86 ROUGH WAYS MADE SMOOTH. 



sinks in obedience to the laws of gravity towards the centi e, 

 yet the greater heat which it encounters as it sinks must 

 vapourise it, notwithstanding increasing pressure, so that it 

 can only remain liquid near the region where rapid radiation 

 allows of sufficient cooling to produce liquefaction. And in 

 the same way we may conceive that the solidification taking 

 place at any portion of the surface of the moon's or the 

 earth's liquid globe, owing to rapid radiation of heat thence, 

 although it might be followed immediately by the sinking 

 of the solidified matter, would yet result in the continuance 

 (rather than the existence) of a partially solid crust. For 

 the sinking solid matter, though subjected to an increase of 

 pressure (which, in the case of matter expanding on lique- 

 faction, would favour solidification), would nevertheless, 

 owing to the great increase of heat, become liquefied, and, 

 expanding, would no longer be so much denser l than the 

 liquid through which it was sinking as to continue to sink 

 rapidly. 



Nevertheless, it is clear that after a time the heat of the 

 interior parts of the liquid mass would no longer suffice to 

 liquefy the solid matter descending from the surface, and 

 then would commence the process of aggregation at the 

 centre described by Dr. Hunt The matter forming the 

 solid centre of the earth consists probably of metallic and 

 metalloidal compounds of elements denser than those form- 

 ing the known portions of the earth's crust 2 In the case of 

 the moon, whose mean density is very little greater than the 

 mean density of the matter forming the earth's crust, we 



1 It would still be somewhat denser, because under the circumstances 

 it would be somewhat cooler. 



2 It is thus, and not by the effects due to increasing pressure (effects 

 which probably do not increase beyond a certain point), that we are to 

 explain the fact that the earth's density as a whole is about twice the mean 

 density of the matters which form its solid surface. It may be that this con- 

 sideration, supported by the results of recent experimental researches, 

 may give a significance hitherto not -noted to the relatively small mean 

 density of the moon. 



