boiling temperatures. Dr. Stoney himself holds that carbon dioxide would give the appearances of vapor, frost and snow, which are seen with the telescope; and there are still heavier gases which might be imagined to be present. In any case, the conditions seem quite incompatible with life of our earthly type.
As already hinted, the atmosphere of a world depends on its gravitation. Gases tend to diffuse into space, unless retained by adequate attraction. Our earth can hardly retain so light and mobile a gas as hydrogen; Mars may have difficulty in retaining the less mobile vapor of water; but the gravitation of the moon is too slight to retain any known gas, hence she has no atmosphere and no water. Yet this is not sufficient reason for assuming the absence of life. The surface of the moon is usually considered to have been for a long time in an inert state. If it had been so, the accumulation of meteoric stones and dust during ages would have covered it with a uniform veil. Instead of this, the surface presents much variety of tint and texture, indicating a still continuing geological activity; and some changes in its markings are said to have been observed in recent years. Professor Lapworth, regarding it with a geologist's eye, feels convinced that the moon is an active and living world. The geological activity may be the result of the extremes of temperature which are produced by the regular alternation of a half-month's sunshine and a half-month's darkness. At the same time such extremes might awaken to vital activity elements which behave as dead on this earth.
In contrast to the moon are the very large planets, Jupiter and Saturn. Owing to the high gravitation, the atmosphere of such planets is very dense, and so loaded with opaque particles that we can not see through it to the body of the planet within. But though the body is beyond our scrutiny, we can infer that it is very hot, even at the surface; for if the solar system was formed (as is assumed) by condensation of a nebula, the heat of condensation must be proportionately greater and longer retained in a large world than in a small one. Thus for the purposes of life on these great planets the energy radiating from within may be available, and indeed may largely exceed the energy received from the sun at so great a distance. The satellites of these planets may resemble our moon, except that they receive much less energy from the sun.
Of Uranus and Neptune we know very little; but their large size leads us to suppose that their physical conditions may have some resemblance to those of Jupiter and Saturn.
What can we say of the possibility of life in the sun? The visible surface or photosphere has a temperature so high that even iron exists there as a gas, and almost all chemical compounds known to us may be dissociated. The deeper parts are doubtless far hotter still, with chemical possibilities or impossibilities beyond our comprehension. In
