Page:Popular Science Monthly Volume 20.djvu/57

Rh molten material well out and sometimes flow outside of the crater, with steam escaping, often in considerable quantities; in some a viscid or semi-liquid substance is seen seething, swelling up and forming gigantic bubbles, which burst under occasional great rushes of steam, carrying fragments of the scum-like surface of the liquid high up into the atmosphere, almost precisely as takes place in a pot of boiling mush. At night, "the smaller cracks and larger openings glow with a ruddy light. Every time a bubble bursts, and the crust is broken up by the escape of steam, a fresh, glowing surface of the incandescent material is exposed. If at these moments we look up at the vapor-cloud covering the mountain, we shall at once understand the cause of the singular appearances presented by Stromboli when viewed from a distance at night; for the great masses of vapor are seen to be lit up with a vivid, ruddy glow, like that produced when an engine-driver opens the door of the furnace and illuminates the stream of vapor issuing from the funnel of his locomotive."

These phenomena differ only in degree from those presented by volcanoes which, like Vesuvius, are spasmodically more active. Occasionally, the violence of the outbursts at Stromboli is temporarily increased; and at Vesuvius a series of small explosions, quite similar to those occurring at Stromboli, were observed for some months before the great eruption of 1872. French geologists, in fact, define the conditions of activity of volcanoes by speaking of the "Strombolian" and the "Vesuvian" stage as two degrees between which the passage is by insensible gradations. The eruption of 1872, of which the accompanying view (Fig. 3) is from a photograph, afforded a fine example of the intense Vesuvian stage. The activity of the forces at work within the mountain had been on the increase for more than a year, and reached its climax on the day the photograph was taken. During the eruption the bottom of the crater was entirely broken up, and the sides of the mountain were rent by fissures in all directions whence liquid matter appeared to be oozing as from every part of the surface, or as Professor Palmieri expressed it, "Vesuvius sweated fire." Enormous volumes of steam rushed out from the crater and from some of the fissures, to the height, as shown by the picture, of twenty thousand feet, or nearly four miles, with a prodigious roaring sound that frightened the inhabitants of Naples into leaving their houses and seeking refuge in the streets. The roaring was produced by explosions or detonations rapidly following one another, each of which sent up a globe of white vapor, out of a mass of which globes the overhanging cloud was formed. Lava, or molten rock, rushed down the sides of the mountain in great streams, whence enormous volumes of steam continually rose, forcing the congealing rock as it escaped from it into great bubbles and blisters, thus giving rise to the formation of innumerable miniature volcanoes.

Other phenomena accompanying this eruption were the prevalence