722 Prof. A. S. Eve on Changes in Velocity in an 
Table I. 
Values of X for aluminium as absorber. /? and y rays. 
Substance 
Primary. 
| 
Secondary. 
... 
Lead. 
Iron. 
Brick. 
Carbon. l 
Atomic Weight 
207 
56 
... 
12 
Secondary ] 
Radiation J 
106 
72 
48 
36 
Thickness 
in cms. Al. 
--009 
48 
48 
71 
70 
90 
-021 
24 
29 
40 
47 
50 
•021- -042 
19 
20 
31 
23 
28 
•042- -063 
14 
16 
20 
18 
16 
•063- -084 
12 
14 
15 
10 
•084- -105 
10 
12 
14 
These figures show that the secondary rays from lead are 
nearly similar to the primary rays from radium, as Allen 
proved. The rays from the lighter substances are more easily 
absorbed, especially at first. The lighter the substance, the 
less is the radiation, the less is the group velocity, and the 
quicker the absorption by the screens. 
It will be proved later by an electrical method that this 
is a case not of selective absorption, but of difference in 
velocity. The table shows also that the more penetrating rays 
from the various substances approximate in character to one 
another and to the primary rays. As the lighter substances 
emit electrons which, regarded as a group, have lower velocities 
than those from the heavier substances, we must conclude that 
the secondary rays are to a large extent intrinsic and projected 
from the radiating substance ; they are not merely diffusely 
scattered primary rays. It may be remarked^ however, that 
Allen has shown that the tertiary rays, from lead-lead-radium, 
are more easily absorbed than the secondary rays from lead. 
It is not difficult to arrange lead plates in such a way as to 
observe tertiary and even quaternary radiations. The relative 
amounts observed by me in one arrangement of apparatus 
were — 
Secondary 70*5 divisions a minute, 
Tertiary 6*7 „ „ 
Quaternary ... 0'5 „ „ 
