22 THE PRESIDENTIAL ADDRESS. 
reasons it may be very difficult, or even impossible, to produce helium 
from hydrogen under laboratory conditions. 
lf this view of the great héat emission in the formation of helium 
be correct, it is clear that the helium nucleus is the most stable of all 
nuclei, for an amount of energy corresponding to three or four « par- 
ticles would be required to disrupt it into its components. In addition, 
since the mass of the proton in nuclei is nearly 1.000 instead of its mass 
1.0072 in the free state, it follows that much more energy must be put 
into the atom than will be liberated by its disintegration into its ultimate 
units. At the same time, if we consider an atom of oxygen, which 
may be supposed to be built up of four helium nuclei as secondary units, 
the change of mass, if any, in its synthesis from already formed helium 
nuclei is so small that we cannot yet be certain whether there will be 
a gain or loss of energy by its disintegration into helium nuclei, but in 
any case we are certain that the magnitude of the energy will be much 
less than for the synthesis of helium from hydrogen. Our information 
on this subject of energy changes in the formation or disintegration 
of atoms in general is as yet too uncertain and speculative to give any 
decided opinion on future possibilities in this direction, but I have 
endeavoured to outline some of the main arguments which should be 
taken into account. 
I must now bring to an end my survey, I am afraid all too brief 
and inadequate, of this great period of advance in physical science. 
In the short time at my disposal it has been impossible for me, even 
if I had the knowledge, to refer to the great advances made during 
the period under consideration in all branches of pure and applied 
science. I am well aware that in some departments the progress made 
may justly compare with that of my own subject. In these great 
additions to our knowledge of the structure of matter every civilised 
nation has taken an active part, but we may be justly proud that this 
country has made many fundamental contributions. With this 
country I must properly include our Dominions overseas, for they have 
not been behindhand in their contributions to this new knowledge. 
It is, 1 am sure, a matter of pride’ to this country that the scientific 
men of our Dominions have been responsible for some of the most 
fundamental discoveries of this epoch, particularly in radioactivity. 
This tide of advance was continuous from 1896, but there was an 
inevitable slackening during the War. It is a matter of good omen 
that, in the last few years, the old rate of progress has not only been 
maintained but even intensified, and there appears to be no obvious sign 
that this period of great advances has come to an end. There has never 
been a time when the enthusiasm of the scientific workers was greater, 
or when there was a more hopeful feeling that great advances were 
imminent. This feeling is no doubt in part due to the great improve- 
