EVOLUTION OF THE UNIVERSE. 575 
red ink into water and leave them to diffuse into one another. The final 
state, as we know, is one in which they are uniformly mixed to form a 
homogeneous pinkish fluid; thus this state of uniform mixture must be 
the state of maximum entropy. Again, I put a kettle of cold water over 
a hot fire. The final state is one in which the water turns into steam. 
Just as the red ink diffused itself equally through all parts of the water, 
so the heat of the fire tends to diffuse itself equally through coal, kettle 
and water. There is a possible state of this miniature universe in which 
the water is turned to ice and the fire is even hotter than before through 
having abstracted heat from the water. This is one of the configurations 
on the map, and we cannot know for certain that it will not be the end of 
the journey—it is perfectly possible in theory for our kettle to freeze 
when we put it on a hot fire. Yet it is almost infinitely improbable that 
it will do so, because the entropy of the frozen configuration is lower than 
that of the starting-point. 
These two simple instances illustrate a very wide principle—the final 
state of maximum entropy avoids concentration, whether of special 
substances (as with the ink) or of energy (as with the heat of the fire)- 
The ‘commonest state’ is one in which both substance and energy are 
uniformly diffused, just as the commonest state in which we find an 
audience is that in which tall people and short, dark and fair, and so on, 
are uniformly diffused. 
Considerations of this kind show that the universe still has a long way 
to go. It can increase its entropy by distributing its radiant energy more 
uniformly ; at present this is still very far from being uniformly dis- 
tributed. Out in the furthest depths of space, the density of radiant 
energy corresponds to a temperature of less than one degree above absolute 
zero: in the interstellar spaces of the galactic system, three or four 
degrees only; near the earth’s orbit about 280 degrees; at the sun’s 
surface about 6,000 degrees ; at the sun’s centre perhaps 40 or 50 million 
degrees. The entropy is increased by equalising these temperatures : 
that is why energy flows from the sun’s hot centre to its cooler surface, 
and why it then streams out into space, past the earth’s orbit, into the 
cold and dark of interstellar and intergalactic space. There can be no 
end until all these regions are at the same temperature, with radiant 
energy diffused uniformly through space. 
Most radiant energy has its origin in atomic disturbances. Hach atom 
is a sort of storehouse of energy or mass—we now believe the two to be 
identical—and at intervals, either spontaneously or through interference 
from outside, an atom may discharge some of its energy or mass into 
space in the form of radiation. This wanders through space like a sort 
of bullet of radiation travelling with the velocity of light; we call it a 
photon. We see the encounters of these photons with free electrons in 
the Compton effect, and with complete atoms in the more ordinary 
phenomena of absorption of radiation. By studying these we can deduce 
the masses of the photons concerned. 
Ordinary spectroscopy tries to correlate the masses of the photons 
with the atomic changes which gave rise to them, the mass of the photon 
being precisely the mass which the atom lost when it ejected the photon 
—the process of ejection is, in fact, a splitting up of the atomic mass into 
