philosopher Kant, who published (1755) a fresh theory of the
concentration of scattered particles into fiery worlds. Then
Laplace (1749-1827) took up the speculation, and gave it the form
in which it practically ruled astronomy throughout the nineteenth
century. That is the genealogy of the famous nebular hypothesis.
It did not spring full-formed from the brain of either Kant or
Laplace, like Athene from the brain of Zeus.

Laplace had one great advantage over the early speculators. Not
only was he an able astronomer and mathematician, but by his time
it was known that nebulae, or vast clouds of dispersed matter,
actually existed in the heavens. Here was a solid basis for the
speculation. Sir William Herschel, the most assiduous explorer of
the heavens, was a contemporary of Laplace. Laplace therefore
took the nebula as his starting-point.

A quarter of an ounce of solid matter (say, tobacco) will fill a
vast space when it is turned into smoke, and if it were not for
the pressure of the atmosphere it would expand still more.
Laplace imagined the billions of tons of matter which constitute
our solar system similarly dispersed, converted into a fine gas,
immeasurably thinner than the atmosphere. This nebula would be
gradually drawn in again by gravitation, just as the dust falls
to the floor of a room. The collisions of its particles as they
fell toward the centre would raise its temperature and give it a
rotating movement. A time would come when the centrifugal force
at the outer ring of the rotating disk would equal the
centripetal (or inward) pull of gravity, and this ring would be
detached, still spinning round the central body. The material of
the ring would slowly gather, by gravitation, round some denser