Thin Films of Elements exposed to Route/ en Rays. 131 



(5) Sadler's remark that the emission of corpuscular 

 radiation is not necessarily associated - with the 

 emission of tertiary X-radiation. This statement 

 will be examined in more detail later. 



There are many points of interest in fig. 7 (PL III.). 

 First, that ordered curves are obtained for the screens used. 

 Again, if the degree of scattering is the same for all screens, 

 -and the phenomenon does not depend upon the actual number 

 of /3-particles involved, then, of the screens used, the silver 

 atom emits the greatest proportion of high-speed electrons 

 when excited by silver X-radiation, and similarly for the 

 barium atom when excited by barium X-radiation. 



Consider tor the moment the effect of silver X-rays on 

 the screens successively. There are two important effects 

 that have to be distinguished. The first is that the K, L, 

 M, etc. radiations are excited in those elements below, and 

 only L, M, etc. radiations are excited in those elements 

 above the atomic weight of silver. The second effect is 

 much more obscure. We do not know the relative numbers 

 of atoms radiating each of the spectral lines. With regard 

 to the latter effect, it appears in photographic images of the 

 X-ray spectra and in ionization work generally that the 

 lines of longer wave-length are the more intense. This can 

 be understood, for the waves of shorter wave-length would 

 soon pass over the atom, and, reacting on an outer ring, be 

 transformed partly into longer waves, and part of the energy 

 would go into the new corpuscles produced. The complete 

 interpretation of the curves in fig. 7 (PI. III.) would 

 necessitate a knowledge of the relative intensity of the 

 various X-ray spectral lines from a thin film of the element. 



There is still another point of very great interest in fig. 7. 

 In 1912, J. C. Chapman, working on homogeneous X-rays, 

 arrived at the formula *" 



W K =l(W L -48), 



where W L is the atomic weight of an element emitting L 

 radiation, W K that of an element emitting K radiation of 

 the same absorbability in aluminium. 



This formula yields the result that the K frequency from 

 ai senic equals the L frequency from gold. Moseley expressed 

 practically the same result in a different form f. He gave 



* Troc. Roy. Soe. A. vol. lxxxvi. p. 447 (1912;, 

 t Phil. Mag. ser. 6. vol. xxvii. p. 712 (1914). 



K2 



