30. Of Mercury we know still less. He receives seven times as much light and heat as the Earth; is much smaller than the earth, but perhaps more dense; and has not, so far as we can tell, any of the conditions which make animal existence conceivable. If it is so difficult to find suitable inhabitants for Venus, the difficulty for Mercury is immensely greater.
31. So far then, we have traversed the Solar System, and have found even here, the strongest grounds that there can be no animal existence, like that which alone we can conceive as animal existence, except in the planet next beyond the earth, Mars; and there, not without great modifications. But we may make some further remarks on the condition of the several planets, with regard to what appears to us to be the necessary elements of animal life.
FOOTNOTES:
[1] More recently, at the meeting of the British Association in September, 1853, Professor Phillips has declared, that astronomers can discern the shape of a spot on the Moon's surface, which is a few hundred feet in breadth.
[2] A person visiting the Eifel, a region of extinct volcanoes, west of the Rhine, can hardly fail to be struck with the resemblance of the craters there, to those seen in the moon through a telescope.
[3] Bessel has discussed and refuted (it was hardly necessary) the conjecture of some persons (he describes them as "the feeling hearts who would find sympathy even in the Moon") that there may be in the Moon's valleys air enough to support life, though it does not rise above the hills.—Populäre Vorlesungen, p. 78.
[4] The doctrine that the interior nucleus of the Earth is fluid, whether accepted or rejected, does not materially affect this argument. It appears, that in some cases, at least, the melting of substances is prevented, by their being subjected to extreme pressure; but the density, the element from which we reason, is measured by methods quite independent of such questions.
[5] Herschel, 512. Bessel, however, holds that the oblateness of Jupiter proves that his interior is somewhat denser than his exterior. Pop. Vorles. p. 91.
[6] Herschel, 513.
[7] A difficulty may be raised, founded on what we may suppose to be the fact, as to the extreme cold of those regions of the Solar System. It may be supposed that water under such a temperature could exist in no other form than ice. And that the cold must there be intense, according to our notion, there is strong reason to believe. Even in the outer regions of our atmosphere, the cold is probably very many degrees below freezing, and in the blank and airless void beyond, it may be colder still. It has been calculated by physical philosophers, on grounds which seem to be solid, that the cold of the space beyond our atmosphere is 100° below zero. The space near to Jupiter, if an absolute vacuum, in which there is no matter to receive and retain heat emitted from the Sun, may, perhaps, be no colder than it is nearer the Sun. And as to the effect the great cold would produce on Jupiter's watery material, we may remark, that if there be a free surface, there will be vapor produced by the Sun's heat; and if there be air, there will be clouds. We may add, that so far as we have reason to believe, below the freezing point, no accession of cold produces any material change in ice. Even in the expeditions of our Arctic navigators, a cold of 40° below zero was experienced, and ice was still but ice, and there were vapors and clouds as in our climate. It is quite an arbitrary assumption, to suppose that any cold which may exist in Jupiter would prevent the state of things which we suppose.