676 Mr. A. 8. Eve on the Secondary Radiation 
It appears that in this case the secondary rays are more 
homogeneous than the rays which cause them, since the 
coefficient of absorption is not constant for the primary 8 or 
y rays. 
A comparison of the absorption by different screens in the 
case of secondary radiation from lead due to 8 and y rays 
may now be made :— 
TABLE IV. 
| Screen. | As |  A'/Density. 
nO 7 a 
Ppa onkc,* seaetsmenes / 152-6 116 
BEaKs «kp eae | 95 | 11:3 
Aluminium ......) 18-5 | Fh 
foro re eee | 23'5 | 9-4 
| Wood (Bass)....../ 2°7 | 68 
| es a 22°5 
big. C2 aie Beg eae | 
The order is still roughly that of density, but the absorption 
is not proportional to it. 
Experiments were made to determine the nature of the 
secondary radiation from surfaces when the rays passed 
through the substances from the other side. The electroscope 
was placed on a thick lead block beneath which was the 
radium. The various plates were placed horizontally, and in 
front of the electroscope, so that the rays struck the lower 
surfaces ; the secondary radiations from the upper surfaces 
were measured by the electroscope. The difficulty of this 
method was found to be that the secondary radiation from 
the air exposed to the full rays was larger than the secondary 
radiation from an interposed plate which cut down the intensity 
of the primary rays. The effect produced depended upon the 
thickness of the plate as well as upon its density. Thus a thin 
layer of wood gave more radiation than a plate of metal of 
equal thickness ; not because the wood radiated better, but 
because the metal cut down the primary and secondary rays 
more effectively. It was not found possible to make a satis- 
factory comparison of the values from different materials. 
Equal radiations were obtained from 50 mm. of wood, 
55 mm. of slate, and 11 mm. of zine. So also 3 mm. of brass 
gave the same amount as 15 mm. of water in a papier-maché 
tray. 3 
Successive layers of paper were placed in front of the 
Se ee 
