310 The Past History of our Moon. [July, 
substances, and notably the various minerals and earthy 
compounds like those which may be supposed to have made 
up the mass of the molten globe, the case is entirely different. 
The numerous and detailed experiments of Charles Deville 
and those of Delesse, besides the earlier ones of Bischof, 
unite in showing that the density of fused rocks is much less 
than that of the crystalline products resulting from their 
slow cooling, these being, according to Deville, from one- 
seventh to one-sixteenth heavier than the fused mass, so 
that if formed at the surface they would, in obedience to the 
laws of gravity, tend to sink as soon as formed.” 
Here it has to be noted that possibly there existed a period 
(for our earth as well as for the moon) during which, not- 
withstanding the relations indicated by Dr. Hunt, the ex- 
terior portions of the moon were solid, while the interior 
remained liquid. A state of things corresponding to what 
we recognise as possible in the sun may have existed. For 
although undoubtedly any liquid matter forming in the sun 
sinks in obedience to the laws of gravity towards the centre, 
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* 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 forming 
* It would still be somewhat denser, because under the circumstances it 
would be somewhat cooler. 
