Hypothesis, to be a philosophic scaffolding to knowledge, must, as Whewell has said, "be close to the facts, and not merely connected with them by arbitrary and untried facts." Yet this appears accepted by Lyell (10th edition, Vol. II., p. 227, and elsewhere); by Phillips ("Vesuvius," pp. 331, 332); by Scrope, if, as I hope, I mistake him not ("Volcanoes," pp. 265, 307-8); though none of these excellent authorities seem either quite clear or quite satisfied with the notion; and in the very passage referred to, Lyell _may_ have possibly a much more philosophic notion in view, where he says: "It is only necessary, in order to explain the action of Volcanoes, to _discover some cause which is capable of bringing about such a concentration of heat as may melt one after the other certain portions of the solid crust_, so as to form seas, lakes or oceans of subterraneous lava." (Vol. II, pp. 226, 227). If by this is meant, that all that is needed to complete a true theory of volcanic action is to discover _an adequate cosmical cause for the heat_--that is to say, a prime mover to which all its phenomena may be traced back, which shall be at once reconcilable with the conditions of our planet as a cooling mass in space and with facts of Vulcanology as they are now seen upon it--then I entirely agree with it. It has been my own object to endeavour to discover and develope that adequate cause in a Paper "On Volcanic Energy, an Attempt to develope its True Nature and Cosmical Relations," read (in abstract) before the Royal Society of London ("Proceedings, Royal Society," Vol. XX., May, 1872), and now (October, 1872) under consideration of Council with a view to publication. I propose concluding this review of the progress of Vulcanology (in which I have had to limit myself to reviewing merely the chief stages of advance towards knowledge of the nature and origin of volcanic heat itself, and have had to pass without notice the vast and important mass of facts and reasonings collected by so many labourers as to its visible phenomena and products, and the still greater mass of speculation, good and bad, on every branch of the subject), by giving a necessarily very brief and imperfect sketch of my own views as in that Paper in part developed. It will first be necessary to retrace our steps a little, in order to gain such a point as shall afford us a fuller view of the whole problem before us. It is not necessary to dilate, even did space allow, upon the many points which bind together Earthquakes and Volcanoes as belonging to the play of like forces. These are generally admitted; and in various ways, more or less obscure, geologists generally have supposed some relations between these and the forces of elevation, which have raised up mountain chains, etc.