84 ROUGH WAYS MADE SMOOTH. 



of the earth, having only indicated the differences just 

 touched upon in order to remind the reader at the outset 

 that we have not in ' the moon ' a representation of the 

 earth at any stage of her history. Other and different 

 relations are presented for our consideration, although it 

 may well be that by carefully discussing them we may 

 learn somewhat respecting our earth, as also respecting the 

 past history and future development of the solar system. 



It appears reasonable to regard the moon, after her first 

 formation as a distinct orb, as presenting the same general 

 characteristics that we ascribe to our earth in its primary 

 stage as a planet. In one respect the moon, even at that 

 early stage, may have differed from the earth. I refer to its 

 rotation, the correspondence between which and its revolu- 

 tion may probably have existed from the moon's first 

 formation. But this would not materially have affected the 

 relations with which we have to deal at present. We may 

 apply, then, to the moon the arguments which have been 

 applied to the discussion of the first stages of our earth's 

 history. 



Adopting this view, we see that at the first stage of its 

 existence as an independent planet, the moon must have 

 been an intensely heated gaseous globe, glowing with in- 

 herent light, and undergoing a process of condensation, 

 * going on at first at the surface only, until by cooling it 

 must have reached the point where the gaseous centre was 

 exchanged for one of combined and liquefied matter.' To 

 apply now to the moon at this stage the description which 

 Dr. Sterry Hunt gives of the earth. 'Here commences 

 the chemistry of the moon. So long as the gaseous con- 

 dition of the moon lasted, we may suppose the whole mass 

 to have been homogeneous ; but when the temperature 

 became so reduced that the existence of chemical com- 

 pounds at the centre became possible, those which were 

 most stable at the elevated temperature then prevailing 

 would be first formed. Thus, for example, while compounds 

 of oxygen with mercury, or even with hydrogen, could not 



