662 Different Kinds of y Rays of Radium. 
relation and breadth of pulse of the pulses of the group 
satisfying very approximately the conditions for best absorp- 
tion for a number of electrons, while the other groups of rays 
do not satisfy these conditions and are therefore not absorbed 
by these electrons. But since the other groups of rays are 
also absorbed to some extent by the substance, the pulses 
satisfy to some extent the conditions for best absorption for 
some other electrons in the atom. The secondary rays pro- 
duced in the substance under these conditions will then be on 
the whole softer than the primary rays, and possess the same 
properties of selective absorption as the primary rays, but in 
a more marked degree. 
Conclusions. 
It has been shown in this paper that the substances Pb, 
Hg, Zn, Cu, Fe, S, Al, and C, whose secondary radiation 
was measured, could be roughly divided into three groups, 
each group of substances radiating rays which differ as a 
whole from those radiated by the other groups. From this 
result and the results of other experiments described in the 
paper, it was deduced that a part of the primary 7 radiation 
from radium and the secondary radiation which it produces 
could be roughly divided into three groups. The ravs of 
each of these groups are not homogeneous, but consist of 
different kinds of rays which may be said to differ less from 
one another than from the rays of some other group. It was 
shown that the three secondary groups of rays possess the 
same properties of selective absorption as the corresponding 
primary groups, but are on the whole more easily absorbed. 
Thus, one of the groups of primary rays is better absorbed 
than the other primary groups by the substances Pb and Hg, 
and one of the other primary groups is better absorbed than 
the other groups by C, and the remaining group of primary 
rays is better absorbed than the other groups by Zn, Cu, Fe> 
S, and Al. The three corresponding secondary groups 
possess the same property, except that each secondarv group 
is softer or more easily absorbed than its corresponding 
primary group. There are probably other rays or groups of 
rays in a primary or secondary beam of rays besides those 
investigated in these experiments. 
It was also found that a substance radiates those secondary 
rays in least amount which it best absorbs. This is due to 
the substance absorbing to a large extent that part of the 
radiation generated in the substance which is easily absorbed 
by the substance. The absorbed radiation — that is, the 
