Transformations in the Active Deposit of Actinium. 693 



taken of the fact that only 65 per cent, of the a particles of 

 thorium C have a range 8'G cm. 



Fig. % 



5C 



15 



0-10 



005 



ff/IDIUM C. 



ThoriumC. 



6-5 



7-0 7-5 8-0 



Centimetres of Rance. 



8-5 



9-0 



It will be seen that the curve for actinium C differs from 

 the others, and indicates that about 0*15 per cent, of the 

 particles can penetrate much further than the ordinary 

 a particles of range 5*4 cm. 



The effect is very small and exact measurement is difficult. 

 With the source used the maximum number of scintillations 

 per minute due to the long-range a particles was seldom 

 more than 20 per minute even when corrected to the initial 

 time, i. e. for the maximum activity of the source. Conse- 

 quently not more than about 40 scintillations could in general 

 be counted for any one point on the curve, and this brings 

 in very considerable probability variations. However, the 

 number of a particles corresponding to the flat part of the 

 curve between 5*5 and 5*9 cm. range only varied in ten 

 experiments betweeu O'lO and 0'20 per cent., which is well 

 within the experimental error and the error due to probability. 

 The mean ratio found was 0*15 per cent., and the range of the 

 new ol particles or the range at which scintillations cease to 

 be produced was about 6*4 cm. 



Various experiments were tried to make sure that the long- 

 range a, particles were not due to radium C or thorium C 

 which might come from a small quantity of radium or radio- 

 thorium in the source. In the case of radium C it is known 

 that its decay for a long exposure is not much different from 

 that of actinium. The absence of radium G in the source 

 was, however, proved by the fact that the ratio of long-range 

 a particles was not altered when the plate A was exposed to 



