1874.] The Past History of our Moon. 311 
the known portions of the earth’s crust.* 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 
must assume that the matter forming the solid nucleus at 
that early stage was relatively less in amount, or else that 
we may attribute part of the difference to the comparatively 
small force with which lunar gravity operated during various 
stages of contraction and solidification. 
In,the case of the moon, as in that of the earth, before 
the last portions became solidified, there would exist a con- 
dition of imperfect liquidity, as conceived by Hopkins, 
*‘ preventing the sinking of the cooled and heavier particles, 
and giving rise to a superficial crust, from which solidification 
would proceed downwards. There would thus be enclosed 
between the inner and outer solid parts a portion of uncon- 
gealed matter,” which may be supposed to have retained its 
liquid condition to a late period, and to have been the princi- 
pal seat of volcanic action, whether existing in isolated 
reservoirs or subterranean lakes, or whether, as suggested 
by Scrope, forming a continuous sheet surrounding the solid 
nucleus. 
Thus far we have had to deal with relations more or less 
involved in doubt. We have few means of forming a satis- 
factory opinion as to the order of the various changes to 
which, in the first stages of her existence as a planet, our 
moon was subject. Nor can we clearly define the nature of 
those changes. In these matters, as with the corresponding 
processes in our earth’s case, there is much room for variety 
of opinion. 
But few can doubt ee by whatever processes such ‘con- 
dition may have been attained, the moon, when her surface 
began to form itself into its present appearance, consisted 
of a globe partially molten surroundéd by a crust at least | 
partially solidified. Some portions of the actual surface 
may have remained liquid or viscous later than others; but 
at length the time must have arrived when the radiating 
surface was almost wholly solid. It isfrom this stage that 
we have to trace the changes which have led to the present 
condition of the moon’s surface. 
It can scarcely be questioned that those seismologists are 
in the right who have maintained in recent times the theory 
* It is thus, and not by the effets due to increasing pressure (effets which 
do not increase beyond a certain point) that we are to explain the fac 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 consideration, supported by the 
results of recent experimental researches, may give a significance hitherto not 
noted to the relatively small mean density of the moon. 
