654 Mr. R. D. Kleeman on Different Kinds of y Rays 
Table V. 
Absorbing 
substance. 
Secondary rays from a carbon plate 
2 cm. thick. (Rays of Groups I. & II.) 
Pb 
(of Group I.). 
No screen over Screen of lead L2 cm. 
radium. thick over radium. 
A = 7"5 \=5"17 
Fe 
(of Group II.). 
No pcreen over Screen of zinc 2-1 cm. 
radium. thick over radium. \ 
\ = 2'72 
A =-80 
| 
Secondary rays from a zinc plate 
•65 cm. thick. (Rays of Groups I. & TIL) 
No screen over 
radium. 
Screen of lead 1*2 cm J 
thick over radium. 
Pb 
(of Group I.). 
X=4-82 
X = 4-5 
the conditions under which each coefficient was determined. 
It will be seen that the coefficient of absorption of each 
substance is decreased with a screen of the same substance 
placed over the radium. The results, therefore, show that 
the primary rays of Group I. and Group II. are transformed 
into corresponding groups of secondary rays. We may take 
it for granted, therefore, that this is the case with each 
primary group of rays. 
Since the secondary y rays consist of at least three groups, 
and, as we have just seen, a group of secondary rays is 
caused by the transformation of a primary group possessing 
similar properties, it follows that the primary rays of radium 
consist of at least three groups, each of which is selectively 
absorbed by one of three groups of substances. We have 
thus established in an entirely different manner a result 
obtained in a previous paper. 
Since the substances in Tables I. and II. fall approximately 
only into groups, the grouping of part of the primary and 
secondary radiation into three groups is therefore only a 
rough one. It appears very probable, therefore, that there 
are other primary and secondary rays which are not included 
in any one of these groups. Some of these rays or groups 
of rays would become apparent by investigating the secondary 
radiation from other substances than those dealt with in this 
