We made our way to the advancing end of one of the lava-streams (like the "snout" of a glacier), which was 20 ft. high, and moved forwards but slowly, in successive jerks. Two hundred yards farther up, where it issued from the sandy ashes, the lava was white-hot and running like water, but it was not in very great quantity and rapidly cooled on the surface and became "sticky." A cooled skin of slag was formed in this way, which arrested the advancing stream of lava. At intervals of a few minutes this cooled crust was broken into innumerable clinkers by the pressure of the stream, and there was a noise like the smashing of a gigantic store of crockery ware as the pieces or "clinkers" fell over one another down the nearly vertical "snout" of the lava-stream, whilst the red-hot molten material burst forward for a few feet, but immediately became again "crusted over" and stopped in its progress. We watched the coming together and fusion of the two streams and the overwhelming and burning up of several trees by the steadily, though slowly, advancing river of fire. Then we climbed up the ash-cone, getting nearer and nearer to the rim of the crater, from which showers of glowing stones were being shot. The deep roar of the mountain at each effort was echoed from the cliffs of the ancient mother-crater, Monte Somma, and the ground shook under our feet as does a ship at sea when struck by a wave. The night was very still in the intervals. The moon was shining, and a weird melancholy "ritornelle" sung by peasants far off in some village below us came to our ears with strange distinctness. It might have been the chorus of the imprisoned giants of Vulcan's forge as they blew the sparks with their bellows and shook the mountains with the heavy blows of their hammers.
As we ascended the upper part of the cone the red-hot stones were falling to our left, and we determined to risk a rapid climb to the edge of the crater on the right or southern side, and to look into it. We did so, and as we peered into the great steaming pit a terrific roar, accompanied by a shuddering of the whole mountain, burst from it. Hundreds of red-hot stones rose in the air to a height of 400 ft., and fell, happily in accordance with our expectation, to our left. We ran quickly down the sandy side of the cone to a safe position, about 300 ft. below the crater's lip, and having lit our pipes from one of the red-hot "bombs," rested for a while at a safe distance and waited for the sunrise. A vast horizontal layer of cloud had now formed below us, and Vesuvius and the hills around Naples appeared as islands emerging from a sea. The brilliant sunlight was reassuring after this night of strange experiences. The fields and lanes were deserted in the early morning as we descended to the sea-level. On our way we met a procession of weird figures clad in long white robes, enveloping the head closely but leaving apertures for the eyes. They were a party of the lay-brothers of the Misericordia carrying a dead man to his grave. Then we found our carriage, and drove quickly back to Naples and sleep!
In the following March I acted as guide to my friend Professor Huxley in expeditions up Vesuvius, now quiescent, and to the Solfatara. Then suddenly, in April, the great eruption of 1872 burst upon us. On the first day of the outbreak some imprudent visitors were killed by steam and gas ejected by the lava-stream. By the next day the violence of the eruption was too great for any one to venture near it. The crater sent forth no intermittent "puffs" as in the preceding November, but a continuously throbbing jet which produced a cloud five miles high, like an enormous cauliflower in shape, suspended above the mountain and making it look by comparison like a mole-hill. Showers of fine ashes, as in the days of Pompeii, fell thickly around, accumulating to the depth of an inch in a few hours even at my house in Pausilippo, nine miles distant across the bay. I was recovering at the time from an attack of typhoid fever, and lay in bed, listening to the deep humming sound and wondering at the darkness until my doctor came and told me of the eruption. I was able to get up and see from the window the great cauliflower-like cloud and the vacant place where the ash-cone was, but whence it had how been scattered into the sky. (It has been gradually re-formed by later eruptions, of which the last of any size was in 1906.) I could also see steam rising like smoke from a long line reaching six miles down the mountain into the flat country below. It was the great lava-stream which had destroyed two prosperous villages in its course.
After ten days I was able to get about, and drove over to one of these villages and along its main street, which was closely blocked at the end by what looked like a railway embankment some 40 ft. high. This was the side of the great lava-stream now cooled and hardened on the surface. It had sharply cut the houses, on each side of the street, in half without setting them on fire, so that the various rooms were exposed in section—pictures hanging on the walls, and even chairs and other furniture remaining in place on the unbroken portion of the floor. The villagers had provided ladders by which I ascended the steep side of the embankment-like mass at the end of the street, and there a wonderful sight revealed itself. One looked out on a great river seven miles long, narrow where it started from the broken-down crater, but widening to three miles where I stood, and wider still farther on as it descended. This river, with all its waves and ripples, was turned to stone, and greatly resembled a Swiss glacier in appearance. A foot below the surface it was still red-hot, and a stick pushed into a crevice caught fire. It was not safe to venture far on to the pie-crust surface. A couple of miles away the campanile of the church of a village called Massa di Somma stood out, leaning like that of Pisa, from the petrified mass, whilst the rest of the village was overwhelmed and covered in by the great stream.
The curious resemblance of the lava-stream to a glacier arose from the fact that it was almost completely covered by a white snow-like powder. This snow-like powder, which often appears on freshly-run lava, is salt—common sea salt and other mineral salts dissolved in the water ejected as steam mixed with the lava. The steam condenses, as the lava cools, into water and evaporates slowly, leaving the salt as crystals. Often these are not white, but contain iron salt, mixed with the white sodium, potassium, and ammonium chlorides, which gives them a yellow or orange colour. Salts coloured in this way have the appearance of sulphur, and are often mistaken for it. The whole of the interior of the crater of Vesuvius when I revisited it in 1875 was thus coloured yellow, and I have a water-colour sketch of the scene made by a friend who came with me for the purpose. As a matter of fact, though small quantities of the choking gas called "sulphurous acid" are among the vapours given off by Vesuvius, there is no deposit of sulphur there. Some large volcanoes (in Mexico and Japan) have made deposits of sulphur, which are dug for commercial purposes; but the sulphur of Sicily is not, and has not been, thrown out or volatilized by Etna. It occurs in rough masses and in splendid crystals in a tertiary calcareous marine deposit, and its deposition was probably due to a chemical decomposition of constituents of the sea water brought about by minute plants, known as "sulphur bacteria." Whether the neighbouring great volcano had any share in the process seems to be doubtful.
It is generally supposed that sea-water makes its way in large quantity through fissures connected with volcanic channels, and is one of the agents of the explosions caused by the subterranean molten matter. Gaseous water, hydrochloric acid, carbonic acid, hydrofluoric acid, and even pure hydrogen and oxygen and argon are among the gases ejected by volcanoes.
The molten matter forced up from the bowels of the earth and poured out by volcanoes is made up of various chemical substances, differing in different localities, and even in different eruptions of the same volcano. It consists largely of silicates of iron, lime, magnesium, aluminium, and the alkali metals, with possible admixture of nearly every other element. Some volcanoes eject "pitch" or "bitumen." When the molten matter cools, interesting crystals of various "species" (i.e., of various chemical composition) usually form in the deeper part of the mass. The lavas of Vesuvius frequently contain beautiful opaque-white twelve-sided crystals of a siliceous mineral called "leucite." I have collected in the lava of Niedermendig, on the Rhine, specimens embedding bright blue transparent crystals (a mineral called Haüynite) scattered in the grey porous rock. The lava-streams, and even the "roots," of extinct volcanoes which are of great geologic age, sometimes become exposed by the change of the earth's surface, and extensive sheets of volcanic rock of various kinds are thus laid bare. Basalt is one of these rocks, and it not unfrequently presents itself as a mass of perpendicular six-sided columns, each column 10 ft. or more high, and often a foot or more in diameter. The "Giant's Causeway," in the North of Ireland, and the "Pavée des Géants," in the Ardêche of Southern France, are examples both of which I have visited. It is not easy to explain how the molten basalt has come to take this columnar structure on cooling. It has nothing to do with "crystallization," but is similar to the columnar formation shown by commercial "starch" and occasionally by "tabular flint". A theoretical explanation of its formation has been given by Prof. J. Thompson, brother of the late Lord Kelvin.
The varieties of volcanoes and their products make up a long story—too long to be told here. There are from 300 to 400 active craters in Existence to-day—mostly not isolated, but grouped along certain great lines, as, for instance, along the Andean chain, or in more irregular tracks. If we add to the list craters no longer active, but still recognizable, we must multiply it by ten. Vesuvius is the only active volcano on the mainland of Europe—Hecla, Etna, Stromboli, Volcano, and the volcanoes of the Santorin group are on islands. The biggest volcanoes are in South America, Mexico, Java, and Japan. Volcanoes and the related "earthquakes" have been most carefully studied with a view to the safety of the population in Japan. The graceful and well-beloved volcano, Fujiyama, is more than 12,000 ft. high, but, unlike others in those islands, it has been quiescent now for just 200 years. The most violent volcanic eruptions of recent times, with the largest "output" of solid matter, are those of the Soufrière of St. Vincent in 1812, of the Mont Pelée of Martinique in 1902, and of Krakatoa in 1883. A single moderate eruption of the great volcano Mauna Loa, in Hawaii, nearly 14,000 feet high, throws out a greater quantity of solid matter than Vesuvius has ejected in all the years which have elapsed since the destruction of Pompeii. Many hundred millions of tons of solid matter were ejected by Mont Pelée in 1902, when also a peculiar heavy cloud descended from the mountain, hot and acrid, charged with incandescent sand, and rolling along like a liquid rather than a vapour. It burnt up the town of St. Pierre and its inhabitants and the shipping in the harbour. In the eruption of the volcano of St. Vincent in 1812 three million tons of ashes were projected on to the Bahamas Islands, 100 miles distant, besides a larger quantity which fell elsewhere. The great explosion at Krakatoa, lasting two days, blew an island of 1400 ft. high, into the air. A good deal of it was projected as excessively fine needlelike particles of pumice with such force as to carry it up thirty miles into the upper regions of the atmosphere, where it was carried by air currents all over the world, causing the "red sunsets" of the following year. The sky over Batavia, 100 miles distant, was darkened at midday so completely that lamps had to be used—as I heard from my brother who was there at the time. The explosions were heard in Mauritius, 3000 miles away. A sea wave 50 ft. high was set going by the submarine disturbance, and reaching Java and neighbouring islands inundated the land and destroyed 36,000 persons. This wave travelled in reduced size over a vast tract of the ocean, and was observed and recorded at Cape Horn, 7800 miles distant from its seat of origin.