628 SCIENCE PROGRESS 



the unmodified theory of electromagnetic radiation when 

 applied to the emission of scattered X-radiation shows that 

 atoms contain electrons in numbers equal to half their respective 

 atomic weights, a view which has been confirmed by other 

 lines of research and is generally accepted. Dr. Barkla pointed 

 out that this result admits of no possible explanation other than 

 the assumption that every electron in the radiating matter is 

 disturbed by every wave of the prima^ radiation ; that the 

 emission of scattered radiation is a continuous process, not 

 dependent on any critical condition of the atom ; that it may 

 take place in any quantity, and that not only is there no sug- 

 gestion of a quantum or entity of radiation, but the phenomena 

 become meaningless on any such theory. Further recent 

 work has demonstrated that scattering takes place by groups of 

 electrons rather than by individual electrons, when the wave 

 length is comparable to the size of a group. 



In contrast with the behaviour of the scattered radiation, 

 the characteristic or fluorescent X-radiation displays a definite 

 quality, an absence of polarisation effects, a uniformity of 

 distribution, and a variation of intensit}' with the wave length 

 of the exciting primary radiation which proves that this process 

 of radiation is absolutely uncontrolled by the primary rays. 

 It arises only indirectly from the passage of the primary beam 

 and is the accompaniment of an exceptional disturbance of a 

 radiating atom, which is in fact the ejection of a high-speed 

 electron. As the character of the radiation, however, and its 

 intensity are independent of the speed of ejection of the electron 

 or its subsequent career, it appears that the radiation does not 

 originate outside the atom thus bereft of an electron, nor does 

 it originate in the electron at all. The origin is to be found in 

 an atom from which an electron has been expelled. When 

 measurements are carried out on the absorption of energy from 

 the primary beam and the emission of energy in the form of 

 characteristic radiation (the energy of scattered radiation being 

 allowed for) and in the form of corpuscular or electronic radia- 

 tion, results of considerable interest for the quantum theory of 

 radiation are discovered. The various qualities of characteristic 

 X-radiation from one and the same substance (which can be 

 excited, as is well known, by primary rays of suitable hardness) 

 are as usual denoted by the terms K fluorescent radiation, L 

 fluorescent radiation, etc. (the K being the hardest). An 



