_Secondly._ Some physicists have asserted that a globe of liquid matter radiating its heat into space, would tend to solidify both at the surface and the centre, at the same time. The consequence of this action would be the production of a sphere with a solid external shell and a solid central nucleus, but with an interposed layer in a fluid or semi-fluid condition. It has been pointed out that if we suppose the solidification to have gone so far, as to have caused the partial union of the interior nucleus and the external shell, we may conceive a condition of things in which the stability and rigidity is sufficient to satisfy both geologists and astronomers, but that in still unsolidified pockets or reservoirs, filled with liquefied rock, between the nucleus and the shell, we should have a competent cause for the production of the volcanic phenomena of the globe. In this hypothesis, however, it is assumed that the cooling at the centre and the surface of the globe would go on at such rates that the reservoirs of liquid material would be left at a moderate depth from the surface, so that easy communication could be opened between them and volcanic vents. [Sidenote: REVIEW OF THE SEVERAL HYPOTHESES.] _Thirdly._ It has been maintained that the earth may have become perfectly solid from the centre to the surface. Those who hold this view endeavour to account for the phenomena of volcanoes in one of two ways. It may be, they say, that the deep-seated rock-masses, though actually solid, are in a state of _potential_ liquidity; that though reduced to a solid state by the intense pressure of the superincumbent masses, yet such is the condition of unstable equilibrium in the whole mass, that the comparatively slight movements and changes taking place at the earth's surface suffice to bring about the liquefaction of portions of its crust and consequent manifestations of volcanic energy. But It may be, as other supporters of the doctrine of the earth's complete solidity have maintained, that the phenomena of volcanoes have no direct connection with a supposed incandescent condition of our planet at all, and that there are chemical and mechanical forces at work within our globe which are quite competent to produce at the surface all those remarkable phenomena which we identify with volcanic action.