Homogeneous Corpuscular Radiation. 349 



The very easily absorbed corpuscular radiation excited by 

 the softer homogeneous beams is almost completely absorbed 

 in the millimetre of air nearest the radiating surface. It 

 was extremely difficult to work with a much smaller distance 

 between the surfaces than 1 mm., for it was impossible to 

 altogether avoid a direct leak from the aluminium window 

 to the insulated plate owing to 1he presence of dust particles. 

 Moreover, the leakage was very irregular, and so could not 

 be allowed for. Under these circumstances, the device was 

 adopted of using a mixture of air and hydrogen as the ab- 

 sorbing medium. With this mixture, readings for the 

 absorption of the soft corpuscular radiation could be taken 

 over a range of about 3 mm., since hydrogen proved a much 

 less efficient absorber than air. Control experiments in which 

 harder secondary beams exciting more penetrating corpuscular 

 radiation were used made it possible to deduce the absorption 

 coefficients of the softer corpuscular radiation in air at atmo- 

 spheric pressure. 



A striking relationship was found to exist between the 

 absorption coefficients in air of the corpuscular radiation 

 excited by the several homogeneous secondary beams in any 

 one substance used as tertiary radiator. 



If the absorption coefficient of the corpuscular radiation 

 excited by each homogeneous secondary beam be plotted as 

 ordinate against the atomic weight of the metal used as a 

 source of the secondary exciting beam as abscissa, it is found 

 that over a wide range the points so obtained lie upon a 

 straight line. This is shown in fig. 8, where iron is used as 

 a tertiary radiator. It will be seen that for the more pene- 

 trating secondary beams there is a departure from this 

 relationship. It has been shown *, however, that from sub- 

 stances whose atomic weights are greater than that of silver, 

 in addition to the emission of a very penetrating homogeneous 

 beam, a softer type of Rontgen rays are produced. From a 

 substance like lead (207) this softer type appears to pre- 

 dominate. In the present experiments the exciting radia- 

 tions from silver and tin were first passed through aluminium 

 screens to sift out the soft radiation. 



Connexion between the Corpuscular and the Exciting 

 Rontgen Radiations. 



But turning now to the main object of the present inves- 

 tigation it is important to learn in what manner the intensity 

 of the corpuscular radiation depends upon the metal emitting 



* Barlda, Phil. Mag. Jan. 1906, pp. 812-823: Barkla and Sadler, Phil. 

 Mag. Oct. 1908, pp. 550-584. 



