694 Mr. E. Marsden and Dr. P. B. Perkins on the 



the actinium preparation for two hours, or for several days, 

 in a completely closed vessel. In the latter case the radium 

 emanation would accumulate and give a far greater quantity 

 of radium C than for a two-hours exposure. Further, the 

 agreement between the various experiments in giving the 

 range as 6*4 cm. in itself shows that no radium C « particles 

 of range 6*9 cm. were present. Difficulty was, however, 

 experienced in the earlier experiments from thorium as an 

 impurity. This was shown by the presence of a. particles of 

 range S'G cm., the number of which did not decay at the 

 same rate as the actinium active deposit, but with the well- 

 known period of thorium B, 10' 6 hours. The number of these 

 particles was with long exposures about 0*03 per cent, of the 

 initial number of « particles from actinium C, but bv exposing 

 for only two hours the number could be considerably reduced 

 and the small necessary correction could be made by counting 

 the number of scintillations at a range greater than 6*4 cm. 

 In the curve given in fig. 1 the thorium C a particles only 

 amounted initially to 0*005 per cent., and have been 

 corrected for. 



Since the range of the observed a particles differs little 

 from that of actinium A, i. e. 6*5 cm., it might be thought 

 possible that they may be due to this product derived from 

 actinium X obtained by recoil from the actinium in the 

 source. The nearly constant ratio obtained in the expe- 

 riments and the rate of decay of the number of long-range 

 a particles, however, negative this idea. 



The above experiments therefore indicate that the a particles 

 of actinium C are complex, and that 0*15 per cent, have a 

 range of 6*4 cm., while the main number have a range of 

 5*4 cm. 



Ionization Curves of a Particles from Polonium, Actinium C, 

 Radium C, and Thorium C. 



In connexion with these experiments it must be recalled 

 that Mile. Blanquies^in 1909 came to the conclusion that 

 two a ray products are present in the active deposit of 

 actinium. This conclusion was based mainly on a comparison 

 of the ionization curves for polonium, actinium C, and 

 radium C. The ionization curves obtained are reproduced 

 in tig. 3. 



It will be noticed that although the curves for polonium 

 and radium C are superposable, the curve for actinium C 

 shows a much less pronounced " knee," as though complex 



* Mile. Blanquies, Le Radium, vi. p. 230 (1909) ; vii. p. 159 (1910). 



