PRESIDENT’S ADDRESS. 95 
as a, 6, and y rays. Thea rays have been shown by Rutherford to 
be positively electrified atoms of helium, moving with speeds which 
reach up to about one-tenth of the velocity of light. The # rays are 
negatively electrified corpuscles, moving in some cases with very nearly 
the velocity of light itself, while the y rays are unelectrified, and are 
analogous to the Réntgen rays. 
The radio-activity of uranium was shown by Crookes to arise from 
something mixed with the uranium, and which differed sufficiently in 
properties from the uranium itself to enable it to be separated by 
chemical analysis. He took some uranium, and by chemical treat- 
ment separated it into two portions, one of which was radio-active and 
the other not. 
Next Becquerel found that if these two portions were kept for 
several months, the part which was not radio-active to begin with 
gained radio-activity, while the part which was radio-active to begin 
with had lost its radio-activity. These effects and many others receive 
a complete explanation by the theory of radio-active change which we 
owe to Rutherford and Soddy. 
According to this theory, the radio-active elements are not permanent, 
but are gradually breaking up into elements of lower atomic weight; 
uranium, for example, is slowly breaking up, one of the products being 
radium, while radium breaks up into a radio-active gas called radium 
emanation, the emanation into another radio-active substance, and so on, 
and that the radiations are a kind of swan’s song emitted by the atoms 
when they pass from one form to another; that, for example, it is when 
a radium atom breaks up and an atom of the emanation appears that the 
rays which constitute the radio-activity are produced. 
Thus, on this view the afoms of the radio-active elements are not 
immortal: they perish after a life whose average value ranges from 
thousands of millions of years in the case of uranium to a second or so in 
the case of the gaseous emanation from actinium. 
When the atoms pass from one state to another they give out large 
stores of energy, thus their descendants do not inherit:the whole of their 
wealth of stored-up energy, the estate becomes less and less wealthy with 
each generation; we find, in fact, that the politician when he imposes 
death duties is but imitating a process which has been going on for ages 
in the case of these radio-active substances. 
Many points of interest arise when we consider the rate at which the 
atoms of radio-active 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 a 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 
