Recoil of Radium C from Radium B. 109 



pressure, which may explain the smallness of the effect 

 obtained by them. The lack of constancy of the amount of 

 radium C emitted from the surface upon which radium B 

 had been deposited is remarkable, seeing that the plates used 

 were always treated in an exactly similar manner. But it 

 was perhaps even more surprising to find that, working with 

 a single surface, the power of emitting radium varied 

 considerably if the surface was allowed to stand untouched. 

 After making due allowance for the decay of radium B, it 

 was found that an active surface after standing for half an 

 hour or an hour, altered its power by emitting radium C, 

 sometimes becoming a more powerful and on other occasions 

 a less powerful radiator. The cause of these changes in the 

 surface is very obscure, and it is difficult in the present state 

 of our knowledge to advance any satisfactory explanation 

 of it. 



Returning to a consideration of the small amount of 

 radium C which was in all cases emitted from the active 

 surfaces, it seems not unreasonable to imagine that, on account 

 of the smallness of the energy of recoil of the radium C 

 particles, it is only those particles which come off normally or 

 in directions making small angles with the normal to the plate, 

 that succeed in getting away from the plate beyond the range 

 of molecular attraction. If this were so, all particles emitted 

 from the plate in directions making large angles with the 

 normal to the surface of the plate would be drawn back, and 

 therefore not be detected as radiant matter. 



To test the correctness of this view, experiments were 

 undertaken to study the variation with distance of the amount 

 of radium C received by a disk exposed in vacuo at different 

 distances from the radiating plate. The apparatus used was 

 that shown in fig. 1. The results obtained are shown in fig. 5 

 (p. 110). On the same diagram are also plotted the curve 

 showing the variation of activity with distance assuming that 

 particles emitted in all directions from the plate are equally 

 likely to escape from the surface, and also a curve assuming a 

 cosine law for the radiation such as could be applicable to the 

 case of light *. It will be seen that the experimental curve 

 agrees with neither of these hypotheses, indicating that the 

 radiation from the plate falls off as the angle which it makes 

 with the normal to the plate increases. The deviation of the 

 experimental curve from that obtained on the supposition that 



* These curves have beeii plotted from values as yet unpublished 

 obtained by Mr. H. Bateman. 



