Scattering of X-rays and Atomic Structure. 231 



by the electric field in the primary wave for only a very 

 short interval of time; its actual displacement is small and 

 the forces called into play within the atom are small ; it 

 emits energy at a rate proportional to that at which primary 

 radiation energy passes over it*. Even in the heavier atoms 

 in which the electrons are more closely packed, and in which 

 they are held by stronger restraining forces as shown by 

 their higher natural frequency of vibration, it appears that this 

 independence in scattering action is maintained provided 

 the primary waves are sufficiently short. Only under such 

 conditions is the scattering coefficient in a given substance 

 independent of the wave-length, and only then does the 

 intensity of the scattered radiation give a direct indication 

 of the relative numbers of electrons in the atom. It appears 

 from the scattering of very short waves that there is little 

 deviation even among the heavier elements from the law 

 of approximate proportionality of the number of electrons 

 per atom and the atomic weight. There is indication that 

 with waves still shorter than those employed, the mass 

 scattering coefficients would have become practically constant 

 for all elements, showing the number of electrons per atom 

 in the heavier elements to be of the order of half the atomic 

 weight, — as in the case of the lighter elements. 



The independent action of the electrons might be expected 

 to disappear as the wave-length of the radiation increased. 

 Interference with this independent scattering action might 

 be expected to take place in the heavier atoms for shorter 

 waves than in the case of the lighter atoms. In such a case 

 the scattering would approximate to a scattering by groups 

 of electrons rather than by the individual electrons. If we 

 considered the extreme case in which n electrons might be 

 considered as a scattering particle of charge ne, we should 

 expect scattered radiation n times as intense as that from n 

 independently scattering electrons f, the rate of radiation 



O 2 -/ 2 



from a charged particle e being given by ,r ' , and the 



acceleration /being identical in the two cases. Though the 

 actual phenomenon may only approximate to this it seems 

 probable that the variation in size (including charge and 

 mass) of what may be called the scattering unit is sufficient 

 to account qualitatively for the above experimental results. 



* This, of course, does not apply to the few electrons actually ejected 

 from atoms. 



f Assuming of course the joint mass of the particle to be n times 

 the mass of a single electron, a condition almost certainly holding with 

 the distribution of electrons in the atom. 



