398 NATURE AND MAN. 



path, — is the identity in elementary composition, not only among 

 the bodies included in our solar system, but throughout the 

 innumerable solid and vaporous masses of which the stellar 

 universe is composed. And it is not a little curious that a link 

 between these two orders should be supplied by those wandering 

 bodies — the Comets — of which many seem to belong to both ; 

 not properly belonging to our system, but presenting themselves 

 within it as occasional visitors from the celestial spaces. Not 

 only does this identity add immensely to the strength of the 

 presumption as to the identity in physical origin of the entire 

 universe, but it also gives an entirely new meaning to the facts 

 previously determined by astronomy in regard to the relative 

 specific gravities of the Sun and Planets. For whilst the Earth 

 weighs niore than five and a half times as much as a globe of 

 water of the same bulk, Mercury rather more in proportion, and 

 Venus and Mars nearly as much, the specific gravity of the Sun 

 is only one-fourth that of the Earth, that of Jupiter a little less, 

 that of Uranus and Neptune only a little above that of water, and 

 that of Saturn so much beloiu it, that if his globe were thrown into 

 water it would float like a cork. Now, so long as nothing what- 

 ever was known about the chemical composition of the heavenly 

 bodies, it might be not -unreasonably surmised that the several 

 planets might be composed of different materials. But now that 

 we have evidence of their identical composition, their differences 

 in density suggest differences in degree of condensation. And 

 this suggestion derives a most remarkable confirmation from the 

 fact, that the greatest density shows itself in those smaller plane- 

 tary bodies which would have cooled the most quickly, and 

 which have therefore more or less nearly reached their final 

 stage ; whilst the least presents itself in the larger masses, whose 

 slower loss of heat would retard their condensation. The smallest 

 planetary body of whose constitution we have any knowledge, 

 — the Moon — is the one whose consolidation is most complete ; 

 even the gases and vapours which form atmospheres round the 

 Earth, Mars, and Venus, being fixed in its solid substance. And 

 of the relative rapidity of its cooling, we have further evidence 

 of the most convincing nature, in the intensity of the former 

 volcanic activity, which shows itself in the multitude of gigantic 



