_Nebular Hypothesis._--The Nebular Hypothesis, which was first, as it
were, tentatively put forward by Laplace as a note in his _Systeme du
Monde_, supposes the solar system to have been a flat, disk-shaped
nebula at a high temperature in rapid rotation. In cooling it
condensed, leaving revolving rings at different distances from the
centre. These themselves were supposed to condense into the nucleus
for a rotating planet, which might, in contracting, again throw off
rings to form satellites. The speculation can be put in a really
attractive form, but is in direct opposition to many of the actual
facts; and so long as it is not favoured by those who wish to maintain
the position of astronomy as the most exact of the sciences--exact in
its facts, exact in its logic--this speculation must be recorded by
the historian, only as he records the guesses of the ancient Greeks--as
an interesting phase in the history of human thought.

Other hypotheses, having the same end in view, are the meteoritic
hypothesis of Lockyer and the planetesimal hypothesis that has been
largely developed in the United States. These can best be read in the
original papers to various journals, references to which may be found
in the footnotes of Miss Clerke's _History of Astronomy during the
Nineteenth Century_. The same can be said of Bredichin's hypothesis of
comets' tails, Arrhenius's book on the applications of the theory of
light repulsion, the speculations on radium, the origin of the sun's
heat and the age of the earth, the electron hypothesis of terrestrial
magnetism, and a host of similar speculations, all combining to throw
an interesting light on the evolution of a modern train of thought
that seems to delight in conjecture, while rebelling against that
strict mathematical logic which has crowned astronomy as the queen of
the sciences.