30 POPULAR SCIENCE MONTHLY. 



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 in- 

 compatible 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 ade- 

 quate 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, indica- 

 ting 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 naoon 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 sur- 

 face; for if the solar system was formed (as is assumed) by condensa- 

 tion 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 radi- 

 ating 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 resem-- 

 blance 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 



