y-jRays of Thorium and Actinium.. 



15] 



up to 5" 7 mm., the limit of the measurements. We have 

 been able to extend the range up to 2' 5 cm. of lead, or 

 including the 3 mm. thickness of lead as base to a total 

 thickness of 2*8 cm. 



Owing to the abnormally high absorption of the rays by 

 lead, as noticed by G-odlewski, Disposition 1 could not be 

 employed, but Disposition 2 was possible. A new arrange- 

 ment (Disposition 5) similar to Disposition 1 was used, in 

 which the walls and base of the lead electroscope were 3 mm. 

 thick and the preparation was mounted 8" 3 cm. below. 

 The absorbing lead and zinc were placed directly over the 

 preparation, and parallel experiments were done with radium 

 for the same disposition. The results* for lead and zinc are 

 plotted in the curves (fig. 3). The abscissae are centimetres 



Rays of Actinium 



DENSITY OF LEAP 



of thickness multiplied by the relative density of the absorber 

 compared with that of lead. The aluminium curve would 

 coincident with that of zinc if plotted. The figure shows 

 how abnormal lead is. Although the measurements were 

 made through a base of lead 3 mm. thick there is still the 

 point of inflexion in the lead curve at 0*3 cm., noticed by 

 Eve, and a still more decided point of inflexion at 0*85 cm. 

 After that the curve is linear up to 2*5 cms. For zinc and 

 aluminium the curves are exponential. In Disposition 2 the 

 curves for copper, aluminium, and zinc are exponential after 



