171 



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 radia- 

 tion was absorbed by Pb domes. In each experiment the 

 dome having been placed in position, two sets of measure- 

 ments were taken, one with, the other without, the radia- 

 tor, and the results of these subtracted. The mean of a 

 number of experiments is shown in table i., and in fig. v., 

 curve .4, the value of the thickness of the dome multiplied 

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

 responding current vertically, the reading with no dome being 

 reduced to 100. It will be seen that the emergence radia- 

 tion from Pb, within the range of thickness of dome, over 

 which it has been measured with sufficient accuracy, is ap- 

 proximately equivalent to a homogeneous radiation for which 

 \ = r33. 



Now these results require correction on account of the 

 absorption by the 5 mm. Pb radiator of the secondary radia- 

 tion from the sides of the cone. As, however, this latter 

 has been shown to have approximately the same value of \, 

 viz., 1'25, it is hardly necessary at the present stage to at- 

 tempt 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 &, fig. v. It will be seen that A /A is of much 

 the same value when the radiation is tested by Zn or Al ; 

 but is considerably greater when tested by Pb. This is simi- 

 lar to the results which are obtained when the original y 

 radiation coming from Ra, which apparently consists of a 

 mixture of hard and soft rays, is tested by small thicknesses 

 of absorbing screen. 



Table I. 



Pb Domes. 



Radiation from Pb 



