[Footnote 21: Of course there remains the question whether, before the
stage here recognized, there had already been produced a high
temperature by those collisions of celestial masses which reduced the
matter to a nebulous form. As suggested in _First Principles_ (Sec. 136 in
the edition of 1862, and Sec. 182 in subsequent editions), there must,
after there have been effected all those minor dissolutions which follow
evolutions, remain to be effected the dissolutions of the great bodies
in and on which the minor evolutions and dissolutions have taken place;
and it was argued that such dissolutions will be, at some time or other,
effected by those immense transformations of molar motion into molecular
motion, consequent on collisions: the argument being based on the
statement of Sir John Herschel, that in clusters of stars collisions
must inevitably occur. It may, however, be objected that though such a
result may be reasonably looked for in closely aggregated assemblages of
stars, it is difficult to conceive of its taking place throughout our
Sidereal System at large, the members of which, and their intervals, may
be roughly figured as pins-heads 50 miles apart. It would seem that
something like an eternity must elapse before, by ethereal resistance or
other cause, these can be brought into proximity great enough to make
collisions probable.]
[Footnote 22: The two sentences which, in the text, precede the
asterisk, I have introduced while these pages are standing in type:
being led to do so by the perusal of some notes kindly lent to me by
Prof. Dewar, containing the outline of a lecture he gave at the Royal
Institution during the session of 1880. Discussing the conditions under
which, if "our so-called elements are compounded of elemental matter",
they may have been formed, Prof. Dewar, arguing from the known habitudes
of compound substances, concludes that the formation is in each case a
function of pressure, temperature, and nature of the environing gases.]
[Footnote 23: At the date of this passage the established teleology made
it seem needful to assume that all the planets are habitable, and that
even beneath the photosphere of the Sun there exists a dark body which
may be the scene of life; but since then, the influence of teleology has
so far diminished that this hypothesis can no longer be called the
current one.]
[Footnote 24: It may here be mentioned (though the principal
significance of this comes under the next head) that the average mean
distance of the later-discovered planetoids is somewhat greater than
that of these earlier-discovered; amounting to 2.61 for Nos. 1 to 35 and
2.80 for Nos. 211 to 245. For this observation I am indebted to Mr.
Lynn; whose attention was drawn to it while revising for me the
statements contained in this paragraph, so as to include discoveries
made since the paragraph was written.]