agined their
infinite number of atoms as scattered originally, like dust, throughout
space and gradually coming together, as dust does, to form worlds. The
way in which they brought their atoms together was wrong, but the genius
of Democritus had provided the germ of another sound theory to the
students of a more enlightened age. Descartes (1596-1650) recalled the
idea, and set out a theory of the evolution of stars and planets from a
diffused chaos of particles. He even ventured to say that the earth was
at one time a small white-hot sun, and that a solid crust had gradually
formed round its molten core. Descartes had taken refuge in Sweden from
his persecutors, and it is therefore not surprising that that strange
genius Swedenborg shortly afterwards developed the same idea. In the
middle of the eighteenth century the great French naturalist, Buffon,
followed and improved upon Descartes and Swedenborg. From Buffon's work
it was learned by the German 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