The phenomena attending an eruption can leave no doubt that below every active volcano a large subterranean cavity must exist in which melted lava accumulates. The partisans of the theory which supposes the earth to consist of a central fluid mass with a solid shell resting upon it, attribute the formation of volcanoes to rents or fissures in this crust through which the lava is cast forth; but the local development of heat by chemical action, or some other unknown cause, is quite sufficient to account for the existence of fiery lakes imbedded in a solid mass, and which, though insignificant when compared with the surface of the globe, may still be large enough to produce volcanic phenomena on the grandest scale. The cause of the reaction of such a reservoir against the surface of the earth must in all probability be sought for in the expansive force of steam; for when water, penetrating through crevices or porous strata, comes in contact with the heated subterranean mass, it is evident that the steam thus generated must press upon the lava, and, when formed in sufficient quantity, ultimately forces it up the duct of the volcano. In other cases, we may suppose a continuous column of lava mixed with liquid water raised to a red-hot, or white-hot, temperature under the influence of pressure. A disturbance of equilibrium may first bring on an eruption near the surface, by the expansion and conversion into gas of the entangled water, so as to lessen pressure. More and more steam would then be liberated, bringing up with it jets of liquid rock, and ultimately ejecting a continuous stream of lava. Its force being spent, a period of rest succeeds, until the conditions for a new outburst (accumulation of steam and melted rock) are obtained, and another cycle of similar changes is renewed. The important part which water plays in volcanic action is moreover sufficiently proved by the enormous quantity of steam which is poured forth during every eruption, or is constantly escaping in the fumaroles of a crater. The various gases (carbonic, muriatic, sulphurous) which are likewise exhaled by volcanoes may also have been rendered liquid by pressure at great depths, and may assist the action of water in causing eruptive outbursts. The great number of active volcanoes on sea-coasts and in islands likewise points to the agency of water in volcanic operations; and in the few cases where eruptive cones are situated far inland, their situation on the borders of a lake, or their cavernous and porous structure, accounts for the absorption of a quantity of atmospheric water, sufficient for the production of volcanic phenomena. [Illustration: ERUPTION OF VESUVIUS, BAY OF NAPLES.] CHAPTER VII. DESTRUCTION OF HERCULANEUM AND POMPEII.