PRESIDENT’S ADDRESS. 27 
radium. The source of this energy is a problem of the deepest interest ; 
if it arises from the repulsion of similarly electrified 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 com- 
parable with the charge on the a particle, could not when they start be 
further apart than the radius of a corpuscle, 10’ cm. 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 moving with this velocity from flying off 
at a tangent are so great that finite charges of electricity could only 
produce them at distances comparable with the radius of a corpuscle. 
One method by which the requisite amount of energy could be obtained 
is suggested by the view to which I have already alluded—thai in the atom 
we have electrified systems of very different types, one small, the other 
large ; the radius of one type is comparable with 10-"cm., 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 radio-active substances, and it is the explosion of these which liberates 
the energy set free during radio-active transformation ? 
The properties of radium have consequences of enormous import- 
ance 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 insig- 
nificant, and that there was nothing to replace the heat which flows from 
the hot interior of the earth to the colder crust. Now when the earth 
first solidified it only possessed a certain amount of capital in the form 
of heat, and if it is continually spending this capital and not gaining 
any fresh heat it is evident that the process cannot have been going 
on for more than a certain number of years, otherwise the earth would 
be colder than it is. Lord Kelvin in this way estimated the age of 
the earth to be less than 100 million years. Though the quantity of 
radium in the earth is an exceedingly small fraction of the mass of 
the earth, only amounting according to the determinations of Professors 
Strutt and Joly to about five grammes in a cube whose side is 100 miles, 
- yet the amount of heat given out by this small quantity of radium is so 
great that it is more than enough to replace the heat which flows from the 
