254 Mr. H. Richardson : Analysis of the 



curve for thicknesses of aluminium greater than 5 cm. The 

 numbers obtained over this range clearly indicated, however, 

 that this more penetrating radiation is complex, consisting 

 of two types. This view is supported by the work of Soddy 

 and Russell*, who have previously examined in detail the 

 absorption of the penetrating 7 rays from uranium X, using 

 large thicknesses of aluminium. They separated the uranium 

 X from 50 kilograms of uranium nitrate, and with this 

 amount of active material they were able to follow the 

 absorption through 12 cm. of aluminium. Their work was 

 specially directed to determine the absorption coefficient of 

 the residual radiation after passing through 2 cm. of 

 aluminium. They were able to show that after a thickness 

 of 5 cm. the absorption curve was quite exponential. The 

 exact value of the absorption coefficient which they obtained 

 varied slightly according to the particular experimental 

 disposition which they employed. The extreme values of the 

 absorption coefficients in aluminium which they obtained 

 were jjl = *130 and /jl = "148. From an inspection of the 

 curves given by Soddy and Russell it is at once seen that a 

 soft type of radiation was also present which was practically 

 absorbed by a thickness of 5 cm. of aluminium. The 

 absorption curve I obtained between 4 mm. and 5 cm. 

 appeared to be in good agreement with the results given by 

 the above curves. I have therefore based the analysis of 

 the absorption curve, shown in fig. 2, on the assump- 

 tion that the most penetrating type of radiation is that 

 observed by Soddy and Russell. Assuming, with sufficient 

 accuracy, a mean value of the absorption coefficient 

 (jl = 0'140, the analysis of the absorption curve between 

 4 mm. and 5 cm. shows that the other type of radiation 

 has an absorption coefficient fi = 0'70 and that it is expo- 

 nentially absorbed. As the examination of the absorption 

 of the penetrating y rays from uranium X had been made 

 with great care by Soddy and Russell, it did not seem 

 necessary in these experiments to repeat any of their work. 



It has been shown that uranium X emits j rays consisting 

 of three types which have absorption coefficients /-t = 24, 

 ^ = O70, and yu, =0*140 in aluminium of density 2'72. 



The recent work of Fajans and Gohringf, Hahn and 

 Meitner +, and Fleck § has shown conclusively that uranium 



* Soddy and Russell, Phil. Mag-, xviii. p. 620 (1909). 

 f Fiijans and Gohring, Die Naturwissenschaften, Heft xiv. p. 339 

 (1913). 



X Hahn and Meitner, Phys. Zeit. xvi. p. 758 (1913). 

 § Fleck, Phil. Mag. xxvi. p. 528 (191 o). 



