170 



seen from fig. i. that the plug would hardly intercept any 

 •y radiation passing directly from the Ra to the ionization 

 chamber. On the other hand, we obtain a rather important 

 result, for assuming, of course, that the interpretation put 

 upon these experiments is correct, even when the Ra is com- 

 pletely surrounded by a thickness of 4 cm. of Pb, a sec- 

 ondary radiation escapes, which has a value of A approx. 

 1*25. Now it has been shown by Wigger (Jahrbuch der Rad. 

 Band 2, '05) that practically all soft y rays coming from 

 the Ra are stopped by a thickness of Pb considerably less 

 than 4 cm., and that the hard y rays which can penetrate 

 such a thickness have a value of A. = '24. 



Again, the amount of the secondary radiation has only 

 been reduced from 570 to 270 by the insertion of the 4 cm. 

 plug of Pb, so that we must suppose most of the secondary 

 radiation we are dealing with to be produced from the hard 

 y rays and very little of it from the soft 7 rays. This re- 

 sult, it will be shown later, is supported by other experiments. 



It is now possible to apply the necessary correction to 

 the curve A, fig. ii. The result is shown by the full line 

 curve C, the proper correction being obtained by adding a 

 quantity 400(1 - e- (i*5xr25)d) to the original values. Part of 

 the secondary radiation from the sides of the cone was in 

 these experiments cut off by suitable Pb screens; the factor 1'5 

 is introduced to allow for the obliquity with which the rays 

 coming from the sides of the cone cross the radiators. 



