576 DISCUSSION ON THE 
two parts, one of which remains with the atom, while the other goes off 
with the photon. Generally, the former part is many millions of times 
greater than the latter. But the so-called ‘ cosmic radiation’ which falls 
on to the earth from outer space is found to consist of photons whose 
masses are comparable with those of complete atoms. The two most 
massive types of photon so far detected in this radiation are found, to 
within errors of measurement, to have precisely the masses of the helium 
and hydrogen atom respectively. The simplest interpretation is that 
these photons originate out of the complete transformation of atoms of 
hehum and hydrogen into radiation. But this is not the only interpreta- 
tion, nor, I think, the most probable. All atoms consist of protons and 
electrons in equal numbers, so that their masses are (approximately) exact 
multiples of the mass of the hydrogen atom, which contains one of each. 
If an electron and a proton were to neutralise one another in any atom 
whatever, the atomic weight of the atom would be reduced by unity, and 
the atom would eject a photon of mass equal to the hydrogen atom. In 
the same way, if an «-particle were to neutralise itself by combining with 
two electrons in any atom whatever, the resulting photon would have a 
mass equal to the helium atom. 
Although not everyone agrees, this seems to me the most plausible 
interpretation of the two most penetrating constituents of the cosmic 
radiation. In any case, these photons are too massive to originate in any 
less drastic transformation than annihilation. I think they provide 
evidence that matter can be annihilated, and point to a general degradation 
of complex atoms in the direction of simplified structure and decreasing 
atomic weight. 
On this view, electrons and protons must be regarded as concentrated 
stores of energy, which are capable of being set loose and dissipated in the 
form of radiation. The entropy of the universe is increased by this 
process, and the final state of maximum entropy is one in which every 
electron and proton which is capable of annihilation has been annihilated. 
The question at once arises as to how many of the 10” or so electrons 
and protons which form the universe are subject to annihilation. They 
may all be, in which case the final state of the universe will be one in 
which all matter is dissolved into radiation, and nothing remains but 
radiation traversing empty space. Or again, only special types of atoms 
may be liable to annihilation, just as only special types are liable to radio- 
active disintegration. In this case, the final state will be one from which 
these atoms, like the radio-active atoms, will all have disappeared. There 
will be a universe of cold inert matter and of cold radiation. 
Amongst a mass of theoretical possibilities, the one certain fact is 
that if the atoms of our earth are undergoing annihilation, their rate of 
destruction must be exceedingly slow. Their average life must be of the 
order of 10" years at least, otherwise the energy generated by their 
annihilation would make the earth too hot for human habitation. If we 
restrict our range of vision to some 10" years or so, the atoms of the 
earth may be treated as permanent—whatever happens to sun and stars, 
the solid earth will endure. 
This suggests the possibility that annihilation can only occur in types 
of atoms which are not found on earth—possibly atoms of higher atomic 
IP OR 8 age 
