154 RATE OF EMISSION OF ENERGY [CH. 
Now it has been shown that for the a rays of radium 
Om: 
eR ae 
My 
The velocity of the @ rays of radium varies between wide 
limits. Taking for an average value 
B= 15 x IO”. 
é 

= Iles $< Oy, 
it follows that the energy of the a particle from radium is almost 
83 times the energy of the 8 particle. If equal numbers of a and 
8 particles are projected per second, the total energy radiated in 
the form of a rays is about 83 times the amount in the form of 
B rays. 
Evidence will be given later to show that the number of 
a particles projected is probably several times greater than the 
number of 8 particles; so that a still greater proportion of the 
energy is emitted in the form of a rays. These results thus lead 
to the conclusion that, from the poimt of view of the energy 
emitted, the a rays are far more important than the @ rays. 
This conclusion is supported by other evidence which is discussed 
in chapter X, where it will be shown that the @ rays play by far 
the most important part in the changes occurring in radio-active 
bodies, and that the @ rays only appear in the last stage of the 
radio-active processes. From data based on the relative absorption 
and ionization of the 8 and y rays in air, it can be shown that the 
y rays carry off about the same amount of energy as the @ rays. 
104. Number and Energy of the a particles. It has 
been shown that the greater part of the energy emitted from 
the radio-elements in the form of ionizing radiations is due to 
the a rays. Rutherford and McClung (loc. cit.) made an estimate 
of the energy of the a rays, radiated into the gas from a thin 
layer of active matter, by determiming the total number of ions 
produced by the complete absorption of the a rays. Taking 
as the value for the energy required to produce an ion in a gas 
