v] RATE OF EMISSION OF ENERGY 157 
a particles, the ionization is produced by a charged body atomic 
in size. It is reasonable to suppose that the maaimum number 
of ions produced by the a particles per cm. of their path cannot be 
greater than the number of molecules in a cylinder of twice the 
diameter of the molecule. The maximum number of ions produced 
per cm. of path in air at 1 mm. pressure cannot in consequence be 
greater than 84. The number per cm. at atmospheric pressure 
and temperature will be 63,800. 
Now half the a rays from radium are absorbed in traversing 
0°75 cm. of air (section 92). The total number of ions produced 
by the rays is about the same as if the ionization at the surface 
of the active matter extended uniformly for a distance of 1:09 cms. 
The number of ions produced on an average by each a particle of 
radium cannot in consequence be greater than 70,000. 
The total number of ions produced for 1 gram of radium is 
10°. This corresponds to an emission of 1°4 x 10" a particles from 
1 gram of radium per second and an emission of energy of 70 gram- 
calories per hour. 
The approximate estimates by the three methods of the 
number of a particles and the rate of emission of energy from 
1 gram of radium are in good agreement. It may be concluded 
that from 1 gram of radium :-— 
(1) about 10" a particles are projected per second ; 
(2) the rate of emission of energy in the form of a particles is 
probably equal to about 50 gram-calories per hour. 
These results will be found to be in harmony with the deduc- 
tions drawn from the observed heat emission of radium discussed 
in the next section. 
Smee radium bromide has an activity (measured by the a rays) 
of about 1,500,000 times uranium, it follows that the number of 
a particles projected from 1 gram of thorium or uranium is only 
7 x 107 of the number from radium. 
In the following table are given the probable number of « 
particles projected per second and the rate of emission of energy 
in the form of a particles from 1 gram of each of the three radio- 
elements. 
