speculative thought of our time that the nebular hypothesis of
Laplace bore to that of the eighteenth century. Outlined in a few words,
it is an attempt to explain all the major phenomena of the universe
as due, directly or indirectly, to the gravitational impact of such
meteoric particles, or specks of cosmic dust, as comets are composed
of. Nebulae are vast cometary clouds, with particles more or less widely
separated, giving off gases through meteoric collisions, internal or
external, and perhaps glowing also with electrical or phosphorescent
light. Gravity eventually brings the nebular particles into closer
aggregations, and increased collisions finally vaporize the entire mass,
forming planetary nebulae and gaseous stars. Continued condensation
may make the stellar mass hotter and more luminous for a time, but
eventually leads to its liquefaction, and ultimate consolidation--the
aforetime nebulae becoming in the end a dark or planetary star.

The exact correlation which Lockyer attempts to point out between
successive stages of meteoric condensation and the various types of
observed stellar bodies does not meet with unanimous acceptance. Mr.
Ranyard, for example, suggests that the visible nebulae may not be
nascent stars, but emanations from stars, and that the true pre-stellar
nebulae are invisible until condensed to stellar proportions. But such
details aside, the broad general hypothesis that all the bodies of the
universe are, so to speak, of a single species--that nebulae (including
comets), stars of all types, and planets, are but varying stages in the
life history of a single race or type of cosmic organisms--is accepted
by the dominant thought of our time as having the highest warrant of
scientific probability.

All this, clearly, is but an amplification of that nebular hypothesis
which, long before the spectroscope gave us warrant to accurately judge
our sidereal neighbors, had boldly imagined the development of stars out
of nebulae and of planets out of stars. But Lockyer's hypothesis does
not stop with this. Having traced the developmental process from the
nebular to the dark star, it sees no cause to abandon this dark star to
its fate by assuming, as the original speculation assumed, that this is
a culminating and final stage of cosmic existence. For the dark star,
though its molecular activities have come to relative stability and
impotence, still retains the enormous potentialities of molar motion;