EVOLUTION OF THE UNIVERSE. 597 
perhaps be called upon to account for this 1/500th part of the cosmic rays 
which possess this great penetrating power. In other words, if one prefers 
to go no further than to seek an answer to the question, What processes 
are able to take place in Nature that are appropriate for supplying the 
energy actually found in the cosmic rays? then the answer is, atom- 
building taking place in interstellar space for supplying more than 99°5 
per cent. of the rays, and proton-, or nucleus-, annihilation for supplying the 
remainder. If we assume them both, then the energy-conditions repre- 
sented in Einstein’s equation can easily be satisfied. 
The reasons, however, for such proton-annihilation in interstellar space 
are not so cogent as are those for atom-building. These latter reasons, 
apart from the direct experimental evidence, may be stated thus. We 
know that all the atoms are actually built up out of hydrogen, and it is 
therefore natural to assume, even apart from direct experimental evidence, 
that somewhere or other the process is going on now. Secondly, Bowen 
has shown that the reason so-called forbidden spectroscopic lines appear 
in the nebulae and not on earth is that, given a long enough time free from 
collisions, an atomic change will take place that cannot take place where 
atomic collisions are frequent. It is not a dissimilar hypothesis, and one 
concordant with modern wave mechanical thinking, too, that a cluster 
of four hydrogen atoms free from collisions for a long enough time will jump 
over a potential wall and find themselves together in the nucleus. The 
reason this does not take place in the atmospheres of the stars, or even 
on earth, is presumably that temperature and density, that is, energy 
and frequency of collisions, destroy the clusters or prevent altogether 
their formation. But at low enough temperatures, under the influence 
of ordinary molecular forces, these clusters must form ; for what is lique- 
faction other than the process of formation of such larger molecular groups ? 
It is such clustering, combined with freedom from energetic collisions, 
which, according to the hypothesis herein presented, provides the neces- 
sary condition for occasional atom-building. It might conceivably take 
place in the very remote regions of our atmosphere, but in that case it 
should also take place in the remote regions of the sun’s atmosphere ; 
and the cosmic rays should then be much stronger during the day than 
at night, a result contrary to fact. Atom-building in interstellar space 
is therefore a natural enough hypothesis. Proton-annihilation, on the 
other hand, must take place, if at all, either within the nucleus or through 
the rushing of an enormously energetic electron into the nucleus. These 
acts ought to be either independent of temperature or else facilitated by 
it. The first cannot be true if cosmic rays are to be explained by 
annihilation, else the sun would be enormously influential. The second 
may be true, and annihilation therefore confined to the interiors of stars 
as has been generally assumed. It is therefore very unnatural, if not quite 
impossible, to assume annihilation to take place in interstellar space and 
not in the atmospheres of the suns. This hypothesis, therefore, I should 
not wish to make until every other avenue of escape from our difficulties 
has been closed. If the ideas presented herewith prove correct, they will 
obviously influence strongly not only present theories but also all future 
theories of the origin and destiny of the universe. 
