Energy may become dissipated. Where does it go? since if it is
indestructible it must still exist. It is easier to ask the question
than to give a final answer, and it is not possible in this OUTLINE,
where an advanced knowledge of physics is not assumed on the part of the
reader, to go fully into the somewhat difficult theories put forward by
physicists and chemists. We may raise the temperature, say, of iron,
until it is white-hot. If we stop the process the temperature of the
iron will gradually settle down to the temperature of surrounding
bodies. As it does so, where does its previous energy go? In some
measure it may pass to other bodies in contact with the piece of iron,
but ultimately the heat becomes radiated away in space where we cannot
follow it. It has been added to the vast reservoir of _unavailable_ heat
energy of uniform temperature. It is sufficient here to say that if all
bodies had a uniform temperature we should experience no such thing as
heat, because heat only travels from one body to another, having the
effect of cooling the one and warming the other. In time the two bodies
acquire the same temperature. The sum-total of the heat in any body is
measured in terms of the kinetic energy of its moving molecules.
There must come a time, so far as we can see at present, when, even if
all the heat energy of the universe is not radiated away into empty
infinite space, yet a uniform temperature will prevail. If one body is
hotter than another it radiates heat to that body until both are at the
same temperature. Each body may still possess a considerable quantity of
heat energy, which it has absorbed, but that energy, so far as reactions
between those two bodies are concerned, _is now unavailable_. The same
principle applies whatever number of bodies we consider. Before heat
energy can be utilised we must have bodies with different temperature.
If the whole universe were at some uniform temperature, then, although
it might possess an enormous amount of heat energy, this energy would be
unavailable.