PROGRESS IN PHYSICS—THOMSON. 203 
atom will be the same whatever its age may be. On this view the 
different atoms of a radioactive substance are not, in all respects, 
identical. 
The energy developed by radioactive substances is exceedingly 
large, 1 gram of radium developing nearly as much energy as would 
be produced by burning a ton of coal. This energy is mainly in the a 
particles, the positively charged helium atoms which are emitted when 
the change in the atom takes place; if this energy were produced by 
electrical forces it would indicate that the helium atom had moved 
through a potential difference of about 2,000,000 volts on its way out 
of the atom of radium. The source of this energy is a problem of 
the deepest interest; if it arises from the repulsion of similarly elec- 
trified systems exerting forces varying inversely as the square of the 
distance, then to get the requisite amount of energy the systems, if 
their charges were comparable with the charge on the a particle, 
could not when they start be farther apart than the radius of a 
corpuscle, 10 em. If we suppose that the particles do not acquire 
this energy at the explosion, but that before they are shot out of the 
radium atom they move in circles inside this atom with the speed 
with which they emerge, the forces required to prevent particles mov- 
ing with this velocity from flying off at a tangent are so great that 
finite charges of electricity could only produce them at distances com- 
parable with the radius of a corpuscle. 
One method by which the requisite amount of energy could be ob- 
tained is suggested by the view to which I have already alluded— 
that in the atom we have electrified systems of very different types, 
one small, the other large; the radius of one type is comparable with 
1078 em., that of the other is about 100,000 times greater. The 
electrostatic potential energy in the smaller bodies is enormously 
greater than that in the larger ones; if one of these small bodies were 
to explode and expand to the size of the larger ones, we should have 
a liberation of energy large enough to endow an a particle with the 
energy it possesses. Is it possible that the positive units of electricity 
were, to begin with, quite as small as the negative, but while in the 
course of ages most of these have passed from the smaller stage to 
the larger, there are some small ones still lingering in radioactive 
substances, and it is the explosion of these which liberates the energy 
set free during radioactive transformation ? 
The properties of radium have consequences of enormous 1m- 
portance to the geologist as well as to the physicist or chemist. In 
fact, the discovery of these properties has entirely altered the aspect 
of one of the most interesting geological problems, that of the age 
of the earth. Before the discovery of radium it was supposed that 
the supplies of heat furnished by chemical changes going on in the 
earth were quite insignificant, and that there was nothing to replace 
