138 NATURE OF THE RADIATIONS [CH. 
proportional to it. Since the absorption of the a rays in gases is 
probably mainly due to the exhaustion of the energy of the rays 
by the production of ions in the gas, it seems probable that the 
absorption in metals is due to a similar cause. 
94. Relation between ionization and absorption in 
gases. It has been shown (section 45) that if the a rays are 
completely absorbed in a gas, the total ionization produced is about 
the same for all the gases examined. Since the rays are unequally 
absorbed in different gases, there should be a direct connection 
between the relative ionization and the relative absorption. This 
is seen to be the case if the results of Strutt (section 45) are com- 
pared with the relative absorption constants (section 92). 
Gua Relative Relative 
absorption ionization 
Aa Me: Whe Il 1 
Hydrogen ... 27 226 
Carbon dioxide 1:43 1°53 
Considering the difficulty of obtaiming accurate determinations 
of the absorption, the relative ionization mm a gas is seen to be 
directly proportional to the relative absorption within the limits of 
experimental error. This result shows that the energy absorbed 
in producing an ion is about the same in air, hydrogen, and carbon 
dioxide. 
95. Theory of the absorption of the a rays by matter. 
As we have seen, experiment shows that the ionization of the gas 
due to the « rays from a large plane surface of radio-active matter 
falls off approximately according to an exponential law until most 
of the rays are absorbed, whereupon the ionization decreases at a 
much faster rate. The ionization of the gas is due to the collision 
of the positively charged particles with the molecules in their path. 
Each projected particle carries with it sufficient energy to produce, 
on an average, several thousand ions in its path before its velocity 
is reduced to a value below which it fails to ionize the gas. This 
minimum velocity for the a and £ particles is probably about 10* cms. 
per second. More experimental data are required on the variation of 
the amount of ionization of the gas with the speed of the projected 

