en. r.] PLANETARY EVOLUTION. 381 



in one direction, but not in the other. Through an enveloping 

 atmosphere the solar rays easily pierce, but return with 

 difficulty. But from the airless surface of the moon the 

 solar radiance must be immediately reflected into space, as 

 from the surface of a polished mirror. Just as on the 

 summits of the Himalayas, where the atmosphere is so rare, 

 the huge snow-masses remain through centuries unmelted, in 

 spite of the sun's blazing heat ; so on the surface or in the 

 deep abysms of the moon, the air and water once frozen must 

 remain frozen forever. 



We have not yet, however, reached a satisfactory inter- 

 pretation of the original disappearance of the lunar atmo- 

 sphere. Granting the disappearance of the atmosphere, the 

 maintenance of a more than arctic cold in spite of the utmost 

 intensity of solar radiation may readily be admitted. But in 

 this explanation the absence of a surface atmosphere is pre- 

 supposed rather than accounted for. Yet I have thought it 

 worth while to introduce the case in this way, as we thus get 

 a more vivid impression of the actual state of things upon 

 the moon. For the original disappearance of the lunar air 

 and water, a far more thoroughgoing explanation was pro- 

 pounded some years since by M. Saemann ; l but in this 

 explanation the extreme cooling of the moon, as just illus- 

 trated, is implicitly involved. According to M. Saemann, 

 the lunar air and water have been literally drunk up by fclus 

 thirsty rocks. On our own globe the tendency of the surface- 

 water is constantly to percolate through the soil of the land 

 or sea-bottom, and thence through the rocks, downward 

 towards the centre of the earth. Yet with our present 

 supply of internal heat, it is not probable that any water can 

 reach more than one hundredth part of the distance towards 

 the earth's centre, without becoming vaporized and thus getting 



1 In a paper on the unity of geological phenomena throughout the solar 

 lystem, translated by Prof. Sterry Hunt, and published in the American 

 journal of Sciet.ce, January, 1862. 



