Transformations of Actinium C. 821 



effect of these particles, it is necessary to subtract from the 

 observed ionization curve a curve similar to the curves for 

 radium .and thorium, but relating to a homogeneous source 

 of range 5*4 cm. 



It will be noticed, however, that the curves for radium 

 C and thorium C differ in that they do not show the same 

 residual ionization at points just beyond, and equidistant from, 

 the end of the range. This residual ionization is presumably 

 due to 7 radiation and indicates a different relative intensity 

 of 7 to ol radiation in the two cases. The exact ionization 

 due to 7 rays in the case of actinium active deposit can only 

 be conjectured, and may as an approximation be taken as the 

 mean of that for radium and thorium. Thus the curve for 

 the a. rays of range 5*4 cm. may be taken as shown in the 

 dotted curve A 2 . The difference curve A 2 gives an estimate 

 of ionization effect of the a rays particles of longer range. 

 The maximum of this curve is at about 5*6 cm., and as it was 

 found that with the apparatus used the maximum of like 

 ionization curves was about 0'9 cm. from the end of the 

 range, we may take the end of the range of the a particles 

 as 5'6-f09 = 6 , 5 cm. This is near to the value 6*4 cm. 

 found in the earlier experiments. The fraction of long range 

 a particles is given by the ordinate of the maximum of the 

 curve A 2 and is seen to be 0'29 per cent. This differs from 

 the value 0*15 per cent, given by the scintillation experiments. 

 However, considering the nature of the deduction of the 

 results, close agreement is not to be expected. The true 

 value is probably not far from 0*20 per cent, or 1 part in 500. 

 That the long range « particles were not due to impurity 

 was verified by observation of the variation with time of 

 the ionization at a distance 5*6 cm. The ionization de- 

 creased in intensity according to a period of practically 

 3tf minutes, in agreement with the known decay of the active 

 deposit of actinium. 



Summary, 



The existence of a branch product in actinium 0, in- 

 volving an emission of a particles of range 6*45 cm. by 

 0*20 per cent, of the atoms is verified. 



We are deeply indebted to Sir E. Rutherford for the loan 

 of the valuable actinium source used, and for his kind interest 

 in the experiments. 



