738 Dr. Tycho E:son Auren on Scattering and 



be expressed by the formula 



K a =k\ b + <Ja, (2) 



where b is a constant, which has closely the same value for 

 all elements ; k a and <r a are constants, which are characteristic 

 for the element in question. The latter term (o- a ) due to the 

 scattering effect is indicated as the atomic scattering co- 

 efficient. The atomic absorption coefficient of a certain 

 element (a x ) in relation to another element (a 2 ) will then be 

 expressed thus : 



*«l/«2— L T,6 I ^ W 



Ka.^A, -f- CTa 2 



At a decreasing wave-length, the term h a \ h will become 

 still smaller and smaller in relation to a a , and, in the lightest 

 elements at very short wave-lengths, will be of no significance 

 as compared with the second term of equation (2). The limit 

 °f K ana 2 a t a decreasing wave-length must therefore be : 



«WA= ^ (*) 



From the experiments, we see also that /c a/ H 2 o for H, 0, ~N, 

 and at a decreasing wave-length seems to approach fixed 

 limits. These limits (see column VI., Table III.) stand very 

 nearly to each other in the ratio of the numbers 1:6:7:8, 

 and these numbers are the same as the atomic numbers of the 

 respective elements. This issue is in good agreement with 

 the theory advocated by J. J. Thomson, according to which 

 the scattering effect is proportional to the number of electrons 

 included in the atom. This gives a support to the current 

 notion of the structure of atoms, which is that the number of 

 electrons corresponds to the atomic number. The supposition 

 I have earlier tentatively suggested that the scattering effect 

 is produced solely by the outer electrons cannot, therefore, 

 be maintained, at least not for the range of wave-lengths made 

 use of here. It then follows that the mass scattering 

 coefficient, except in some of the lightest elements, must be 

 notably higher than I previously thought to be the case. In 

 accordance with what Barkla and Dunlop * have shown, we 

 see that the relation between the mass scattering coefficients 

 for Cu and Al at short wave-lengths is fairly equal to one, 

 whence it follows that the relation between the scattering 

 coefficients of these elements also becomes very nearly the 



* Barkla and Dunlop, Phil, iftag. xxxi. p. 229 (1916). 



