42 



ROYAL SOCIETY OF CANADA 



Lead, aluminium and carbon plates were compared with screens 

 of lead and iron. 



In this case the radium was in vessel H, figure 1, and screens 

 were interposed near H. Zinc, copper, carbon and paraffin screens 

 were also used and these resembled iron, not lead, in the effects pro- 

 duced. It is then easy to produce a change with less than a centi- 

 metre of lead which cannot be brought about by many centimetres 

 of iron. This peculiarity was first noted by Bragg and Madsen^ 

 and it cannot be attributed to selective absorption because a varia- 

 tion in the character of the screens causes corresponding modifica- 

 tions in the radiations from other pairs of plates, such as copper- 

 carbon or copper-aluminium. 



After the 7- rays have passed through iron screens similar changes 

 can be produced by a moderate thickness of lead. Thus: 



Iron, however thick, appeared incapable of producing equivalent 

 results. 



It is possible that the true explanation is a little complex. When 

 primary ;- rays pass through iron they give rise to secondary 

 Y rays, softer than the primary which caused them, but nevertheless 

 capable of penetrating a considerable thickness of iron. The same 

 is true of zinc, copper and other substances of a like order of atomic 

 weight. When, however, primary ^ rays pass through lead, second- 

 ary ;- rays are produced to a less extent than with iron, or they 

 are so rapidly absorbed that they are more difficult to detect. Hence 



1 Trans. Roy. Soc. S. Aust. 32, 190S. 



