the Radiations from Radioactive Substances. 5 



has shown that the absorption of cathode-rays in substances 

 depends only on the density of the material through which 

 they pass, and is approximately independent of its chemical 

 constitution. On account of the constancy of the uranium 

 rays, it is possible to determine their absorption in different 

 media with accuracy. 



A few experiments were consequently made to see how 

 closely the absorption varied with the density for the high- 

 velocity particles emitted by uranium. 



The experimental arrangement is shown in fig. 2, where 

 the dotted lines represent insulators. 



Fig. 2. 



£/TRTfl 





300 V 



CU. Oiude 



IF--1-] 



ZflVIl, 



A thick layer of uranium was uniformly spread over a 

 shallow rectangular groove 6 cms. square in lower plate A. 

 The plate A was charged to 300 volts by a battery of small 

 accumulators, the other pole of which was to earth. The 

 current was observed between the plates A and B by means 

 of the sensitive Dolezalek electrometer previously described, 

 with, if necessary, a suitable capacity in parallel. 



In order to completely absorb the a radiation an aluminium 

 plate *003 cm. in thickness was fastened tightly over the 

 layer of uranium. The P.D. of 300 volts between A and B 

 (6 cms. apart) was sufficient to carry over all the ions to the 

 electrodes before appreciable recombination occurred. 



The rate of movement of the electrometer-needle was 

 observed, for different layers of material of uniform thickness 

 successively placed over the uranium. 



If \ is the coefficient of absorption of the radiation in a 

 material, the intensity I of the radiation after passing through 

 a thickness cl is given by 



I=V-^, 



where I is the intensity of the radiation at the surface before 



