Iv] NATURE OF THE RADIATIONS 115 
of matter, and J, is the initial intensity. For uranium rays the 
current is reduced to half its value after passmg through about 
‘5 mm. of aluminium. 
If a compound of thorium or radium is examined in the same 
way, 1t is found that the current does not decrease regularly 
according to the above equation. Results of this kind for radium 
rays have been given by Meyer and Schweidler.. The amount of 
absorption of the rays by a certain thickness of matter decreases 
with the thickness traversed. This is exactly opposite to what is 
observed for the a rays. This variation in the absorption is due to 
the fact that the 8 rays are made up of rays which vary greatly in 
penetrating power. The rays from uranium are fairly homogeneous 
in character, v.e. they consist of rays projected with about the same 
velocity. The rays from radium and thorium are complex, 2.e. they 
consist of rays projected with a wide range of velocity and con- 
sequently with a wide range of penetrating power. The electrical 
examination of the deviable rays thus leads to the same results as 
their examination by the photographic method. 
Results on the absorption of cathode rays have been given by 
Lenard’, who has shown that the absorption of cathode rays is 
nearly proportional to the density of the absorbing matter, and is 
independent of its chemical state. Ifthe deviable rays from active 
bodies are similar to cathode rays, a similar law of absorption is to 
be expected. Strutt®, working with radium rays, has determined 
the law of absorption and has found it roughly proportional to the 
density of matter over a range of densities varying from 0:041 for 
sulphur dioxide to 215 for platinum. In the case of mica and 
cardboard, the values of X divided by the density were 3°94 and 
3°84 respectively, while the value for platinum was 7°34. In order 
to deduce the absorption coefficient, he assumed that the radiation 
fell off according to an exponential law with the distance traversed. 
As the rays from radium are complex, we have seen that this is 
only approximately the case. 
78. Since the 8 rays from uranium are fairly homogeneous, 
and are at the same time penetrating in character, they are more 
1 Phys. Zeit. pp. 90, 113, 209, 1900. 
2 Wied. Annal. 56, p. 275, 1895. 
* Nature, p. 539, 1900. 
