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



different absorbing layers when the rays were deflected 

 upwards and away from the chamber, are given in columns 

 VI., VII., and VIII. of Table II., and curves E', F', and G', 

 which were drawn from these numbers, are shown in fig. 5, 

 and represent the conductivity when the rays were deflected 

 in the opposite direction. 



The short sharp rise in the curve E shows that with tinfoil 

 1*254 mms. in thickness the ft rays were still able to pene- 

 trate the absorbing layer. A slight rise, as can be seen from 

 the figure, occurs in the curve F, but with the curve G no 

 evidence exists of any rise in conductivity. This curve, 

 moreover, coincides with the curve G', which is drawn from 

 values of the ionization obtained when the rays were deflected 

 upwards; and this coincidence of the two curves G and G' 

 shows that with the absorbing layer with which the results 

 illustrated by those curves were obtained, a thickness of 

 tinfoil was finally reached which could not be penetrated by 

 the ft rays, and by the secondary rays which were produced 

 by them in the metal. 



Tn order to find the precise thickness of tinfoil necessary to 

 stop all the /3and ft secondary radiations, a curve shown in fig. 6 

 was plotted, taking as ordinates the ionization in the chamber 

 duo to the maximum ft and ft secondary rays for each thick- 

 ness of tinfoil, and as abscissae the thickness of the tinfoil 

 screen with which each maximum was obtained. The 

 .maximum ft and ft secondary ionization for each thickness 

 was determined in the following manner. Taking the results 

 for a particular thickness,, the limiting value of the ordinate 

 •of the curve drawn for a deflexion of the ft rays upward was 

 subtracted from the maximum value of the ordinate of the 

 curve drawn for deflexions of the ft rays downwards. Inas- 

 much as the limiting value of the ordinate of the former 

 curve represented the ionization in the chamber due to 7 rays 

 and 7 secondary together with that due to natural causes, and 

 the maximum value of the ordinate of the latter, the ionization 

 due to the maximum ft and ft secondary, 7 and 7 secondary 

 radiations with that due to natural causes, the difference 

 gave the ionization due to maximum ft and ft secondary 

 ionization for the particular thickness of tinfoil over the top 

 of the ionization-chamber. 



In Table III. there is collected and given in row I. the 

 maximum saturation currents in the chamber due to the 

 ft and ft secondary, 7 and 7 secondary radiations, and that due 

 to natural causes, for the screens of different thicknesses of 

 tinfoil ; in row II. the saturation-currents due to the 7 and 

 7 secondary radiations and that due to natural causes, and 



