﻿a. Particle from Radium in passing through Master. 143 



makes the observed value of the velocity of: the a particles of 

 radium A '88V , which is in good agreement with the value 

 *86V , previously deduced from its known range in air. 



Another photograph was taken in which the active wire 

 was covered with a layer of mica equivalent in stopping 

 power to 3*5 cms. of air. The measurements in this case were 

 complicated by the fact that the rays from radium C on the 

 wire produced also a comparable photographic effect. Two 

 broal bands were consequently obtained, the outer parts of 

 which were due to the rays from radium A. Allowing for 

 the known width of the trace of the undeflected pencil, the 

 velocity of the a particles from radium A after passing through 

 3*5 cms. of air was found to be '54V . The value deduced 

 from measurement of the range of the rays in air, viz. 

 1*3 cm., is *56V . Considering the difficulty of measuring 

 such weak photographs, the observed and calculated values 

 are in as good agreement as could be expected. 



Radium F. 



It has not been thought necessary to determine directly the 

 magnetic deflexion of the a. rays from radium F (polonium), 

 for this has already been done with accuracy by Mackenzie 

 (loc. cit). Using a rod covered with a thin film of polonium, 

 he found the value of Up for the polonium rays to be 

 3*30 x 10"'. The value (see previous table) deduced from 

 the range and known value of Hjo for radium C is 3"2Q x 10% 

 which is in substantial agreement with the value determined 

 by Mackenzie. 



I have not so far separately determined the magnetic 

 deflexion of the ravs from the emanation. I think, however, 

 that there can be little doubt that the velocity *8lY , deduced 

 from the known range of the rays, is close to the true value. 

 Experiments are in progress to obtain if possible on a single 

 photographic plate the separate traces of the rays from each 

 of the four a, ray products of radium in equilibrium. This 

 should be theoretically possible using a very thin film of 

 radium in equilibrium as a source of rays. It is doubtful, 

 however, whether it will be practically possible to obtain a 

 complete separation of the traces due to the ravs from the 

 emanation and radium A, on account of their velocities of 

 projection being u early the same. 



Scattering of the a. Rays. 



When Bragg first put forward his theory of the absorption 

 of the a rays, which has received such a remarkable measure 



