428 Dr. J. P. V. Madsen on 



completely surrounded by a thickness of 4 cm. of Pb, a 

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

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

 JRad. Bd. ii. 1905) that practically all soft 7 rays coming 

 from the Ra are stopped by a thickness of Pb considerably 

 less than 4 cm., and that the hard 7 rays which can pene- 

 trate 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 

 7 rays and very little of it from the soft y rays. This result, 

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



It is now possible to apply the necessary correction to 

 the curve A, fig. 2. The result is shown by the full line 

 curve C, the proper correction being obtained by adding a 

 quantity 400(1 — e -^" oXl ' 2 ' od )) 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. 



In order to investigate now the quality of the secondary 

 radiation which comes from the Pb radiators, a Pb plate 

 5 mm. thick was used as radiator, and the emergence 

 radiation was absorbed by Pb domes. In each experiment 

 the dome having been placed in position, two sets of 

 measurements were taken, one with, the other without, the 

 radiator, and the results of these subtracted. The mean of 

 a number of experiments is shown in Table I., and in fig. 5,. 

 curve A, the value of the thickness of the dome multiplied 

 by its density is shown horizontally, and the log. of the 

 corresponding current vertically, the reading with no dome 

 being reduced to 100. It will be seen that the emergence 

 radiation from Pb, within the range of thickness of dome,, 

 over which it has been measured with sufficient accuracy 

 is approximately equivalent to a homogeneous radiation for 

 which A = 1*33. 



Now these results require correction on account of the 

 absorption by the 5 mm. Pb radiator of the secondary 

 radiation from the sides of the cone. As, however, this 

 latter has been shown to have approximately the same value 

 of A, viz., 1*25, it is hardly necessary at the present stage 

 to attempt to apply the correction. 



The quality of the radiation from Pb was now tested by 

 Zn and by Al domes. The results are shown in the curves 

 B and C, fig. 5. It will be seen that A/A is of much the 



