Hacxerr— The Secondary Radiation excited by y Rays. 215 
face, as the 8 particles are initially projected in the direction of the y rays. 
Further, as is shown by the equations (Cand D), the radiation from the incident 
face is approximately the fraction p of that from the emergence face, where p 1s 
the ratio of secondary to primary radiation when the primary consists also of 
B particles. Since, therefore, the secondary radiation from the emergence face 
when y rays are used is constant, at least for substances of low atomic weight, the 
radiation from the incident face should vary in the same general way as for 
B rays. Such is the natural interpretation of Kleeman’s results on Bragg’s theory 
of the y rays. But, as pointed out earlier in this paper, further measurements are 
required of the secondary radiation from the incident face, care being taken to 
use a primary beam of rays free from 8 particles. And further it is clear that 
equations similar to @ and D will hold on any theory of the y rays, the ether- 
pulse theory included, which admits the possibility of the B particles being 
projected initially in the direction of the y rays. 
Ture PENETRATING POWER OF THE SECONDARY RADIATION. 
The apparatus used in making the above observations also served for making 
an examination of the penetrating power of the secondary rays. A measurement 
of the secondary radiation was made in the manner described with one sheet of 
tinfoil, covering the end of the ionisation cylinder and a second measurement 
with five sheets of tinfoil as a covering. The ratio of these two measurements (p) 
gives an approximate value for the relative penetrating powers of the rays from 
different substances. To standardize these observations the radium was removed, 
and a similar observation was made on the penetrating power of the rays from a 
tray of uranium oxide of exactly the same size placed in the same place as the 
plates used. The penetrating power of secondary radiation from uranium oxide 
itself was also determined in the same way as for the other substances. The ratio 
for the natural radiation from uranium oxide was found to be ‘77. If we take its 
coefficient of absorption as 100, we can obtain the relative coefficients of absorp- 
tion [ Ag’, \g”, As” for the secondary rays excited by y rays of different penetrating 
powers. ‘These are easily obtained by placing over the conical hole lead plates 
and blocks of lead cast to fit the opening. 
An auxiliary series of observations, using the same arrangement of screening 
with the y rays falling directly on an ionisation cylinder closed with a sheet of 
lead 2 mm. thick, gave an approximate measure of the intensity of the y rays. 
The coefficient of absorption of the rays after transmission through the lead 
screens was measured by placing a thick sheet of metal in front of the cylinder, 
and then interposing a second sheet of the same thickness in the path of the 
rays. | 
