y-Rays of Thorium and Actinium. 135 



another part of the paper, the penetrating powers of all three 

 types of 7-radiation are very similar. 



A second series of experiments, which embraced in addition 

 the actinium active deposit, was started in order to investigate 

 various influences which are likely to affect the vaiues of the 

 <y//3 ratio. These are the difference in penetrability of the 

 hard ftrays of each element and the effect of scattering of 

 such rays by the air between the preparation and the 

 electroscope. Some metal must necessarily intervene between 

 the preparation and the electroscope, but the amount used 

 was the minimum, so that the absorption of ftrays by it 

 was small. When the ratio of 7- and ftrays for uranium X 

 was measured in a former experiment it had been necessary 

 to place the preparation at a great distance from the 

 electroscope in order not to give too large a ftray effect. 

 This was not essential in the case of active deposits having a 

 quick rate of decay, so that the values of the ftactivity of 

 the preparations, placed at five different distances from the 

 electroscope, has been measured in order to see how the 

 distance affects the values of the y//3 ratio of thorium D, 

 mesothorium 2 and actinium active deposit relatively to that 

 of radium C. The distances used were 51*1 cm., 35*9 cm., 

 20*6 cm , 13"0 cm., and 5'37 cm. These positions are 

 denoted as ft, ft, ft, ft, and ft respectively. The dispo- 

 sition 7j, used before, was used again. The mesothorium 2, 

 separated chemically as before, was this time mounted on a 

 brass disk, exactly the same as those on which the active 

 deposits of the other bodies were collected, placed in a brass 

 cell and covered with the usual aluminium foil. The sub- 

 stitution of the brass for the glass had only a small effect on 

 the value of the ftrays. The active deposits were prepared 

 and mounted as before, the actinium active deposit being 

 obtained in the same manner as that of thorium. The 

 very fine actinium preparation, lent by Messrs. Buchler & Co., 

 and described in Section III., was employed. Measurements 

 were started in the y x and ft positions, and, when the pre- 

 paration had become too weak to be easily measurable in the 

 latter, it was placed in the ft position, and so on till the 

 ft position. The residual activity was also measured in 

 the ft position ; but since the ftrays of radium C and meso- 

 thorium 2 reach a minimum and then again increase, owing 

 to the formation of later ftray products, the correction for 

 the residual activity cannot be directly applied, but must 

 be extrapolated. From the decay curves of all the bodies 

 experimented with in the five different dispositions, corrected, 

 where necessary, for residual activity, the 7, 73 ratios in the 



