Iv] NATURE OF THE RADIATIONS 133 
curves of absorption are given in Fig. 27. For the purpose of 
comparison in each case, the initial current with the bare active 
compound is taken as 100. A very thin layer of the active 
substance was used, and, in the case of thorium and radium, the 
emanations given off were removed by a slow current of air through 
the testing vessel. A potential difference of 300 volts was applied 
between the plates, which was sufficient to give the maximum 
current in each case. 
Curves for the minerals organite and thorite were very nearly 
the same as for thoria. 
For the purpose of comparison, the absorption curves of the 
excited radiations of thorium and radium are given, as well as the 
curve for the radio-elements uranium, thorium, radium, and polo- 
nium. The @ radiations may be arranged in the following order, 
as regards their power of penetration, beginning with the most 
penetrating. 
ee excited radiation. 
Radium _ 
Thorium. 
Radium. 
Polonium. 
Uranium. 
The same order is observed for all the absorbing substances 
examined, viz., aluminium, Dutch metal, tinfoil, paper, and air and 
other gases. The differences in the absorption of the a rays from 
the active bodies are thus considerable, and must be ascribed either 
to a difference of mass or of velocity of the a particles or to a 
variation in both these quantities. 
Since the a rays differ either in mass or velocity, it follows 
that they cannot be ascribed to any single radio-active impurity 
common to all radio-active bodies. 
92. Absorption of the a rays by gases. The a rays from 
the different radio-active substances are quickly absorbed in their 
passage through a few centimetres of air at atmospheric pressure 
and temperature. In consequence of this, the ionization of the air, 
due to the a rays, is greatest near the surface of the radiating body 
and falls off very rapidly with the distance (see section 90). 
