214 Hackett—The Secondary Radiation excited by y Rays. 
TasxeE III. 
Relative die rence 
Element. Tonisation Atomic weight. : 
Payee Atomic weight. 
Hydrogen, 6 : 0:18 1:008 | 180 
Carbon, : 0 0:46 12:00 | 038 
Nitrogen, : 6 0°45 14:04 | 032 
Oxygen, . : 0:58 16:00 | 036 
Sulphur, : 5 | 1:60 32:06 | 050 
Chlorine, ; & | 1:44 35°45 | 046 
Bromine, : . 2°81 79°96 | 035 
Iodine, 6 : 4°50 126°85 035 
From the numbers in the fourth column of Table III. it will be seen that the 
ratio of the relative ionisation per atom to the atomic weight tends towards a 
constant value for a change of atomic weight from 12 to 126. It was deduced 
that the relative ionisation is proportional to 7 (1 + @) + 0’; and direct measure- 
ments on the penetrating secondary radiation have shown that o increases at the 
same rate as the atomic weight. Both lines of work will be in complete accord 
if we make either of the probable assumptions that the easily absorbed secondary 
radiation is either small in proportion to o, or bears a constant ratio to it. 
KLEEMAN’S OBSERVATIONS. 
Kleeman* has recently determined the secondary radiations from different 
substances by another method. He allows the y rays, after passing through a 
sheet of lead and the side of an aluminium box, to fall directly on the plate, and 
measures the secondary radiation produced. In such a method the exciting 
radiation consists not only of the y rays, but also of the B rays produced by them 
in passing through the aluminium. If the latter were unimportant, the method 
would be accurate. But this is not so, as the author found in some preliminary 
experiments which led him to adopt the method described in this paper. The 
numbers given by Kleeman differ widely from those given in ‘Table I. ; instead 
of the secondary radiation remaining nearly constant for a wide range of atomic 
weight, he finds it increases with the atomic weight in the same manner as it 
does for 8 rays or X rays, which would be the result obtained if in the exciting 
radiation there were a large amount of 6 rays. 
Bragg offers another explanation of K]eeman’s results from the point of view 
of his theory of the y rays. On this theory the secondary radiation from the 
face of a plate on which the rays are incident should be less than from the other 
* R. D. Kleeman, Phil. Mag., Noy., 1907. 
