﻿the Mass of the a, particles from Thorium. 375 



source of radiation. The following relative values were 

 observed : — 



4'= 1-09; y='91; $=1-20. 



«3 »3 «2 



Remembering that the rauges in air of the a rays from 

 thorium B, thorium C, and radium C are 5*0 cms.. 8*6 cms., 

 and 7'06 cms. respectively, and that the amount of deflexion 

 of a pencil of rays is inversely proportional to the velocity of 

 the rays, we should expect the following values of the 

 ratios : — 



<h_ i /7-06 + 1-25 - tK 

 J 3 -V 5-0+1-25 =1 ' 15 ' 



d 2 _ / 7-06 + l-2 5_ Qg 

 d,~V 8-6 + 1-25 "' ' 



h - , /?? 



rf, V 5-0 



+ 1-25 - OK - 

 = 1-255. 



+ 1-25 



Taking into consideration the difficulty of determining the 

 edge of the weak photographic impression due to the rays 

 from thorium B, the relative velocities observed experimentally 

 substantially agree with those deduced from the known ranges 

 of the a particles. The a rays from thorium C are projected 

 with a velocity about 9 per cent, greater than those from 

 radium C, while the a particles from thorium B are projected 

 with a velocity about 13 per cent, less than those from 

 radium C. 



Experiment 2. — The experiments were repeated with a 

 thickness of aluminium over the active wires equivalent in 

 stopping- power to about 2'5 cms. of air. A good photograph 

 was obtained which showed clearly the bands due to the rays 

 from thorium C. bounded on the edge by the weaker bands 

 due to the rays from thorium B. The effect of covering both 

 the thorium and radium wire with the screen is to bring out 

 more clearly the difference in velocities of the three types of 

 rays. The actual photographs obtained are reproduced in 

 fig. 2 A & b. (PL V.) Fig. 2 A shows the thorium bands. The 

 well-defined dark band due to the a rays from thorium C is 

 seen to be bounded on the outer edge by a hazy band due to 

 the a rays from thorium B. The original negative showed 

 the two sets of rays very clearly, but much of the definition 

 is lost in the reproduction, which is magnified about 1"4 

 times. It will be seen that the corresponding radium photo- 

 graph (fig. 2 b) shows two clear well-defined bands. 



