cles a body contains in proportion to other bodies.
Thus the planet Jupiter is very much larger than the earth, but his
density is less. That does not mean to say that if Jupiter were in one
scale and the earth in the other he would weigh less, because he is so
very much bigger he would outweigh the earth still; his total _mass_
would be greater than that of the earth, but it means that a piece of
Jupiter the same size as a piece of the earth would weigh less under the
same conditions.

Now, before there were any planets at all or any sun, in the place of
our solar system was a vast gaseous cloud called a nebula, which slowly
rotated, and this rotation was the first impulse or force which God gave
it. It was not at all dense, and as it rotated a part broke off, and
inheriting the first impulse, went on rotating too. The impulse would
have sent it off in a straight line, but the pull of gravity from the
nebula held it in place, and it circled round; then the nebula, as it
rotated, contracted a little, and occupied less space and grew denser,
and presently a second piece was thrown off, to become in time another
planet. The same process was repeated with Saturn, and then with the
huge Jupiter. The nebula was always rotating and always contracting. And
as it behaved, so did the planets in their turn; they spun round and
cooled and contracted, and the moons were flung off from them, just as
they--the planets--had been flung off from the parent nebula.

Now, after the original nebula had parted with the mighty mass of
Jupiter, it never again made an effort so great, and for a long time
the fragments that were detached were so small as hardly to be worth
calling planets; they were the asteroids, little lumps and fragments
that the nebula left behind. But as it still contracted in time there
came Mars; and having recovered a little, the nebula with more energy
got rid of the earth, and next Venus, and lastly little Mercury, the
smallest of the eight planets. Then it contracted further, and perhaps
you can guess what the remainder of it is--the sun; and by spinning in a
plastic state the sun, like the earth, has become a globe, round and
comparatively smooth; and its density is now too great to allow of its
losing any more fragments, so, as far as we can see, the solar system is
complete.

This theory of the origin of the planets is called the nebula theory. We
cannot prove it, but there are so many facts that can only be exp