1 
130 NATURE OF THE RADIATIONS [CH. 
with successive layers of aluminium foil interposed, each 00034 cm. 
in thickness. In order to get rid of the ionization due to the @ 
rays from radium, the radium chloride employed was dissolved in 
water and evaporated. This renders the active compound, for the 
time, nearly free from 8 rays. 






Polonium. Radium. 
Ratio of | Ratio of 
meh pnekt Current | decrease for reve Hed Current | decreasefor 
each layer each layer 
0 100 (0) 100 
4] “48 
1 4] 1 48 
| Sill “48 
2 12°6 2 23 
O17 “60 
3 Dill | 3 13°6 
‘067 47 
4 14 4 674 
| 39 
5 0 5 WAS) 
36 
6 9 
7 0 | 







The initial current with 1 layer of aluminium over the active 
material is taken as 100. It will be observed that the current due 
to the radium rays decreases very nearly by half its value for each 
additional thickness until the current is reduced to about 6 °/, of 
the maximum. It then decays more rapidly to zero. Thus, for 
radium, over a wide range, the current decreases in an exponential 
law with the thickness of the screen, 
or ~=e 
where 2; is the current for a thickness ¢, and 7, the initial current. 
In the case of polonium, the decrease is far more rapid than would 
be indicated by the exponential law. By the first layer, the 
current is reduced to the ratio ‘41. The addition of the third 
layer cuts the current down to a ratio of 17. For most of the 
active bodies, the current diminishes slightly faster than the 
exponential law would lead one to expect, especially when the 
radiation is nearly all absorbed. 
