far as the theory of gravitation carries us, they do guarantee its
permanence; but something more remains to be said on the subject in a
future lecture (XVIII).

And now, finally, we come to a sublime speculation, thrown out by
Laplace, not as the result of profound calculation, like the results
hitherto mentioned, not following certainly from the theory of
gravitation, or from any other known theory, and therefore not to be
accepted as more than a brilliant hypothesis, to be confirmed or
rejected as our knowledge extends. This speculation is the "Nebular
hypothesis." Since the time of Laplace the nebular hypothesis has had
ups and downs of credence, sometimes being largely believed in,
sometimes being almost ignored. At the present time it holds the field
with perhaps greater probability of ultimate triumph than has ever
before seemed to belong to it--far greater than belonged to it when
first propounded.

It had been previously stated clearly and well by the philosopher Kant,
who was intensely interested in "the starry heavens" as well as in the
"mind of man," and who shewed in connexion with astronomy also a most
surprising genius. The hypothesis ought by rights perhaps to be known
rather by his name than by that of Laplace.

The data on which it was founded are these:--Every motion in the solar
system known at that time took place in one direction, and in one
direction only. Thus the planets revolve round the sun, all going the
same way round; moons revolve round the planets, still maintaining the
same direction of rotation, and all the bodies that were known to rotate
on their own axis did so with still the same kind of spin. Moreover,
all these motions take place in or near a single plane. The ancients
knew that sun moon and planets all keep near to the ecliptic, within a
belt known as the zodiac: none strays away into other parts of the sky.
Satellites also, and rings, are arranged in or near the same plane; and
the plane of diurnal spin, or equator of the different bodies, is but
slightly tilted.

Now all this could not be the result of chance. What could have caused
it? Is there any connection or common ancestry possible, to account for
this strange family likeness? There is no connection now, but there may
have been once. Must have been, we may almost say. It is as though they
had once been parts of one great mass rotating as a whole; for if such a
rotating mass broke up, its parts would retain its direct