1874.] The Past History of our Moon. 309 
It appears reasonable to regard the moon, after her first 
formation as a distin¢t orb, as presenting the same general 
characteristics that we ascribe to our earth in its primary 
stage asa 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 revolution 
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, 
“‘soing 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, forexample, while compounds 
of oxygen with mercury, or even with hydrogen, could not 
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, 
asin 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 
VOL. IV. (N.S.) aR 
