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THE PRESIDENTIAL ADDRESS. 21 
It may be urged that the artificial disintegration of certain elements 
by bombardment with swift « particles gives definite evidence of a 
store of energy in some of the ordinary elements, for it is known that 
a few of the hydrogen nuclei, released from aluminium for example, 
are expelled with such swiftness that the particle has a greater indi- 
vidual energy than the « particle which causes their liberation. Un- 
fortunately, it is very difficult to give a definite answer on this point 
until we know more of the details of this disintegration. 
On the other hand, another method of attack on this question has 
become important during the last few years, based on the comparison 
of the relative masses of the elements. This new point of view can 
best be illustrated by a comparison of the atomic masses of hydrogen 
and helium. As we have seen, it seems very probable that helium is 
not an ultimate unit in the structure of nuclei, but is a very close com- 
bination of four hydrogen nuclei and two electrons. The mass of the 
helium nucleus, 4.00 in terms of O=16, is considerably less than the 
mass 4.03 of four hydrogen nuclei. On modern views there is believed 
to be a very close connection between mass and energy, and this loss 
in mass in the synthesis of the helium nucleus from hydrogen nuclei 
indicates that a large amount of energy in the form of radiation has 
been released in the building of the helium nucleus from its components. 
It is easy to calculate from this loss of mass that the energy set free 
in forming one gramme of helium is large even compared with that 
liberated in the total disintegration of one gramme of radium. For 
example, calculation shows that the energy released in the formation 
of one pound of helium gas is equivalent to the energy emitted in the 
complete combustion of about eight thousand tons of pure carbon. 
It has been suggested by Eddington and Perrin that it is mainly to 
this source of energy that we must look to maintain the heat emission 
of the sun and hot stars over long periods of time. Calculations of 
the loss of heat from the sun show that this synthesis of helium 
need only take place slowly in order to maintain the present rate of 
radiation for periods of the order of one thousand million years. It 
must be acknowledged that these arguments are somewhat speculative 
in character, for no certain experimental evidence has yet been obtained 
that helium can be formed from hydrogen. 
The evidence of the slow rate of stellar evolution, however, certainly 
indicates that the synthesis of helium, and perhaps other elements of 
higher atomic weight, may take place slowly in the interior of hot stars. 
While in the electric discharge through hydrogen at low pressure we 
can easily reproduce the conditions of the interior of the hottest star 
as far as regards the energy of motion of the electrons and hydrogen 
nuclei, we cannot hope to reproduce that enormous density of radiation 
which must exist in the interior of a giant star. For this and other 
