the Absorption of X-Rays. 185 



•degree may the impurities disadvantageous^ influence the 

 X-ray absorption of the lightest elements, a case which was 

 already pointed out in respect to carbon. 



III. Distribution of Electrons betioeen the inner and 

 outer region. 



In accordance with what has been shown in previous 

 •experiments, the atomic absorption coefficient, at least within 

 a not too wide wave-length range, may be expressed by the 

 formula 



Ka = Ca + k a \\ (4) 



where C a is a constant depending on the absorption due to 

 the diffusedly reflected radiation, k ' scattering," k a is a 

 constant exclusively depending on the chemical nature of 

 the substance, and b has very nearly the value 3. 



It we proceed from the theory that absorption is exclu- 

 sively determined by the number of electrons in the atom 

 and by the manner in which these are attached to the 

 nucleus, it seems highly probable that the electrons arranged 

 in the periphery of the atom, which in the following are 

 designated as outer electrons (p), must play quite another part 

 than the inner, more closely-attached electrons. When, in 

 the lines below, the expression inner electrons (c) is used it 

 does not designate those hypothetic electrons being a part of 

 the very nucleus, but only those arranged nearer to the 

 nucleus and the number of which corresponds to the atomic 

 number on deducting the number of the outer electrons. 

 These outer electrons *, on which the optical and chemical 

 qualities of the atom are generally supposed to depend, and 

 which, therefore, are usually designated as fk dispersions ,J or 

 respectively " valence-electrons," must be considered much 

 more elastically attached to the nucleus than the others. 

 Working with X-ray crystal analysis of different sub- 

 stances, also, Hull f makes the conclusion that certain 

 electrons constituting the atom are more closely attached to 

 the nucleus than others which show T a certain degree of 

 freedom. 



The theory brought forward by J. J. Thomson on the 

 scattering phenomenon is founded on the assumption that 

 the electrons are quite independent of each other when sub- 

 jected to the primary radiation. This assumption may be 

 thought to be fulfilled in respect to the electrons of the 

 surface part of the atom, but not in respect to the electrons 



* Oomp. Sommerfeld, Ann. d. Phi/s. B. 53, p. 513 (1917). 

 t Hull, Phys. Rev. x. ser. 2, p. 66:3 (1917). 



