8 McCiLeLLanp— On Secondary Radiation and Atomie Structure. 
for the primary rays the depth of the substance penetrated, and the coefficient of 
absorption of the secondary rays determines the depth from which they can come. 
It is easy to show, however, that the secondary radiation given off by a plate, 
arranged as in fig. 1, is proportional to 
Ay 
h+Q? 
where \ is the coefficient of absorption of the primary rays, and )’ that of the 
secondary rays. If, then, \=’, the secondary radiation will not depend on the 
coefficient of absorption of the substance used. Experiments described in the 
previous paper showed that in the case of lead, at any rate, this relation was 
approximately fulfilled. 
Another point to be noticed is that the secondary radiation from uranium is 
not abnormal in any way; the secondary radiation is what was to be expected 
from an element of its atomic weight, and the fact that it is a radio-active 
substance has produced no abnormal effect. It must be remembered, however, 
that the first stage in radio-activity consists in the emission of « particles, while 
the radiation we are dealing with is one of 6 particles only. ‘The breaking up of 
the atom of a radio-active substance may arise, not from the instability of electrons, 
but from the instability of positive parts. 
It may also be mentioned that phosphorus was tested both in the amorphous 
form and as yellow phosphorus, but no difference in radiating power was detected. 
I have pleasure in thanking Dr. Adeney, who supplied me with most of the 
elements used, and also Mr. J. H. Pollok, to whom I am greatly indebted for the 
trouble he took in preparing some metallic uranium. 
