26 PRESIDENT’S ADDRESS. 
something from outside which struck old and young indiscriminately ; 
in a battle, for example, the chance of being shot is the same for old and 
young; so that we are inclined at first to look to something coming from 
outside as the cause why an atom of radium, for example, suddenly 
changes into an atom of the emanation. But here we are met with the 
difficulty that no changes in the external conditions that we have as yet 
been able to produce have had any effect on the life of the atom; as far 
as we know at present the life of a radium atom is the same at the tem- 
perature of a furnace as at that of liquid air—it is not altered by sur- 
rounding the radium by thick screens of lead or other dense materials 
to ward off radiation from outside, and, what to my mind is especially 
significant, it is the same when the radium is in the most concentrated 
form, when its atoms are exposed to the vigorous bombardment from 
the rays given-off by the neighbouring atoms, as when it is in the most 
dilute solution, when the rays are absorbed by the water which separates 
one atom from another. This last result seems to me to make it some- 
what improbable that we shall be able to split up the atoms of the non- 
radio-active elements by exposing them to the radiation from radium; 
if this radiation is unable to affect the unstable radio-active atoms, it is 
somewhat unlikely that it will be able to affect the much more stable non- 
radio-active elements. 
The evidence we have at present is against a disturbance coming 
from outside breaking up of the radio-active atoms, and we must 
therefore look to some process of decay in the atom itself; but if 
this is the case, how are we to reconcile it with the fact that the expecta- 
tion of life of an atom does not diminish as the atom gets older? We 
can do this if we suppose that the atoms when they are first produced 
have not all the same strength of constitution, that some are more 
robust than others, perhaps because they contain more intrinsic energy 
to begin with, and will therefore have a longer life. Now if when 
the atoms are first produced there are some which will live for one year, 
some for ten, some for a thousand, and so on; and if lives of all 
durations, from nothing to infinity, are present in such proportion that 
the number of atoms which will live longer than a certain number of 
years decrease in a constant proportion for each additional year of life, 
we can easily prove that the expectation of life of an atom will be the 
same whatever its age may be. On this view the different atoms 
of a radio-active substance are not, in all respects, identical. 
The energy developed by radio-active substances is exceedingly large, 
one gramme 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 poten- 
tial difference of about two million volts on its way out of the atom of 
