ure
would be enough to have a serious effect on Mercury, an effect that
would certainly have been noticed. There can, therefore, be no such mass
of matter near the sun, and though there is no doubt a certain number of
meteors do fall into his furnaces day by day, it is not nearly enough to
account for his continuous radiation. It seems after this as if nothing
else could be suggested; but yet an answer has been found, an answer so
wonderful that it is more like a fairy tale than reality.

To begin at the beginning, we must go back to the time when the sun was
only a great gaseous nebula filling all the space included in the orbit
of Neptune. This nebula was not in itself hot, but as it rotated it
contracted. Now, heat is really only a form of energy, and energy and
heat can be interchanged easily. This is a very startling thing when
heard for the first time, but it is known as surely as we know anything
and has been proved again and again. When a savage wants to make a fire
he turns a piece of hard wood very very quickly between his
palms--twiddles it, we should say expressively--into a hole in another
piece of wood, until a spark bursts out. What is the spark? It is the
energy of the savage's work turned to heat. When a horse strikes his
iron-shod hoofs hard on the pavement you see sparks fly; that is caused
by the energy of the horse's leg. When you pump hard at your bicycle you
feel your pump getting quite hot, for part of the energy you are putting
into your work is transformed into heat; and so on in numberless
instances. No energetic action of any kind in this world takes place
without some of the energy being turned into heat, though in many
instances the amount is so small as to be unnoticeable. Nothing falls
to the ground without some heat being generated. Now, when this great
nebula first began its remarkable career, by the action of gravity all
the particles in it were drawn toward the centre; little by little they
fell in, and the nebula became smaller. We are not now concerned with
the origin of the planets--we leave that aside; we are only
contemplating the part of the nebula which remained to become the sun.
Now these particles being drawn inward each generated some heat, so as
the nebula contracted its temperature rose. Throughout the ages, over
the space of millions and millions of miles, it contracted and grew
hotter. It still remained gaseous, but at last it got to an immense
temperature, and is the s