diffused, gradually condensed to their
present volume with development of heat and light, and, it may plausibly
be added, with the separation of dependent globes. The data furnished by
spectrum analysis, too, favour the supposition of a common origin for
sun and planets by showing their community of substance; while gaseous
nebulae present examples of vast masses of tenuous vapour, such as our
system may plausibly be conjectured to have primitively sprung from.

But recent science raises many objections to the details, if it supplies
some degree of confirmation to the fundamental idea of Laplace's
cosmogony. The detection of the retrograde movement of Neptune's
satellite made it plain that the anomalous conditions of the Uranian
world were due to no extraordinary disturbance, but to a systematic
variety of arrangement at the outskirts of the solar domain. So that,
were a trans-Neptunian planet discovered, we should be fully prepared to
find it rotating, and surrounded by satellites circulating from east to
west. The uniformity of movement, upon the probabilities connected with
which the French geometer mainly based his scheme, thus at once
vanishes.

The excessively rapid revolution of the inner Martian moon is a further
stumbling-block. On Laplace's view, _no_ satellite can revolve in a
shorter time than its primary rotates; for in its period of circulation
survives the period of rotation of the parent mass which filled the
sphere of its orbit at the time of giving it birth. And rotation
quickens as contraction goes on; therefore, the older time of axial
rotation should invariably be the longer. This obstacle can, however, as
we shall presently see, be turned.

More serious is one connected with the planetary periods, pointed out by
Babinet in 1861.[1163] In order to make them fit in with the hypothesis
of successive separation from a rotating and contracting body, certain
arbitrary assumptions have to be made of fluctuations in the
distribution of the matter forming that body at the various epochs of
separation.[1164] Such expedients usually merit the distrust which they
inspire. Primitive and permanent irregularities of density in the solar
nebula, such as Miss Young's calculations suggest,[1165] do not, on the
other hand, appear intrinsically improbable.

Again, it was objected by Professor Kirkwood in 1869[1166] that there
could be no sufficient cohesion in such an enormously diffused mass as
the planets are su