What is going on within the crater of Stromboli we may perhaps best illustrate by the boiling of a stiff porridge over a hot fire. Any one who has made corn mush over a hot camp fire is fully aware that in proportion as the mush becomes thicker by the addition of the meal, it is necessary to stir the mass with redoubled vigor if anything is to be retained within the kettle. The thickening of the mush increases its viscosity to such an extent that the steam which is generated within it is unable to make its escape unless aided by openings continually made for it by the stirring spoon. If the stirring motion be stopped for a moment, the steam expands to form great bubbles which soon eject the pasty mass from the kettle. For the crater of Stromboli this process is illustrated by the series of diagrams in Fig. 112. As the lava rises toward the surface, presumably as a result of convectional currents within the chimney of the volcano, the contained steam is relieved from pressure, so that at some depth below the surface it begins to separate out in minute vesicles or bubbles, which, expanding as they rise, acquire a rapidly accelerating velocity. Soon they flow together with a quite sudden increase of their expansive energy, and now shooting upward with further accelerated velocity, a layer of liquid lava with its cover of scum is raised on the surface of a gigantic bubble and thrown high into the air. Cooled during their flight, the quickly congealed lava masses become the tuff or volcanic ash which is the material of the cinder cone. [Illustration: FIG. 112.—Diagrams to illustrate the nature of eruptions within the crater of Stromboli.] =Grander volcanic eruptions of cinder cones.=—Most cinder and composite cones, in the intervals between their grander eruptions, if not entirely quiescent, lapse into a period, of light activity during which their crater eruptions appear to be in all essential respects like the habitual explosions within the Strombolian crater. This phase of activity is, therefore, described as _Strombolian_. By contrast, the occasional grander eruptions which have punctuated the history of all larger volcanoes are described in the language of Mercalli as _Vulcanian_ eruptions, from the best studied example. Just what it is that at intervals brings on the grander Vulcanian outburst within a volcano is not known with certainty; but it is important to note that there is an approach to periodicity in the grander eruptions. It is generally possible to distinguish eruptions of at least two orders of intensity greater than the Strombolian phase; a grander one, the examples of which may be separated by centuries, and one or more orders of relatively moderate intensity which recur at intervals perhaps of decades, their time intervals subdividing the larger periods marked off by the eruptions of the first order. [Illustration: