RECENT ADVANCES IN SCIENCE 103 



substance containing only light elements, the radiation pro- 

 ceeding from this substance is almost purely a scattered radia- 

 tion, and no characteristic radiation is excited ; further, the 

 scattering particles are not the ions or atoms, but the constituent 

 electrons, and the intensity of the scattered radiation is pro- 

 portional to the quantity and independent of the quality of the 

 matter traversed by the primary radiation. The experimental 

 results led to the conclusion that for the light elements, say 

 from hydrogen to sulphur, the number of electrons per atom 

 is almost exactly half the atomic weight. When elements of 

 higher atomic weight were subjected to Rontgen radiation this 

 conclusion appeared to be invalidated as far as they were 

 concerned ; for copper was found to scatter about twice as 

 much, and silver about six times as much, as an equal mass 

 of one of the light elements. But it must be borne in mind 

 that the scattering can only be taken as proportional to the 

 number of scattering electrons when these act independently 

 of one another, and this condition is approximately satisfied 

 for light atoms traversed by Rontgen radiation of any wave- 

 length within wide limits. In the heavier atoms the electrons 

 are more closely packed, and are held by stronger retaining 

 forces, and this independence of action is no longer possible 

 in general. However, such independence will be more closely 

 approached the shorter the wave-length of the primary 

 radiation. With such a condition it might be expected that 

 the intensity of the scattered radiation would give a similar 

 indication of the number of electrons in an atom as that ob- 

 tained for the light atoms. This expectation has been put to 

 the test by Prof. Barkla and Miss Dunlop in the paper referred 

 to, and the results justify the anticipation. They show that 

 when the wave-length of the primary radiation is reduced to 

 the order -3 x io~ 8 cm., there is little deviation even among 

 the heavier elements from the law of proportionality of number 

 of electrons per atom and atomic weight ; further, there are 

 indications that with waves still shorter, the mass scattering 

 coefficients would 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. 



