130 Tracks of a. Particles from Radium A in Sensitive Films. 



A microphotograph of one of the plates is reproduced in 

 PI. IY. fig. 1; enlarged 120 diameters. Irregular dark areas 

 seen in this figure are the spots at which the plate was struck 

 in the process above mentioned. Around these areas there are 

 to be seen a number of spots, each of which consists of a set 

 of a-ray tracks radiating from a common centre. In these 

 cases, fine dust particles which had adhered to the ball must 

 have been set free by the shock and have settled down on 

 the plate, thus forming the nuclei of the a-ray radiation. 

 Rubbing the ball with a fine emery-paper before exposure 

 to the emanation was found to be effective in obtaining a 

 larger number of spots. 



In order to show the spots more clearly, two of them, 

 marked P and Q in fig. 1, are reproduced in figs. 2 and 3 

 respectively, magnified 345 diameters. The halo in fig. 2 

 has a mean radius of '0507 mm., and is, as we have shown 

 in the previous paper, due to radium C. When closely 

 examined, another concentric halo is to be seen inside this 

 halo, evidently due to a set of homogeneous a rays from 

 radium A, so that the outer one appears as a corona, resem- 

 bling a pleochroic halo. The halo due to radium A is more 

 clearly illustrated in fig. 3, in which, however, the halo due 

 to radium C is feeble. In the above two cases, the radii of 

 the outer and inner haloes are "0507 and '0348 mm. respec- 

 tively, so that their ratio 1: '6S6 is very nearly the same as 

 that of the ranges of the respective a rays in air, viz. 

 6*94 cm.: 4*75 cm. or 1:*685. This result shows that the 

 radiant nuclei can be regarded practically as points. 



It may be remarked that haloes produced simultaneously 

 on the very same plate are somewhat different ; the halo due 

 to radium C is most conspicuous in P while it is very feeble 

 in Q. This singularity seems to have been due to the 

 manner of exposing the iron ball to the emanation. In 

 the above experiments, the emanation was preserved in a 

 glass vessel inverted over mercury, through which the ball 

 was introduced. Under this circumstance, the active deposit 

 found on the ball would consist of two different parts : one 

 deposited directly from the emanation during the exposure, 

 and the other, a portion of the deposit which had already 

 been accumulated on the surface of the mercury in contact 

 with the emanation, and which contained radium C in such a 

 proportion that it was in equilibrium with radium A. It 

 would be natural to suppose that the second part of the 

 deposit covered the ball not uniformly all over the surface, 

 but adhered to it rather irregularly. Consequently, which 

 of the tracks of the a rays from radium A or radium C 



