Velocity of Electrons expelled by X-rays, 749 



emission caused by X-rays. The maximum velocity of 

 emergence from a substance is dependent simply on the 

 frequency of the exciting X-radiation. 



The experiments of Sadler * showed that the emission of 

 a characteristic X-radiation by a substance under exposure 

 to a primary X-radiation is accompanied by an emission of 

 a corpuscular (electronic) radiation from that substance. 

 One of us has indicated f that not only is this so, but that a 

 well defined corpuscular radiation is definitely associated 

 with each characteristic X-radiation — that is, with the cha- 

 racteristic X-radiation of each series. 



The point needs further explanation. It was first shown 

 by one of the writers \ that each element has its own 

 characteristic line spectrum in X-rays. A characteristic 

 radiation may be excited in any substance by the incidence 

 on that substance of X-radiation of shorter wave-length 

 only. 



These homogeneous X-radiations have been called the 

 K, L, M, etc. radiations, each letter corresponding to a 

 spectral line — or rather, as shown later by interference ex- 

 periments §, a strong line with several fainter lines about it 

 — in the X-ray spectrum. When a primary X-radiation is 



transmitted through a substance and excites this fluorescent 



... 

 radiation, the primary radiation absorbed may be definitely 



analysed as (a) that scattered (usually though by no means 



universally small), (b) that absorbed in association with the 



emission of K radiation, (c) that absorbed in association with 



the emission of L radiation, and so on. 



There is thus what we shall call " K absorption w of the 

 primary beam, meaning the absorption which is clearly con- 

 nected with the emission of the K fluorescent characteristic 

 X-radiation, though it is not entirely re-emitted as K radia- 

 tion ; L absorption similarly associated with the emission of 

 L radiation, and so on. 



In the same way the ionization produced in a given 

 substance under the action of X-rays may be divided into 

 several distinct portions, each portion associated with || the 



* Phil. Mag. March 1910. 



t Barkla, 'Nature,' Teh. 18 and March 4, 1915. 



X Barkla, " The Spectra of Fluorescent Rdntgen Radiations " (Phil. 

 Mag-. Sept. 1911). This paper gave a summary of the results of experi- 

 ments made by the writer with Sadler, Xicol, and others. 



§ W. H. & W. L. Bragg-, Proc. Rov. Soc. A. lxxxviii, 1913 ; Moseley 

 & Darwin, Phil. Mag. Jan. 1913, and later papers. 



i| Distinct from that produced by the characteristic X-radiation itself 

 when this is absorbed. 



