Secondary Homogeneous X Radiation. 901 



Since performing this experiment a paper has been 

 published by Sadler * showing that with silver there is a 

 corpuscular radiation in the absence o£ the characteristic 

 secondary X radiation which has been observed. This also 

 shows that electrons ejected with speeds approaching the 

 velocity of emissions of the electrons which accompany the 

 characteristic homogeneous silver radiation when just excited, 

 cannot produce homogeneous secondary radiation by collision 

 with an atom. 



The above considerations point to the conclusion that the 

 atom from which the electron is ejected is the seat of pro- 

 duction of the radiation. As the result of the loss of an 

 electron the atomic system will have to adjust itself, and in 

 so doing the electron or an electron may vibrate with an 

 emission of the homogeneous secondary radiation. It seems 

 probable, therefore, that, as has been pointed out, an electron 

 is made to vibrate by the passage of a Rontgen ray pulse 

 over it, with the consequent emission of secondary X rays in 

 somewhat the same way that certain fluorescent bodies give 

 out radiations when suitable light is incident upon them. 

 The agreement between the law found by Barkla and Sadler 

 in the case of secondary radiation and Stokes' law of fluor- 

 escence, and the fact, discovered by Stark f, that fluorescence 

 is accompanied by a corpuscular radiation, both strongly 

 support this theory. The possibility of an appreciable 

 phosphorescent effect, that is, a persistence of secondary 

 radiation after the removal of the radiating atoms from the 

 exciting beam, as observed in the case of ordinary light by 

 Becquerel, led to the following experiment. 



An Attempt to Detect a Possible Persistence of Secondary 

 X Radiation. 



A brass cylinder (5*5 cm. in diameter and 6 cm. high) 

 mounted on ball bearings was rotated rapidly about a vertical 

 axis (0 in figure). An X ray bulb inside a lead box was placed 

 as shown in the diagram so that the rays fell directly on the 

 cylinder. Lead screens, S 1? S 2 , S 3 , were adjusted so that with 

 the X ray bulb working and the cylinder at rest no leak of 

 the electroscope, as shown by a gold leaf, was observed. 



Between Sj and S 3 a particular part of the cylinder was 

 subject to the X rays and gave off the homogeneous X radia- 

 tion. After passing S 2 and S 3 this part was no longer subject 

 to the rays but came into a position in which it could affect 



* Phil. Mag-. March 1910. | Pht/s, Zeif. ix. pp. 481-495, 1908. 



