Now within the crater of Stromboli we have precisely the necessary conditions for the display of the same series of operations. In the apertures at the bottom there exists a quantity of imperfectly fluid materials at a higher temperature, containing water entangled in its mass. As this water passes into the state of steam it tends to escape, and in so doing puts the whole mass into violent movement. When the steam rises to the surface, bubbles are formed, and the formation of these bubbles is promoted by the circumstance that the liquid mass, where exposed to the atmosphere, becomes chilled, and thereby rendered less perfectly fluid. By the bursting of these bubbles the pressure is partially relieved, and a violent escape of the pent-up steam takes place through the whole mass. Equilibrium being thus restored, there follows a longer or shorter interval of quiescence, during which steam is being generated and collected within the mass, and the series of operations which we have described then recommences. There is one other consideration which must be borne in mind in connection with this subject. It is well known that if water be subjected to sufficiently great pressure it may be raised to a very high temperature and still retain its liquid condition. When this pressure is removed, however, the whole mass passes at once into the condition of steam or water-gas; and the gas thus formed at high temperatures has a proportionably high tension. In a Papin's digester water confined in a strong vessel is raised to temperatures far above its ordinary boiling-point, and from any opening in such a vessel the steam escapes with prodigious violence. Now, at considerable depths beneath the earth's surface, and under the pressure of many hundreds or thousands of feet of solid rock, water still retaining its liquid condition may become intensely heated. When the pressure is relieved by the formation of a crack or fissure in the superincumbent mass of rock, the escape of the superheated steam will be of very violent character, and may be attended with the most striking and destructive results. In the existence of high temperatures beneath the earth's surface, and the presence in the same regions of imprisoned water capable of passing into the highly elastic gas which we call steam, we have a cause fully competent to produce all the phenomena which we have described as occurring at Stromboli. It may at first sight appear that the grand and terrible displays of violence witnessed during a great volcanic eruption differ fundamentally in their character and their origin from those feeble outbursts which we are able to examine closely and analyse rigorously at Stromboli. But that such is not the case a few simple considerations will soon convince us. [Sidenote: STROMBOLI COMPARED WITH VESUVIUS.]