of Radium, and Secondary Rays which they produce. 645 
that the value o£ the radiation from zinc has been put equal 
to 100 for each screen, and the values of the radiations from 
the other substances changed correspondingly. 
It will be seen from Table I. that when Pb is put equal to 
100, the values of the radiation from Pb with the different 
screens (given in the horizontal column containing Hg) 
become approximately equal to one another. Prom Table II. 
it will be seen that when zinc is put equal to 100, the values 
of the radiations (with the different screens) from each of 
the substances Al, S, Fe, Cu, Zn, become approximately 
equal to one another. But the values of the radiations from 
C do not become equal to one another in either case. Thus 
the substances appear to fall roughly into three groups. 
The interpretation of this is that each member of a group 
radiates approximately the same kind of secondary rays, but 
the radiation from a group is as a whole different from that 
from some other group. Thus, if each of the substances of 
a group radiates the same kind of rays, a screen will diminish 
the radiation in the same proportion in each case whatever 
the selective absorbing nature of the screen. The relative 
radiations will therefore not be altered by a screen, and 
therefore when the radiations with the different screens from 
one of the substances of the group is put equal to 100, the 
values of the radiations from each of the other substances of 
the group will become approximately equal to one another. 
These considerations also show that the substances which do 
not belong to a group radiate rays which differ as a whole 
from the rays radiated by the group. 
For convenience of reference we will denote by Group I. 
the substances Pb and Hg, including the substances which 
radiate the same kind of rays as Pb or Hg, if any exist. 
Similarly we will denote by Group II. the substances which 
radiate the same kind of rays as Zn, Cu, Fe, S, and Al; and 
by Group III. the substances which radiate the same kind 
of rays as C. 
The problem that presented itself next was to determine 
if there is any relation between the rays from the various 
groups of substances. This was done by the following method. 
The radiations from two substances A and B belonging to 
two different groups were compared — when the screen used 
was thin, and when it was much thicker. Now, if one of 
the substances, say A, radiates more rays which are well 
absorbed by the screen, than the other substance B, the ratio 
of radiation from A to that from B will decrease when the 
thickness of the screen is increased. In that case, we can 
Phil. Mag. S. 6. Vol. 15. No. 89. May 1908. 2 X 
