ed eye; we
must also believe that there are single stars so enormous that though
removed to this same distance they remain visible. If we accept the
other alternative, and say that many nebulae are no further off than our
own stars of the eighth magnitude; then it is requisite to say that at
a distance not greater than that at which a single star is still
faintly visible to the naked eye, there may exist a group of a hundred
thousand stars which is invisible to the naked eye. Neither of these
suppositions can be entertained. What, then, is the conclusion that
remains? This only:--that the nebulae are not further from us than parts
of our own sidereal system, of which they must be considered members;
and that when they are resolvable into discrete masses, these masses
cannot be considered as stars in anything like the ordinary sense of
that word.[12]

And now, having seen the untenability of this idea, rashly espoused by
sundry astronomers, that the nebulae are extremely remote galaxies; let
us consider whether the various appearances they present are not
reconcilable with the Nebular Hypothesis.

* * * * *

Given a rare and widely-diffused mass of nebulous matter, having a
diameter, say, of one hundred times that of the Solar System,[13] what
are the successive changes that may be expected to take place in it?
Mutual gravitation will approximate its atoms or its molecules; but
their approximation will be opposed by that atomic motion the resultant
of which we recognize as repulsion, and the overcoming of which implies
the evolution of heat. As fast as this heat partially escapes by
radiation, further approximation will take place, attended by further
evolution of heat, and so on continuously: the processes not occurring
separately as here described, but simultaneously, uninterruptedly, and
with increasing activity. When the nebulous mass has reached a
particular stage of condensation--when its internally-situated atoms
have approached to within certain distances, have generated a certain
amount of heat, and are subject to a certain mutual pressure,
combinations may be anticipated. Whether the molecules produced be of
kinds such as we know, which is possible, or whether they be of kinds
simpler than any we know, which is more probable, matters not to the
argument. It suffices that molecular unions, either between atoms of the
same kind or between atoms of different kinds, will finally ta