The conclusions arrived at by astronomers and physicists is one quite in accord with those which geologists have reached by totally different methods. It is now very generally admitted that if the earth were not a rigid mass, its behaviour under the attract live influences of the surrounding members of the solar system would be very different to what is found to be the case.
ARGUMENTS AGAINST LIQUID INTERIOR.
That the earth is in a solid condition to a great depth from the surface, and possibly quite to the centre, is a conclusion concerning which there can be little doubt; and in the next chapter we shall endeavour to show that such a condition of thirds is by no means incompatible with those manifestations of internal energy, the phenomena of which we are considering in this work. The question, therefore, of the complete solidity of our globe, or of its consisting of a solid and a liquid portion, is one of speculative interest only, and is in no way involved in our investigations concerning the nature and origin of volcanic activity. We may conclude this chapter by enumerating the several hypotheses which have at different times been maintained concerning the nature of the interior of our globe.
First. It has been suggested that the earth consists of a fluid or semi-fluid nucleus surrounded and enclosed in a solid shell. Some have maintained this shell to be of such insignificant thickness, as compared with the bulk of the interior liquid mass, that portions of the latter are able to reach the earth's surface through movements and fractures of the outer shell, and that in this manner volcanic manifestations originate. Others, impressed with the general stability and rigidity of the globe as a whole, have maintained that the outer solid shell must have a very considerable thickness, amounting probably to not less than several hundreds of miles. But through a shell of such thickness it is difficult to conceive of the liquid masses of the interior finding their way to the surface, and those who have held this view are driven to suggest some other means by which local developments of volcanic action might be brought about.
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.
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.
From this summary of the speculative views which have been entertained upon the subject of the physical condition of the earth's interior, it will be clear that at present we have not sufficient evidence for arriving at anything like a definite solution of the problem. The conditions of temperature and pressure which exist in the interior of a globe of such vast dimensions as our earth, are so far removed from those which we can imitate in our experimental enquiries, and it is so unsafe to push the application of laws arrived at by the latter to the extreme limits required by the former, that we shall do well to pause before attempting to dogmatise on such a difficult question.
In the next chapter we shall endeavour to grapple with a somewhat more hopeful task, to point out how far observation and experiment have enabled us to offer a reasonable explanation of the wonderful series of phenomena which are displayed during outbursts of volcanic activity.