﻿370 The a 'particles expelled from Radium and Actinium. 



emission of energy is mechanically equivalent to l'16xl0 y 



ergs per second. Since the average energy of the expelled 



a particle is 3*11 x 10 u e, the number of a particles expelled 



per second from one gram of radium in equilibrium is 



3*65 



-—y- . The number previously found by the writer by 



measuring the total charge carried by the a particles was 



2*82 



■TTzz- » *• e - 77 per cent, of the theoretical number. The 



We 



agreement between theory and experiment is thus fairly 

 good. In the above estimate, it is assumed that the heating 

 effect is due entirely to the kinetic energy of the expelled 

 a particles. It is known experimentally that the heating 

 effect of the ft and 7 rays is only a small percentage of that 

 due to the a. rays. The expulsion of an a particle from an 

 atom should lead to the recoil of the residue of the atom. 

 Assuming that the momentum of the atom is equal and 

 opposite to that of the a particle, the velocity of recoil of the 

 atom can be simply calculated. Taking the mass of the a par- 

 ticle as 4 and of the radium atom as 225, the velocity of 

 recoil of the disintegrated radium atom, for example, is 

 1/55 x 1*56 x 10 9 or 2*8 X 10 7 cms. per second. The heating- 

 effect resulting from this recoil is thus only about 2 per cent, 

 of that due to the a particle. 



Assuming that each u particle carries a single ionic charge 

 of 1*13 x 10 -20 electromagnetic units, the number of a par- 

 ticles which must be expelled per second from one gram of 

 radium in order to account entirely for the heating effect is 

 32 x 10 11 . The experimental number is 2'5xlO u . If it is 

 assumed that the a particle carries twice the usual ionic charge, 

 each of these numbers is reduced by one half. 



It is of interest to calculate the distribution of the heating 

 effect of radium in equilibrium amongst the various a ray 

 products. The theoretical percentages of the total heating- 

 effect are given in column 1. These are calculated from 

 the known energy of the a particles expelled from each 

 product. The observed percentages are deduced from the 

 experimental numbers and curves given by Rutherford and 

 Barnes (Phil. Mag. Feb. 1904). 



Product. 

 Radium 



Calculated 

 heating effect, 



19-2 



" Observed 

 lieating effect. 



23 



Emanation . . . 

 Radium A . . . 

 Radium C ... 



... 22-71 



24-5 j 4 ' J 

 33-6 



45 

 32 



