We have thus briefly reviewed the chief hypotheses which have been suggested in order to account for the two great factors in all volcanic phenomena--namely, the presence of highly heated rock-masses within the earth's crust, and the existence of various vapours and gases in a state of most intimate mechanical, but not chemical, union with these incandescent materials. It must be admitted that we do not at present appear to have the means for framing a complete and consistent theory of volcanic action, but we may hopefully look forward to the time when further observation and experiment shall have removed many of the existing difficulties which beset the question, and when by the light of such future researches untenable hypotheses shall be eliminated and the just ones improved and established. But if we are constrained to admit that a study of the observed phenomena and established laws of volcanic action have not as yet enabled us to frame any complete and satisfactory theory on the subject, we cannot lose sight of the fact that all modern speculation upon this question appears to be tending in one definite direction. It is every day becoming more and more clear that our earth is bound by ties of the closest resemblance to the other members of that family of worlds to which it belongs, and that the materials entering into their constitution, and the forces operating in all are the same. We have had occasion in a previous chapter to point out that there are the strongest grounds for believing the interior of our globe to consist of similar materials to those found in the small planetary bodies known as meteorites. That the comets are merely aggregations of such meteorites, and that the planets differ from them only in their greater dimensions, may be regarded as among the demonstrated conclusions of the astronomer. The materials found most abundantly in meteorites and in the interior of our globe are precisely the same as those which are proved to exist in an incandescent state in our sun. Hence we are led to conclude that the whole of the bodies of the solar system are composed of the same chemical elements. [Sidenote: ERUPTIVE ACTION IN THE SUN.]