v| RATE OF EMISSION OF ENERGY 155 
1:90 x 10-” ergs, it was calculated that the amount of energy, 
radiated into the gas, from 1 gram of uranium oxide, spread over 
a plate im a thin layer, corresponded to 0:032 gram-calories per 
year. Taking the activity of pure radium chloride as 1,500,000 
times that of uranium, the corresponding rate of emission of energy 
from radium is 48,000 gram-calories per year. This is an under- 
estimate, for it mcludes only the energy radiated into the gas. 
The actual amount of energy released in the form of a rays is 
evidently much greater than this on account of the absorption of 
the a rays in the active matter itself. 
It is very important to form an estimate of the total energy 
emitted in the form of @ rays, and also of the number of a particles 
expelled per second from a known weight of an active substance. 
Three different methods of estimating these quantities will now 
be considered. 
Method 1. It can be deduced from the results of Wien 
(section 74) that the number of 8 particles projected from 
1 gram of radium bromide is 6°6 x 10* per second. In this calcu- 
lation no correction has been made for the 8 rays absorbed in 
the envelope of the active matter and in the surrounding glass 
tube. Assuming that about half of the 8 particles escape, it 
follows that the number of 8 particles projected per second from 
1 gram of radiwm is about 2 x 10” per second. Now it will be 
shown later, in chapter xX, that probably four a particles are pro- 
jected from radium for each 8 particle. The number of a particles 
projected per second is thus about 8 x 10". Taking the energy of 
each a particle (section 86) as 5:9 x 10~ ergs, this corresponds to 
a rate of emission of energy from 1 gram of radium of 40 gram- 
calories per hour. 
Method 2. In the case of an active substance in the solid 
or liquid state, most of the « rays emitted are absorbed in the 
active material. The total ionization produced by all the a rays 
from 1 gram of radium, when there is no absorption in the 
active substance itself, was experimentally deduced in the follow- 
ing way. A weight of 0:26 milligrams of pure radium bromide was 
dissolved in water and the solution, spread uniformly over a plate 
about 100 sq. cms. in area, was evaporated to dryness. A few 
