140 Mr. V. E. Pound on the Absorption of the 



and represented graphically by the curve A in fig. 11, were 

 taken to represent the ionizations produced in the chamber 

 by the primary ft rays and by the secondary rays excited by 

 them in the lead reflectors. From a consideration of these 

 values and of the form of the curve in fig. 11, it is evident 

 that a maximum secondary radiation, due to the impact of 

 j3 rays on the lead reflectors, was obtained with a thickness 

 of '16 mm. of this metal. 



From these results, then, it is clear that the secondary 

 radiation emitted by the front side of a lead plate upon which 

 the /3 rays from radium fall, do not come from a depth of the 

 metal greater than *16 mm. It is also established by the 

 results that a plate of lead *9 mm. in thickness will com- 

 pletely absorb all the primary ft rays from radium, as well as 

 all the secondary radiation excited by these rays in the lead 

 plate. 



V. Experiments on the Absorption and Eeflexion 

 of ft Rats by Aluminium-Foil. 



A. Measurements on Transmitted Rays, 



A series of readings was also made with a number of 

 different thicknesses of absorbing layers of aluminium-foil 

 over the top of the chamber, in order to find the minimum 

 thickness of aluminium necessary to stop the j3 rays. The 

 bottom of the chamber was closed by the same sheet of 

 aluminium-foil, *0065 mm. thick, used throughout these 

 experiments. As before, the first series of measurements 

 was taken without any cover over the top of the chamber ; 

 and this series is given in column I. of Table XL The 

 results obtained with layers '0065 mm., '28 mm., l'184mm., 

 3*41 mm., 4*73 mm., and 8' 14 mm. in thickness respectively, 

 are given in columns II., III., IV., V., VI., VII. of the 

 same Table. 



The curves A, B. C, D, and E, shown in figs. 12 and 13, 

 are plotted from the results given in columns I., II., IV., 

 VI., and VII. of Table XI. ; and curves D' and E', fig. 13, 

 from the results given in columns VI. and VII. of Table XII. 



On comparing the results obtained when there was no 

 metallic covering over the opening at the top of the ionization- 

 chamber with the results when a covering of O0065 mm. of 

 aluminium was used, it is readily seen that the addition of 

 the covering considerably increased the maximum ionization 

 in the chamber as the /§ .rays were deflected into it. This 

 effect is also brought out very clearly by the curves A and B 



