v] RATE OF EMISSION OF ENERGY 151 
If it is assumed that the same amount of energy is required to 
produce an ion by either the @ or the 8 ray, and that the same 
proportion of the total energy is used up in producing ions, an 
approximate estimate can be made of the ratio of the energy 
radiated by the a and 8 rays by measuring the ratio of the total 
number of ions produced by them. If 2% is the coefficient of 
absorption of the 8 rays in air, the rate of production of ions 
per unit volume at a distance w from the source is q,e~** where q 
is the rate of ionization at the source. | 
The total number of ions produced by complete absorption of 
the rays is 
em aac £ : 
Now 2 is difficult to measure experimentally for air, but an 
approximate estimate can be made of its value from the known 
fact that the absorption of 8 rays is approximately proportional to 
the density of any given substance. 
For 8 rays from uranium the value of for aluminium is about 
14, and 2 divided by the density is 5-4. Taking the density of air 
as (0012, we find that 
» for air = 0065. 
The total number of ions produced in air is thus 154 q, when 
the rays are completely absorbed. 
Now from the above table the ionization due to the @ rays 
is ‘0074 of that produced by a rays, when the 8 rays passed 
through a distance of 5°7 cms. of air. 
Thus we have approximately 
Total number of ions produced by 6 rays _ ‘0074 
: ie — = 54 = 0°20. 
Total number of ions produced by a rays 5°7 ae 

Therefore about 1/6 of the total energy radiated into air by a 
thin layer of uranium is carried by the 8 rays or electrons. The 
ratio for thorium is about 1/22 and for radium about 1/14, assum- 
ing the rays to have about the same average value of 2. 
This calculation takes into account only the energy which is 
radiated out into the surrounding gas; but on account of the ease 
with which the « rays are absorbed, even with a thin layer, the 
