£18 Mr. R. W. Varder and Dr. E. Marsden on the 



and Darwin * for the relative penetrating power of the 

 J3 radiations from Th and Th D, and knowing the equi- 

 valent thickness of the bottom of the electroscope, the above 

 ratio is reduced to 1*6 : 1. The mean of a considerable number 

 of such experiments was 1*5. This value for the ratio of the 

 activities of Th and Th D is in fairly good agreement 

 with that obtained by Marsden and Darwin f , and supports 

 the scheme of disintegration of the thorium series as sug- 

 gested by these authors. 



My warmest thanks are due to Prof. Wilberforce for his 

 kind interest in these experiments, and for placing the 

 facilities of his laboratory at my disposal. 

 Holt Physics Laboratory, 

 The University, Liverpool. 

 June 1914. 



LXXXIX. The Transformations of Actinium C. 

 By R. W. Varder, M.A., and E. Marsden, D.Sc.t 



1) ECENT experiments by Wilson, Perkins §, and one of 

 \j us have suggested that the atoms of actinium C have 

 two possible modes of transformation. The evidence for 

 this conclusion was based on the observation that the a 

 particles from the active deposit of actinium have not all the 

 same range, a small fraction about 0*15 per cent, penetrating 

 1 cm. farther than the main portion. 



By analogy with the transformation in the active deposits of 

 thorium and radium the following scheme was put forward: 



fly 



JL(S'7) ^(6-5) */ 



Jcz£m. — > Mr. /I > /Ict.B — 



3-3 sees, -OOB sea. 36-/mms, 



•005 sec^) 



The experiments referred to above practically amounted 

 to counting the a particles at different distances in air from 

 a source of actinium active deposit. The results showed that 

 although the majority of particles are stopped 5*4 cm. from 

 the source, a few travel to a distance of about 6*4 cm. 



The chief uncertainty of the experiments arises from the 



* Marsden and Darwin, Proc. Roy. Soc. A. Ixxxvi. (1912). 

 t Loc. cit. 



% Communicated by the Authors. 



§ Marsden and Wilson, Nature, xcii. p. 29 (1913) ; Marsden and 

 Perkins, Phil. Mag-, xxvii. p. 692 (1914). 



