of Radium, and Secondary Rays which they produce . 649 
that the substances of both Group I. and Group II. radiate a 
greater proportion of rays which are well absorbed by a 
substance of Group III., than the substances of Group III. 
themselves. 
Thus we have the general result that the substances of any 
two groups radiate a greater proportion of rays which are 
well absorbed by the substances of the remaining group, than 
the substances of this group radiate themselves. It will now 
be shown that it follows from this result that each group of 
substances radiates two groups of rays, each of which is best 
absorbed by one of the remaining groups of substances. 
Let us begin with considering the radiation from the sub- 
stances of Group I. We are given that Group I. and 
Group II. radiate a greater proportion of rays which are 
well absorbed by Group III., than Group III. itself; and also 
that Group I. and Group III. radiate a greater proportion of 
rays which are well absorbed by Group II., than Group II. 
itself. Hence Group I. radiates rays which are well absorbed 
by Group III., and rays which are well absorbed by Group II. 
These two groups of rays are different, since the former group 
is radiated in a greater proportion by Group II. than by 
Group III., and the latter group is radiated in a greater 
proportion by Group III. than by Group II. In a similar 
manner, the above statement may be proved for the other 
groups of substances. 
Thus the substances of Group I. radiate two groups of 
rays, one of which is best absorbed by the substances of 
Group II. and the other best absorbed by the substances 
of Group III., and the substances of Group II. radiate two 
groups of rays, one of which is best absorbed by the sub- 
stances of Group III. and the other best absorbed by the 
substances of Group I. It follows from this that the sub- 
stances of Groups I. and II. radiate one group of rays in 
common, the group being best absorbed by the substances of 
Group III. In the same manner it can be shown that any 
other pair of groups of substances radiate a group of rays in 
common. Since there are three groups of substances, each 
of which radiates two groups of rays, there are at least three 
different groups of secondary 7 rays. But since the sub- 
stances in Tables I. and II. fall approximately only into 
three groups, it follows that the rays of a group consist of 
different kinds of rays, but which differ less from one another 
than from the rays radiated by some other group. We will 
return to this point about the grouping of the rays later on. 
It will be convenient to distinguish these groups of rays 
from one another by names. Thus the secondary rays which 
