Secondary 7 Radiation. 447 



rays from thorium have an absorption curve, measured by 

 Pb, very nearly the same as that for Ra. We should expect, 

 then, that thorium gave out two homogeneous sets of 7 rays 

 as does Ra, the values of X by Pb for these groups being *32 

 and 1*35. It has also been found by Eve that the value of 

 X for the 7 rays given out by Ur is 1*4, and that these rays 

 appear nearly homogeneous. The 7 rays of Ur appear, then, 

 to correspond very closely with soft bundle of rays emitted 

 by both Th and Ra. 



Again, it seems rather striking that the secondary rays 

 produced by the hard 7 rays of Ra produce in Pb,, and 

 apparently in all substances, a secondary, for which the value 

 of X is of the same order as for the Ur rays and the soft rays 

 of Ra and Th. 



In the case of the secondary rays produced from the hard 

 7 rays of Ra it has been possible to treat them as practically 

 homogeneous over the range of thickness of screens which 

 have been used. It will, however, require much more 

 careful research to determine the exact quality of this 

 radiation. 



Again, the value of X/A for the soft primary bundle of 

 rays has been shown to be nearly four times that of the 

 hard. It seems possible that this may have some connexion 

 with the result obtained previously by Professor Bragg and 

 myself for the /3 rays which are produced from the 7 rays 

 of Ra. It was found that the value of X for the hard and 

 soft /3 rays was approximately as one to four. 



Summary. 



1. The 7 rays of Ra and possibly of Th appear to con-i-t 

 of two distinct homogeneous bundles, the value of \/A for 

 the soft set being approximately four times that for the 

 hard. 



2. For each set of rays the value of X/A is constant and 

 practically independent of the nature of the absorbing 

 material with which X is measured, provided that in the case 

 of the soft rays secondary effects be excluded. 



3. Secondary 7 radiation appears on both sides of a plate 

 which is penetrated by a stream of 7 rays. There exists a 

 marked lack of symmetry between the amount of secondary 

 radiation which proceeds from the two sides. 



4. A lack of symmetry exists in the case of some sub- 

 stances between the quality of the radiation on the two sides. 



