It is the weight of these several objections that has led geologists in recent years to regard with greater favour those hypotheses which seek to account for the production of high temperatures within parts of the earth's crust, without having recourse to a supposed incandescent nucleus. If it can be shown that there are any chemical or mechanical forces at work within the crust of the globe which are capable of producing local elevations of temperature, then we may conceive of a condition of things existing in the earth's interior which is free from the objections raised by the astronomer on the score of the earth's proved rigidity, and by the geologist on the ground of its general stability, and which at the same time seems to harmonise better with the observed facts of the distribution of temperature within the earth's crust. How far the existence of such chemical and mechanical agencies capable of producing high temperatures within the crust of the globe have been substantiated, we have already endeavoured to point out. It must be admitted, then, that the questions of the nature of the earth's interior and the cause of the high temperatures which produce volcanic phenomena, are still open ones. We have not yet got beyond the stage of endeavouring to account for the facts observed by means of tentative hypotheses. Some of these, as we have seen, agree with the facts, so far as they are at present known, much better than others; but the decision between them or the rejection of the whole of them in favour of some new hypothesis, must depend on the results of future observation and enquiry. It may be well, before leaving this subject, to remark that they are all equally reconcilable with the nebular theory of Kant and Laplace. Granting that the matter composing our globe has passed successively through the gaseous and liquid conditions, it is open to us to imagine the earth as now composed of a liquid nucleus with either a thick or a thin solid shell; of a solid nucleus and a solid shell with more or less liquid matter between them; or, lastly, to conceive of it as having become perfectly solid from the centre to the surface. [Sidenote: CAUSE OF THE PRESENCE OF WATER IN LAVAS.] But it is not upon the existence of a high temperature within certain parts of the earth's crust that the production of volcanic activity alone depends. The presence of water and other liquid and gaseous substances in a state of the most intimate admixture with the fused rock-masses, is, as we have seen, the main cause of the violent displays of energy exhibited at volcanic centres. And We shall now proceed to notice the hypotheses which have been suggested to account for the presence of these liquid and gaseous bodies in the midst of the masses of incandescent materials poured out from volcanic vents.