^73-] Condition of the Moon's Surface. 33 



state. Whatever objections suggest themselves to such a 

 view are precisely the objections which oppose themselves 

 to the simple nebular hypothesis, and may be disposed of by 

 those who accept that hypothesis. But better, to my view, 

 it may be reasoned, that the processes of contraction and of 

 the gathering in of matter from without, which maintained 

 the heat of the nebulous masses, operated to produce all the 

 processes of disturbance which brought the moon to her 

 present condition, and that thus there was not necessarily 

 any combustion whatever. Indeed, in any case, combustion 

 can only have commenced when the heat had been so far 

 reduced that any oxygen existing in the lunar spheroid 

 would enter into chemical combination with various com- 

 ponents of the moon's glowing substance. If there were 

 no oxygen (an unlikely supposition, however), the moon's 

 heat would nevertheless have been, maintained so long as 

 meteoric impact: on the one hand, and contraction of the 

 moon's substance on the other, continued to supply the 

 requisite mechanical sources of heat-generation. In this 

 case there would not necessarily have been any gaseous or 

 vapourous matter, other than the matter retained in the 

 gaseous condition by intensity of heat, and becoming first 

 liquid and afterwards solid, so soon as the heat was 

 sufficiently reduced. 



It must here be considered how far we have reason to 

 believe that the heat of the various members of the solar 

 system — including the moon and other secondary bodies — 

 was originally produced, and thereafter maintained, by col- 

 lisions ; because it is clear that, as regards the surface 

 contour of these bodies, much would depend on this circum- 

 stance. There would be a considerable difference between 

 the condition of a body which was maintained at a high 

 temperature for a long period, and eventually cooled, but 

 slowly, under a continual downfall of matter, and that of a 

 body whose heat was maintained by a process of gradual 

 contraction. It is true that in the case of a globe like the 

 earth, whose surface was eventually modelled and re-modelled 

 by processes of a totally different kind, by deposition and 

 denudation, by wind and rain, river-action and the beating 

 of seas, the signs of the original processes of cooling would 

 to a great extent disappear ; but if, as we are supposing in 

 the case of the moon, there was neither water nor air (at 

 least in sufficient quantity to produce any effect corresponding 

 to those produced by air and water on the earth), the prin- 

 cipal features of the surface would depend largely on the 



vol. in. (n.s.) f 



