652 Mr. E. D. Kleeman on Different Kinds of y Rays 
Table IV. 
Absorbing 
substance. 
Secondary Secondary 
rays from Zn rays from Pb 
(of Group II.). ; (of Group I.). 
1 
Secondary 
rays from C 
(of Group III.) 
Primary 
rays. 
Pb (Group I.) ... 
A = 4-S4 
X=l-85 
A = 7-5 
X=-77 
, Cu y (Group II.) . 
X = -365 
A = -794 
A = -628 
A- -894 
X=272 
A = -28 
A --31 
i 
\ZnJ X=-69 
A = -28 
C (Group III.)... 
A — -049 
: 

contains the substance. It will be seen that on the whole 
the coefficients of absorption of the secondary rays are much 
greater than those of the primary rays. The secondary rays 
we are dealing with, it will be observed, consist in each case 
of at least two different groups. Thus it follows that the 
three groups of primary rays of radium with which we are 
dealing in this paper are transformed by a substance into 
three groups of secondary rays, which possess the same 
property of selective absorption as the primary rays, but 
which are, on the whole, softer or more easily absorbed than 
the primary rays. This result shows that we must distinguish 
between selective absorption and softness of y rays. The 
distinction is best illustrated as follows : — Suppose we were- 
able to isolate a group of primary y rays, and then measure 
its absorption by three different substances, each of which 
belongs to one of the groups defined in this paper. We 
would then find that each of the substances absorbs the 
group of rays to some extent, but one substance to a greater 
extent than either of the remaining two substances. In the 
case of the corresponding group of secondary rays, we would 
find that each of the substances absorbs the group of rays to 
a greater extent than the primary group, and that the same 
substance as before absorbs to a greater extent the group 
than either of the remaining two substances. 
It may be pointed out that the results contained in 
Table IV. bear out some of the conclusions drawn from the 
results in Table III. Thus, the secondary radiation from 
lead, a substance of Grroup I., is better absorbed by iron and 
copper, substances of Group II., than the secondary radiation 
from zinc, a substance of the latter group. Also, the 
secondary radiation from carbon, a substance of Group III... 
