202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Many points of interest arise when we consider the rate at which 
the atoms of radioactive substance disappear. Rutherford has 
shown that whatever be the age of these atoms, the percentage of 
atoms which disappear in one second is always the same; another way 
of putting it is that the expectation of life of an atom is independent 
of its age—that an atom of radium one thousand years old is just as 
likely to live for another thousand years as one just sprung into 
existence. 
Now this would be the case if the death of the atom were due to 
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 com- 
ing 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 temperature of a furnace as at that of liquid 
air—it is not altered by surrounding 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 neighbor- 
ing 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 somewhat improbable that we shall 
be able to split up the atoms of the nonradioactive elements by expos- 
ing them to the radiation from radium; if this radiation is unable to 
affect the unstable radioactive atoms, it is somewhat unlikely that it 
will be able to affect the much more stable nonradioactive elements. 
The evidence we have at present is against a disturbance coming 
from outside breaking up of the radioactive atoms, and we must there- 
fore 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 expectation 
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 num- 
ber of years decrease in a constant proportion for each additional 
year of life, we can easily prove that the expectation of life of an 
