218 Hacxerr—TZhe Secondary Radiation excited by y Rays. 
It will be merely remarked that Brage’s theory, in its present simple form, 
demands that the secondary § particles from all substances should have the same 
velocity, and so the same penetrating power, since they are only the negative 
particles of the revolving couple which constitute the y rays, moving on alone 
after the collision with their original velocity. This does not at all agree with 
the above measurements. As the author is at present engaged in pursuing this 
investigation, the further discussion of the subject is postponed until a later 
paper. 
SUMMARY. 
1. An account of the direct measurement of the secondary radiation emitted 
from the upper surface of a plate, when y rays are incident on the lower surface, 
is given. ‘The pencil of y rays passes up through the plate at an angle of 45° to 
the normal. The intensity of the radiation emitted in the plane of incidence in 
a direction normal to the y rays is measured by means of an ionisation cylinder. 
2. The secondary radiation thus measured is nearly constant for all atomic 
weights ranging from 27 to 120, but increases for the gold-lead-bismuth group of 
elements, and there is a further increase for uranium. 
3. A discussion of these results leads to the conclusion that the secondary 
radiation per atom is roughly proportional to the atomic weight when this does 
not exceed 120, and increases more rapidly than the atomic weight for elements 
of high atomic weight such as lead and platinum. 
4, It is shown that this result is in general agreement with the work of 
Kleeman on the relative ionisation of gases per atom by y rays, which also 
increases at the same rate as the atomic weight. 
5. The penetrating power of the secondary § radiation diminishes with the 
increase of atomic weight for soft y rays, but increases with the atomic weight for 
hard y rays. 
6. For elements of low atomic weight, as aluminium, the penetrating power 
of the secondary 8 radiation does not alter with the penetrating power of the 
primary ; but it increases with the latter for elements of high atomic weight. 
7. The results of this paper do not afford any decisive test between the ether- 
pulse theory and Bragg’s theory. 
In conclusion, the author desires to record his great indebtedness to Professor 
McClelland, who suggested the present research, and whose interest and advice 
facilitated the course of the investigation. 
