Page:The evolution of worlds - Lowell.djvu/80

54 Thus we are sure that free hydrogen exists in large quantities in the atmospheres of the two outermost planets and most so in the one farthest off. Helium, too, apparently is there, and other gases which in part may be those of long-period stars, decadent suns, in part substances we do not know.

From the fact that these bands are not present in the Sun and apparently in no type of stars, we may perhaps infer that the substances occasioning them are not elements but compounds to us unknown. And from the fact that free hydrogen exists there alongside of them, and apparently helium, too, we may further conclude that they are of a lighter order than can be retained by the Earth.

But now, we may ask, why should these lighter gases be found where they are? It cannot be in consequence simply of the kinetic theory of gases from which a corollary shows that the heaviest bodies would retain their gases longest, because the strange gases are not apportioned according to the sizes of their hosts. Jupiter, by all odds the biggest in mass, has the least, and Saturn, the next weightiest, the next in amount. Nor can title to such gaseous ownership be lodged in the planet's present state. For though Jupiter is the hottest and Saturn the next so, the increased mass more than makes up in restraint what increased temperature adds in molecular volatility—as we perceive in the cases of the Sun and Earth.