650 Mr. R. D. Kleeman on Different Kinds of 7 Rays 
are best absorbed by the substances of Group I. will be 
called Group I. rays, and a similar name given to each of 
the other groups of rays corresponding to the group of sub- 
stances by which the rays are best absorbed. According to 
this notation the substances of Group I. radiate secondary 
rays of Group II. and Group III., and so on. 
We have seen (Table III.) that the increase in the ratio of 
the radiations from two substances with an increase of thick- 
ness of screen, becomes more marked when the thickness of 
one of the radiators (or of both) is increased. This is due 
to secondary rays of all three groups being generated in a 
substance, but one of the groups being more easily absorbed 
by the substance than either of the other groups. The 
radiation from a substance would then consist principally of 
two groups of rays, and a small proportion of rays of the 
remaining group; and the difference in the nature of the 
radiations from two substances would therefore become more 
marked the greater their thickness. 
The changes observed in the ratios of the radiations with 
the thickness of screen are small, but they can be accurately 
measured. The reason for their smallness will now be ap- 
parent from the deductions made from the results. Since 
two substances of two different groups radiate (at least) one 
group of rays in common, this group of rays will mask to 
some extent the relative change in the radiations produced 
by an increase of the thickness of the screen. Moreover, the 
fact that all the groups of rays are absorbed more or less by 
a substance will also have the effect of making the change 
small. 
It will be profitable now to compare the results so far 
obtained with the result quoted in the beginning of this 
paper from a previous paper, namely, that the primary 
7 rays of radium consist in part of three groups of rays. In 
the paper mentioned it was shown that one of the groups of 
rays is best absorbed by lead, mercury, and bismuth, and one 
group best absorbed by tin, zinc, copper, nickel, iron, sulphur, 
and aluminium, and the remaining group best absorbed by 
carbon. It will be seen that each of these three groups of 
substances includes one of the three groups of substances 
defined in this paper. It appears, therefore, that the primary 
7 rays of radium consist of at least three groups which 
possess the same property of selective absorption as the three 
groups of secondary rays discussed in this paper. 
It became of interest next to investigate if the secondary 
7 rays possess the same penetrating power as the primary 
rays. The measurements for determining the coefficients of 
