THE PAST HISTORY OF OUR MOON. 85 



exist, oxides of silicon, aluminium, calcium, magnesium, 

 and iron, might be formed and condensed in a liquid form 

 at the centre of the globe. By progressive cooling still 

 other elements would be removed from the gaseous mass, 

 which would form the atmosphere of the non-gaseous 

 nucleus.' ' The processes of condensation and cooling 

 having gone on until those elements which are not volatile 

 in the heat of our ordinary furnaces were condensed into a 

 liquid form, we may here inquire what would be the result 

 on the mass of a further reduction of temperature. It is 

 generally assumed that in the cooling of a liquid globe of 

 mineral matter congelation would commence at the surface, 

 as in the case of water ; but water offers an exception to 

 most other liquids, inasmuch as it is denser in the liquid 

 than in the solid form. Hence, ice floats on water, and 

 freezing water becomes covered with a layer of ice which 

 protects the liquid below. Some metals and alloys resemble 

 water in this respect. With regard to most other earthy 

 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 Bischoff, 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 



