814 The Recoil oj Radium Jj from Radium C. 



It is interesting to note that the line due to the recoiT 

 stream does not appear to be accurately midway between the 

 two a ray lines, which is remarkable if radium D carries a single 

 positive charge when it recoils from radium C ; but careful 

 consideration of the experimental conditions under which the 

 photograph was taken leads to the conclusion that a slight 

 dissymmetry such as is observed is to be expected. For when 

 the wire is exposed to the emanation, there is deposited on its 

 surface radium A, which subsequently disintegrates into 

 radium B and then into radium C. During the process of 

 disintegration of the radium A, which is accompanied by the 

 expulsion of a particles, the radium B is shot into the wire 

 by recoil, with the result that the radium G subsequently 

 formed is situated, not on the surface of the wire, but at 

 varying depths below its surface. The recoil stream of 

 radium D subsequently emerging from the wire is therefore 

 heterogeneous; for the particles of radium D must have 

 traversed different thicknesses of platinum and so escape with 

 all possible velocities. The densest part of the recoil line in 

 the photograph is therefore produced in the region corre- 

 sponding with the most probable velocity of emission of the 

 particles*. In support of this view it will be noticed that 

 the left-hand edge of the recoil line is much sharper than the 

 right-hand edge, indicating that there are no particles 

 travelling with velocities greater than that corresponding 

 with the left-hand edge of the line. On the right-hand 

 side the line gradually fades away, indicating the presence 

 of recoil particles of all possible velocities. If instead of 

 fixing attention on the regions of maximum density in the 

 lines, the positions of the left-hand edges of the three lines 

 are measured, it is found that the recoil line is much more 

 nearly midway between the two a ray lines. But since even 

 the left-hand edge of the recoil line is not perfectly sharp, 

 and since the displacement of the line from the centre is small, 

 it is not possible to make this measurement with very great 

 accuracy. 



From these considerations there seems to be no room for 

 doubt that the particles of radium D which leave the surface 

 of the wire traverse the magnetic field in a path the radius 

 of curvature of which is double that of the a rays from 

 radium C. Thus the radium D is projected with a single 

 positive charge, just as is the case when radium B recoils 

 from radium A. It should, however, be noticed that no 



* If this view of the displacement of the recoil line is correct, the 

 efficiency of recoil of radium D from radium C should he low, and 

 experiments are in progress to test this point. 



