a Particles ivitli Light Ate 



54£ 



aluminium foil were interposed between the silver plate and 

 the screen, and the scintillations counted. 



The results obtained are shown in fig. 4, curve A, where 

 the ordinates represent number on an arbitrary scale and the 



Fig. 4. 







A. obsei 



-ved 











b 



! 



V, 



















\<S> 













-o 2 





















13 16 19 22 25 



Range of H atoms in cms. of air 



28 



abscissae the thickness of absorbing material measured in* 

 terms of cms. of air for a particles. The equivalent absorption 

 in the silver plate and in the hydrogen is included. The 

 latter was taken as equivalent to 8 mm. of air or one quarter 

 the length of the path of the a particles in hydrogen. The 

 correction due to the natural scintillations from the source 

 and silver plate was small. 



It is seen that there is no diminution in the number of 

 scintillations for absorptions between 9 and 19 cm. of air*. 

 After 19 cm., there was a slow decrease followed by a rapid 

 fall near the end of the curve. No scintillations were observed 

 beyond 28 cm., i. e. for a range four times that of the a par- 

 ticles from radium C. 



The shape of the absorption curve is entirely different 

 from that to be expected theoretically. The latter is shown 

 in curve B, calculated from the data given in § 7, the same 

 ordinate being taken for an absorption of 10 cm. Between 

 9 and 19 cm. absorption, the number of scintillations 

 according to theory should fall from 100 to 28. 



* It should be remarked that, for the distances employed, the width 

 of the testing vessel (fig-. 1) was sufficient to give the correct average 

 distributions of H atoms with velocity, corresponding to a source at the 

 centre of a sphere. 



