorption, showing the possibility of a 

 similar rise in the absorption having occurred with a much softer 

 radiation. 



Again — and this is a most important point — the mass absorption 

 coefficients for a given radiation are in the order of atomic weight 

 of the absorbing substance when the corresponding radiations are 

 excited. 



Thus using the very soft radiation from Cr, - in C, Mg and Al 



are in order of atomic weight, but there is a step back for the 

 elements Fe, Ni, Cu, Zn, Ag, Sn because in these the radiation of 

 group B is not excited, and there is a further step back for Pt and 



* Reproduced from paper on " The Absorption of Eontgen Rays," Barkla and 

 Sadler. 



