41 



rays which have passed through 2'8 cm. of lead are then 

 absorbed by different substances according to a density law 

 simply : they do not recognize atomic groupings. We may 

 safely assume that the first condition is then effectively rea- 

 lized also. We therefore placed a lead plug (1"61 cm. in 

 thickness) in the conical opening (see fig. 1), and hoped to 

 find that the emergence radiation, which we then proceeded 

 to measure, would depend only on the absorption of the ^ 

 rays. For according to our theory equal quantities of y rays 

 would be converted into /3 rays in equal weights of different 

 substances ; and the subsequent emergence of these 13 rays into 

 the ionization-chamber would be governed only by their abi- 

 lity to penetrate the layers intervening between their place 

 of origin and the chamber. It is, perhaps, important to ob- 

 serve that we are implicitly making another assumption, viz., 

 that the ^8 rays originating in different substances have the 

 same speed. On the material theory of the y rays the assump- 

 tion is natural ; it is justified by the general nature of our 

 results, and by special experiments to be described later. 



Since the absorption of jS rays by substances of small 

 atomic weight is m.uch less than in the case of the large atomic 



1600 



i|oo 



800 



1.61cm. Pb 



C AIS^O FeCuZn^O ^g IgSOxe 



Atomic Weight of Screen — * 



)0p^ 



Fig. 2. 



