In both these accounts we clearly see aëriform agents acting from beneath. The _hissing_ noise of steam escaping from a boiler, convinces of the _nature_ of the body escaping. The _upheaving_ of the melted lava proves, not only, that the agent acts from _beneath_ by expansion, but also, by its resemblance to the common phenomenon observed in boiling liquids, that the agent is formed below, and rises through the melted lava, heaving it up in swells and waves, until it escapes in a gaseous state, like vapor from boiling liquids. We must come to the same conclusion from the experiments, observations, and reports of the celebrated, and intrepid Spallanzani, who visited and examined the crater of the ever-burning Stromboli. His words are nearly these: Fluid lava, resembling melted brass red-hot, and liquid filled the crater to a certain height, and this matter appeared to be influenced by two distinct impelling powers; the one whirling and agitated; and the other upwards, terminating in an explosion like a short clap of thunder. Immediately before the explosion occurred, the lava appeared _inflated_, and _large bubbles_, some several feet in diameter, rose and burst, the detonation followed and the lava sunk. During the rising, _a sound issued from the crater like that produced by a liquid boiling violently in a caldron_. In this case we have every evidence of an _aëriform agent acting from beneath_. An aëriform agent is detected also by examining the _structure_ of volcanic products, which have been ejected in a melted state. They are found to be _vesicular_, _cellular_, and _porous_. This structure proves, incontestibly, that these cavities and cells were filled with an aëriform body, which escaped upon cooling. This position might be sustained by other proofs, but it is unnecessary. It remains only to ask, _whether these elastic agents are sufficient to produce the astonishing amount and products of volcanic action and earthquakes_? The force which elastic agents are known to possess, when generated suddenly, and raised to a high temperature, answers this question promptly in the _affirmative_. A very few grains of gunpowder, when converted into gas by sudden ignition in a gun-barrel, by their expansive force drive a bullet with astonishing power and velocity. A few cubic feet of water converted into steam, will burst the strongest metallic barrier which man can construct, unless it find vent. As we have seen sufficiently clearly that aëriform bodies, as steam and gases, are the elastic agents in producing earthquakes and volcanos, it remains to inquire into the _production_ and _action_ of those agents.