nsation.

If my views are tenable, we should be prepared to find the newer
planets--that is to say, those nearer the Sun--more luminous than those
older and more remote:--and the extreme brilliancy of Venus (on whose
dark portions, during her phases, the Auroras are frequently visible)
does not seem to be altogether accounted for by her mere proximity to
the central orb. She is no doubt vividly self-luminous, although less so
than Mercury: while the luminosity of Neptune may be comparatively
nothing.

Admitting what I have urged, it is clear that, from the moment of the
Sun's discarding a ring, there must be a continuous diminution both of
his heat and light, on account of the continuous encrustation of his
surface; and that a period would arrive--the period immediately previous
to a new discharge--when a _very material_ decrease of both light and
heat, must become apparent. Now, we know that tokens of such changes are
distinctly recognizable. On the Melville islands--to adduce merely one
out of a hundred examples--we find traces of _ultra-tropical_
vegetation--of plants that never could have flourished without immensely
more light and heat than are at present afforded by our Sun to any
portion of the surface of the Earth. Is such vegetation referable to an
epoch immediately subsequent to the whirling-off of Venus? At this epoch
must have occurred to us our greatest access of solar influence; and,
in fact, this influence must then have attained its maximum:--leaving out
of view, of course, the period when the Earth itself was discarded--the
period of its mere organization.

Again:--we know that there exist _non-luminous suns_--that is to say, suns
whose existence we determine through the movements of others, but whose
luminosity is not sufficient to impress us. Are these suns invisible
merely on account of the length of time elapsed since their discharge of
a planet? And yet again:--may we not--at least in certain cases--account
for the sudden appearances of suns where none had been previously
suspected, by the hypothesis that, having rolled with encrusted surfaces
throughout the few thousand years of our astronomical history, each of
these suns, in whirling off a new secondary, has at length been enabled
to display the glories of its still incandescent interior?--To the
well-ascertained fact of the proportional increase of heat as we descend
into the Earth, I need of course, do nothing more than refer:--it comes
in