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.