652 Dr. H. Geiger and Mr. J. M. Nuttall on Ranges of 



experimental results, and gives the ionization curve of the 

 a particles from actinium X, actinium C, actinium emanation, 

 and actinium A. Since in this case there is do substance 

 giving a particles of prominent range, it was more difficult 

 to obtain the ranges of the different products by subtraction. 

 The standard ionization curve was fitted on to the end of 

 Curve I. (of fig. 3), so that the maximum coincided with the 

 inflexion in Curve I. After subtracting the standard curve 

 three times in succession, the ionization Curves II., III., 1Y. 

 were obtained. Curve II. gives the ionization curve due to 

 actinium X, actinium C, and actinium emanation ; Curve III. 

 that due to actinium X and actinium C; and Curve IV. that of 

 actinium X. The Curve IV. finally remaining was slightly 

 higher than the standard curve, but this deviation was pro- 

 bably within the experimental error. Since the period of 

 actinium X is long compared with the periods of the suc- 

 ceeding products, no correction had to be made for the 

 deviation from true equilibrium. 



The ranges obtained from fig. 3 and given later in Table I. 

 are in satisfactory agreement with the measurements taken 

 previously by one of us * by the scintillation method, and 

 also with those obtained by Hahn f by Bragg's method. 



In our previous discussion of the ranges of the u particles 

 from the actinium products we have taken for the range of 

 the a particles from radioactinium the value obtained by 

 Hahn, which is 4*80 em., and slightly greater than that of 

 actinium X. This result appears to be an exception to the 

 relation we have given between range and period, since the 

 a particles from actinium X, which has the shorter life, 

 should have the longer range. 



To decide whether this was actually an exception to the 

 rule we have carried out experiments under exactly the 

 same conditions with a film of radioactinium. This was 

 prepared according to the method given by Hahn, and was 

 at the time of measurement practically free from all sub- 

 sequent products. The curve obtained indicated a value for 

 the range little different from that found by Hahn, and 

 showed that the a particles from radioactinium had actually 

 a range somewhat longer than those from actinium X. The 

 deviation from the range-period rule is perhaps only ap- 

 parent, since it seems quite possible that radioactinium 

 consists of two products, one of which emits /3 rays and has 

 the period of about lS^ days, and one which has a con- 

 siderably shorter life and emits the a rays. Some preliminary 



* H. Geiger, Pliil. Mag. xxi. p. 201 (1911). 

 t O. Hahn, Phil. Mag. xii. p. 244 (1900). 



