Ranges of <x particles from Thorium and Actinium. 647 



calculation o£ the diffusion coefficient. Hahn and Sackur * 

 obtained the same value by following the change of activity 

 directly by means of an electrometer. Similarly the period 

 of transformation, 54 seconds or A, = 1'29 x 10~ 2 sec. -1 found 

 by Bronson f, was adopted for thorium emanation. 



My thanks are due to Professor Rutherford for his help 

 and interest in these experiments. 



Physical Laboratory, 

 The Victoria University, Manchester. 



LXIL Tlie Ranges of the a. particles from the Thorium and 

 Actinium Products. By H. Geiger, Ph.D., Lecturer in 

 Physics, and J. M. Nuttall, Al.Sc, University of Man- 

 chester X • 



IN two previous papers § we have shown that there exists 

 a quantitative relation between the range of an a particle 

 and the transformation constant of the product from which 

 it arises. For the products of the uranium-radium series 

 the values of the ranges of the a particles were partly taken 

 from papers by Bragg ||, and were in part redetermined. 

 As regards the members of the actinium and thorium series, 

 with a few exceptions, only the initial measurements of 

 HahnU were available. 



A determination of the ranges of the a particles from most 

 of the products of these families is difficult, since, owing to 

 the short periods of some of them, it is impossible to separate 

 the products from each other. Thus the actinium emanation 

 and actinium A will always be associated with actinium X, 

 and therefore complicate a determination of the range of the 

 on particles from the latter substance. Similar difficulties 

 arise in the thorium series. 



In the following we have undertaken the redetermination 

 of the ranges of the a particles emitted by the products of 

 the thorium and actinium series, making use of the experi- 

 mental arrangement we have previously employed for the 

 determination of the ranges of the a particles from uranium. 



* Hahn and Sackur, Ber. deutsch. diem. Ges. 38. ii. p. 1943 (1905). 



t Bronson, Amer. Journ. of Sc. xix. p. 185 (1905). 



% Communicated by Prof. E. Rutherford, F.R.S. 



§ H. Geiger and J. M. Nuttall, Phil. Mag. xxii. p. 613 (1911) ; xxiii. 

 p, 439 (1912). 



|| W. H Bragg and E. D. Kleeman, Phil. Mag. viii. p. 726 (1904) ; 

 x. p. 318 (1905). 



f] 0. Hahn, Phil. Mag. xii. p. 244 (1906). 



2 U 2 



