Homogeneous Corpuscular Radiation. 339 



advantages to be gained by the substitution of these homo- 

 geneous secondary beams for heterogeneous primary beams. 



In a course of some previous experiments *, in which these 

 homogeneous radiations were used to excite tertiary radia- 

 tions in metals belonging to the chromium-tin group, the 

 emission of the tertiary Rontgen radiation was found to be 

 governed by the following laws : — 



(1) With a given substance as radiator, its characteristic 



radiation is only excited by those secondary beams 

 which are more penetrating than the tertiary radia- 

 tion characteristic of the substance. 



(2) When the secondary beam is only just more penetrating 



than the tertiary, the intensity of the latter is small, 

 but, as the secondary beam becomes more penetrating, 

 a very rapid increase in the intensity of the tertiary 

 radiation to a maximum takes place. 



(3) As the secondary beam becomes more penetrating still 



the intensity of the tertiary radiation decays as a 

 linear function of the ionization produced in a given 

 volume of air by the secondary beam. 



(4) When these secondary beams are absorbed by thin 



sheets of metals from the group chromium-tin, a 

 large increase in the absorption takes place when 

 the secondary beams become more penetrating than 

 the radiation characteristic of the absorber, this 

 increase in the absorption being intimately connected 

 with the emission of tertiary radiation by the absorber. 

 The fraction of this increase in the absorption of the 

 energy of the secondary beam which is re-emitted 

 as tertiary radiation is not constant, but decreases as 

 the secondary beam becomes more penetrating, slowly 

 at first and then more rapidly when a relatively 

 penetrating secondary beam is used. 

 The results obtained with these homogeneous beams ap- 

 peared to be sufficiently regular to warrant an attempt being 

 made to find in what manner the emission of corpuscular 

 radiation by a substance depended upon the exciting radia- 

 tion. It was thought probable that when the intensity of 

 the tertiary Rontgen radiation decreased as the secondary 

 beam became more penetrating, an increasing fraction of the 

 energy of the secondary beam absorbed by the tertiary 

 radiator would be expended in corpuscular radiation. ■ 



To investigate this point was the main object of the present 

 research. 



* Sadler, Phil. Mag. Julv 1909. 

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