If, however, careful and exact observations, like those carried on at Stromboli, compel us to reject the popular notions concerning the supposed resemblance between volcanic action and the combustion of sulphur or other substances, they nevertheless suggest analogies with certain other simple and well-known operations. And in pursuing these analogies, we are led to the recognition of some admirable illustrations both of the attendant phenomena and of the true cause of volcanic outbursts. No one can look down on the mass of seething material in violent agitation within the fissures at the bottom of the crater of Stromboli, without being forcibly reminded of the appearances presented by liquids in a state of boiling or ebullition. The glowing material seems to be agitated by two kinds of movements, the one whirling or rotatory, the other vertical or up-and-down in its direction. The fluid mass in this way appears to be gradually impelled upwards, till it approaches the lips of the aperture, when vast bubbles are formed upon its surface, and to the sudden bursting of these the phenomena of the eruption are due. Now if we take a tall narrow vessel and fill it with porridge or some similar substance of imperfect fluidity, we shall be able, by placing it over a fire, to imitate very closely indeed the appearances presented in the crater of Stromboli. As the temperature of the mass rises, steam is generated within it, and in the efforts of this steam to escape, the substance is set in violent movement. These movements of the mass are partly rotatory and partly vertical in their direction; as fresh steam is generated in the mass its surface is gradually raised, while an escape of the steam is immediately followed by a fall of the surface. Thus an up-and-down movement of the liquid is maintained, but as the generation of steam goes on faster than it can escape through the viscid mass, there is a constant tendency in the latter to rise towards the mouth of the vessel. At last, as we know, if heat continues to be applied to the vessel, the fluid contents will be forced up to its edge and a catastrophe will occur; the steam being suddenly and violently liberated from the bubbles formed on the surface of the mass, and a considerable quantity of the material forcibly expelled from the vessel. The suddenness and violence of this catastrophe is easily accounted for, if we bear in mind that the escaping steam acts after the manner of a compressed spring which is suddenly released. Steam is first formed at the bottom of the vessel which is in contact with the fire; but here it is under the pressure of the whole mass of the liquid, and moreover, the viscidity of the substance tends to retard the union of the steam bubbles and their rise to the surface of the mass. But when the pressure is relieved by the bursting of bubbles at the surface, the whole of the generated steam tends to escape suddenly. [Sidenote: ESCAPE OF STEAM-BUBBLES FROM LAVA.]