Absorption of 7 Rays. 607 



the effects of both true absorption and scattering are super- 

 posed. However, it is generally supposed * that the softer 

 constituents of 7 rays are more easily scattered than the 

 harder ones, so that scattering gives no ready explanation of 

 the softening obtained in these experiments. 



It is possible that softening may be due in some way to 

 the effect of excited 7 rays in the absorption-sheets — i. e n a 

 transformation of part of the original 7 rays. Such excited 

 rays will be in all cases softer than the rays producing them, 

 and the issuing rays will reach a state of equilibrium when 

 the proportion of primary rays and of softer excited rays has 

 a certain definite value. In the case of aluminium absorber, 

 for example, when the rays have previously passed through 

 a hardener of high atomic weight, they will have been robbed 

 of the greater part of the soft constituents, so that as they 

 pass through the aluminium more and more soft radiation 

 will be excited until equilibrium is established. The 

 absorption coefficient should therefore increase to a constant 

 value. The numbers given in the tables tend to support this 

 conclusion. 



The question next arises as to whether, when this supposed 

 equilibrium has been established, the absorption coefficient 

 has the same value independent of the hardener. Leaving 

 out of consideration the rays from RaB, this obviously ought 

 to be the case if the rays from RaC are strictly homogeneous, 

 i. e. consist of rays of one wave-length only. If the rays, 

 for example, consist of two homogeneous beams of nearly 

 equal wave-length, as the experiments of Moseley f suggest, 

 the hardeners might be expected to show some selective 

 effect which would permanently alter the absorption 

 properties of the beam. The range of thickness used in 

 these experiments is not large enough to decide the question 

 definitely, although the results with Al appear to be in 

 favour of the latter hypothesis. 



In conclusion I desire to express my best thanks to 

 Professor Sir Ernest Rutherford, at whose suggestion these 

 experiments were carried out. 



too 



* Florance, loc. tit. 



t Moseley, Phil. Mag. xxvi.p. 1024 (1913). 



2S2 



