established, some are probable, some have a sort of plausibility,
others--to which we have refrained from giving assent--may
possibly be true; but none are irretrievably beyond the
jurisdiction of scientific tests. No suggestion has so far been
broached which a very little further increase of our scientific
knowledge may not show to be either eminently probable or
eminently improbable. We have kept pretty clear of mere
subjective guesses, such as men may wrangle about forever without
coming to any conclusion. The theory of the nebular origin of our
planetary system has come to command the assent of all persons
qualified to appreciate the evidence on which it is based; and
the more immediate conclusions which we have drawn from that
theory are only such as are commonly drawn by astronomers and
physicists. The doctrine of an intermolecular and interstellar
ether is wrapped up in the well-established undulatory theory of
light. Such is by no means the case with Sir William Thomson's
vortex-atom theory, which to-day is in somewhat the same
condition as the undulatory theory of Huyghens two centuries ago.
This, however, is none the less a hypothesis truly scientific in
conception, and in the speculations to which it leads us we are
still sure of dealing with views that admit at least of definite
expression and treatment. In other words, though our study of the
visible universe has led us to the recognition of a kind of
unseen world underlying the world of things that are seen, yet
concerning the economy of this unseen world we have not been led
to entertain any hypothesis that has not its possible
justification in our experiences of visible phenomena.

We are now called upon, following in the wake of our esteemed
authors, to venture on a different sort of exploration, in which