It is now generally recognised that the power which forces up lava from a depth of miles through narrow and crooked fissures broken across the solid crust of the globe, is no other than steam, developed in the interior of the lava by vaporisation of water intimately disseminated throughout its substance. I am not aware that previously to the publication of my volume on Volcanos in 1825. But I had derived a conviction of 1822, and a study of Etna and Stromboli in the years 1819–21. This was subsequently confirmed by the discoveries of Sorby, Scheerer, and others, of the existence of water in intimate molecular combination with the crystalline or granular minerals that compose the granitic rocks from the fusion of which lava is supposed to proceed. I had expressed my belief that the aqueous particles so confined, whether in a liquid or a solid state, would upon the occurence of increased heat, or diminished pressure, exert an elastic separating force upon the solid crystals of granules, in the manner of a flux, and thus assist the liquefaction of the mass, and drive it up any fissure that might open for its passage, in a pasty or semi-liquid state, short of complete fusion; so that on reaching the outer air, and parting with the contained steam, together with the heat which the expansion of this element would carry off in a latent from, its surface would consolidate instantly; and the solidification extending rapidly inwards by the formation of crevices giving an outlet to further vapour, the resulting rock would exhibit a cellular or porous structure, and a granular or crystalline texture, in lieu of the compact and glassy one which, if completely fused, it should possess.