ke place.
When they do take place, they will be accompanied by a sudden and great
disengagement of heat; and until this excess of heat has escaped, the
newly-formed molecules will remain uniformly diffused, or, as it were,
dissolved in the pre-existing nebulous medium.

But now what may be expected by and by to happen? When radiation has
adequately lowered the temperature, these molecules will precipitate;
and, having precipitated, they will not remain uniformly diffused, but
will aggregate into flocculi; just as water, precipitated from air,
collects into clouds. Concluding, thus, that a nebulous mass will, in
course of time, resolve itself into flocculi of precipitated denser
matter, floating in the rarer medium from which they were precipitated,
let us inquire what are the mechanical results to be inferred. Of
clustered bodies in empty space, each will move along a line which is
the resultant of the tractive forces exercised by all the rest, modified
from moment to moment by the acquired motion; and the aggregation of
such clustered bodies, if it eventually results at all, can result only
from collision, dissipation, and the formation of a resisting medium.
But with clustered bodies already immersed in a resisting medium, and
especially if such bodies are of small densities, such as those we are
considering, the process of concentration will begin forthwith: two
factors conspiring to produce it. The flocculi described, irregular in
their shapes and presenting, as they must in nearly all cases,
unsymmetrical faces to their lines of motion, will be deflected from
those courses which mutual gravitation, if uninterfered with, would
produce among them; and this will militate against that balancing of
movements which permanence of the cluster pre-supposes. If it be said,
as it may truly be said, that this is too trifling a cause of
derangement to produce much effect, then there comes the more important
cause with which it co-operates. The medium from which the flocculi have
been precipitated, and through which they are moving, must, by
gravitation, be rendered denser in its central parts than in its
peripheral parts. Hence the flocculi, none of them moving in straight
lines to the common centre of gravity, but having courses made to
diverge to one or other side of it (in small degrees by the cause just
assigned, and in much greater degrees by the tractive forces of other
flocculi) will, in moving towards the central region