Transformations in the Active Deposit of Actinium. 699 



the distance between the maximum and the end of the range 

 is least for polonium and greatest for thorium 2 . This has 

 apparently not been observed previously, though it is to be 

 anticipated theoretically. It is known from experiments on 

 the magnetic deflexion of a particles that an initially homo- 

 geneous beam becomes slightly less homogeneous as ab- 

 sorption proceeds *. This phenomenon was first predicted 

 by Darwin f and termed by him " straggling." It is due to 

 the fact that some a particles suffer more severe encounters 

 with the atoms of matter traversed than others. Consider 

 a pencil of a particles from thorium C 2 which have traversed a 

 distance from the source in air of 8*6 — 3*77 = 4*83 cm. These 

 a particles will have the same residual range, 3*77 cm., 

 and consequently the same average velocity as a pencil of 

 a particles at the moment of their projection from atoms 

 of polonium. Yet whereas in the latter case the A^elocities of 

 every a particle in the beam will be exactly the same, in the 

 former case the velocities will have values grouped round 

 the average, and in general differing from it by small 

 amounts. It is easy to see that the ionization curve will be 

 different in the two cases, and that the " knee " will be more 

 pronounced in the case of polonium a particles in agreement 

 with the results shown in fig. 6. 



As the a particles of actinium and radium C have ranges 

 intermediate between those of thorium C 2 and polonium, it is 

 to be expected that the curves should show intermediate 

 characteristics. Our results are in general agreement with 

 this expectation. The experimental curves are, however, 

 almost identical; whereas theoretically the actinium curve 

 should be nearer to that for polonium, since the range of the 

 a particles of actinium C is smaller than the range of the 

 a particles of radium C. It must be remembered, however, 

 that owing to the rapid decay of the active deposits of actinium 

 and radium, the experimental error is somewhat increased 

 and is probably greater than any deviation from what is to be 

 anticipated theoretically. 



A comparison of figs. 3 and 6 will show that our measure- 

 ments do not agree with those of Mile. Blanquies, and that 

 no definite evidence of further anomaly in the a particles of 

 actinium C, than those indicated in the first section of this 

 paper, has been established. 



* Marsden & Taylor, Proc. Roy. Soc. A. lxxxviii. p. 443 (1913). 



t Darwin, Phil. Mag. xxiii. p/901 (1912). Cf. Marsden & Richardson. 

 Phil. Mag. xxv. p. 184 (1913); also Bianu, Le Radium, x. p. 122 

 (1913). 



