Transformations in the Active Deposit of Actinium, 691 



/ 



a (4,16). ,a (4.75) £y a(?) >* Ra,C, — >? 



/* >^ >*/* ? Vrnnq 1.4 mins. 



^\9997 ^l (6.94) 

 3.85 6%g«„ S.QiwXs.. 26.8 mins.. P ^ Ka'C 1 — KaD.cfe. 



-' 19.5 mins. 10"° sees. 



From the close analogy of the thorium and actinium series 

 it is to be anticipated that actinium C should also show two 

 modes of disintegration, and the present experiments were 

 made to test this point *. 



It will be noticed, in the cases o£ both thorium C and 

 radium C, that the branch disintegration involves the emission 

 of a long-range u particle, 8*6 cm. in the former case and 

 6*94 cm. in the latter. Further, with the whole series of 

 products in equilibrium, the total number of a particles of the 

 two ranges emitted by the " C " products is equal to the 

 number from the emanation and A products on the main 

 series. From the experiments of Geigert such a result 

 must also hold for actinium C if a branch occurs in this 

 product. 



Experiments were consequently made to test whether any 

 long-range a particles were emitted from actinium C, the 

 general evidence indicating that the number of such particles 

 must be small compared with the number of range 5*4 cm., 

 which constitute the main radiation J. The simplest method 

 of making such an experiment would be to take a source 

 of actinium active deposit, and to count the scintillations 

 on a zinc sulphide screen placed in air at different distances 

 from it. Such an experiment had already been tried by 

 Geiger §, and we have repeated it without finding any a par- 

 ticles beyond 5*4 cm. However, with the sources of actinium 

 available the number of a particles falling on an area of 

 1 sq. mm. (the area of the field of the microscope) at distances 

 greater than 5 cm. is very small. Consequently, a sheet of 

 mica of air equivalent over 5 cm. was interposed between 



* The first part of these experiments was carried out by R. H. Wilson 

 and one of us, and the results were given briefly in i Nature/ xcii. p. 29 

 (Sept. 11,1913). 



+ Geiger, Phil. Mao-. xx ii. p. 201 (1912). 



t Cf. R. Swinne, Phys. Zcit. xiv. p. 142 (1913). 



k Geiger, loc. cit. 



