410 Prof. W. H. Bragg on the Consequences of 



therefore, we make a number of ionization vessels of different 

 materials but the same form, and cause 7 rays to pass into 

 them, the amount of ionization produced inside becomes a 

 measure of the kd of that substance. The experiment may 

 conveniently be carried out by making a thick lead ionization- 

 chamber and inserting different linings. The 7 rays must 

 of course be kept at the same strength inside each lining, or 

 if not any differences must be allowed for. 



Mr. H. L. Porter has recently carried out some experi- 

 ments for me in this way, the results of which are shown in 

 the table below. The first column gives the material of the 

 lining, and the second its thickness, which was enough to 

 give the true value of Jed in all cases except perhaps those of 

 aluminium and cardboard. The third column gives the 

 results obtained when the 7 rays had to pass through little 

 more than the lead wall of the ionization-chamber, which was 

 0*47 cm. thick, and the fourth the results when the rays had 

 to pass through a screen of lead 1*1 cm. thick in addition. 

 The figures are corrected for differences in volume and for 

 differences in the strength of the 7 rays due to absorption in 

 the linino-s. 



I. 



Metal. 



II. 



Thickness 

 of screen. 



•16 



•21 



•155 



•21 



•24 



III. 

 Ra unscreened. 



IV. 



Ra screened. 





100 

 58 

 47 

 45 

 40 

 39 



100 

 68 

 55 

 54 



49 

 46 



Tin 



Zinc 



Iron 



Aluminium 



Card. 





The height of the chamber, which was cylindrical in form, 

 was 15 cm., and the diameter 9 cm. The radium was placed 

 on the axis of the cylinder, 10 cm. away from one end. 



The differences between the figures in the last two columns 

 are really due to a change in the relative value of lead only. 

 The rays have been so hardened by passing through the extra 

 cm. of lead that the absorption coefficient of the lead lining- 

 has fallen to the same value as that of the other metals. In 

 the first case there is a special production of softer /3 rays by 

 the lead which does not take place in the second. 



In these experiments the strength of the 7 rays is not the 

 same all over the cavity as it ought to be ; but the inequality 



