If the public journals, in describing great eruptions, often state the shape of Vesuvius to have undergone an entire change, and if these assertions appear to be confirmed by picturesque views sketched at Naples, the cause of the error consists in the outlines of the margin of the crater having been confounded with those of the cones of eruption accidentally formed in the middle of the crater on its floor or bottom which has been upheaved by vapours. Such a cone of eruption, consisting of loosely heaped-up rapilli and scoriæ, had in the course of the years 1816-1818 gradually risen so as to be seen above the south-eastern margin of the crater; and the eruption of the month of February 1822 augmented it so much, that it even became from 100 to 110 (about 107 to 117 English) feet higher than the north-western margin of the crater (the Rocca del Palo). This remarkable cone, which it had become customary in Naples to regard as the true summit of the mountain, fell in, with a dreadful noise, in the last eruption, on the night of the 22d of October (1822): so that the floor of the crater, which had been constantly accessible since 1811, is now 750 (almost 800 English) feet lower than the northern, and 200 (213 English) feet lower than the southern edge of the volcano. Variations in the form and relative position of the cones of eruption,—the openings of which ought not to be confounded, as they often are, with the crater of the volcano itself,—give to Vesuvius at different epochs a different appearance, which would enable a person well acquainted with the history of the volcano, on a mere inspection of Hackert’s paintings in the palace of Portici, to tell from the outlines of the summit, according as the northern or the southern side of the mountain is represented as the highest, in what year the artist had taken the sketch from which the picture was made.

In the last eruption, in the night of the 23d to the 24th of October, twenty-four hours after the falling in of the great cone of scoriæ which has been mentioned, and when the small but numerous currents of lava had already flowed off, the fiery eruption of ashes and rapilli commenced: it continued without intermission for twelve days, but was greatest in the first four days. During this period the detonations in the interior of the volcano were so violent that the mere concussion of the air, (for no earthquake movement was perceived), rent the ceilings of the rooms in the palace of Portici. In the neighbouring villages of Resina, Torre del Greco, Torre del Annunziata, and Bosche Tre Case, a remarkable phenomenon was witnessed. Throughout the whole of that part of the country the air was so filled with ashes as to cause in the middle of the day profound darkness, lasting for several hours: lanterns were carried in the streets, as has so often been done at Quito during the eruptions of Pichincha. The flight of the inhabitants had never been more general: lava currents are regarded by those who dwell near Vesuvius with less dread than an eruption of ashes, a phenomenon which had never been known to such a degree in modern times; and the obscure tradition of the manner in which the destruction of Herculaneum, Pompeii, and Stabiæ took place, filled the imaginations of men with appalling images.

The hot aqueous vapours which rose from the crater during the eruption and spread themselves in the atmosphere, formed, in cooling, a dense cloud, surrounding the column of fire and ashes, which rose to a height of between nine and ten thousand feet. So sudden a condensation of vapour, and even, as Gay-Lussac has shewn, the formation of the cloud itself, augmented the electric tension. Flashes of forked lightning, issuing from the column of ashes, darted in every direction; and the rolling thunders were distinctly heard, and distinguished from the sounds which proceeded from the interior of the volcano. In no other eruption had the play of the electric forces formed so striking a feature.

On the morning of the 26th of October, a surprising rumour prevailed, to the effect that a torrent of boiling water was gushing from the crater, and pouring down the slope of the cone of ashes. The learned and zealous observer of the volcano, Monticelli, soon discovered that this erroneous rumour had arisen from an optical illusion. The supposed torrent of water was in reality a flow of dry ashes, which, being as loose and moveable as shifting sands, issued in large quantities from a crevice in the upper margin of the crater. The cultivated fields had suffered much from a long-continued drought which had preceded the eruption; towards its close the “volcanic thunder-storm” which has been described produced an exceedingly violent and abundant fall of rain. This phenomenon is associated in all climates with the close of a volcanic eruption. As during the eruption the cone of ashes is generally enveloped in cloud, and as it is in its immediate vicinity that the rain is most violent, torrents of mud are seen to descend from it in all directions, which the terrified husbandman imagines to consist of waters which have risen from the interior of the volcano and overflowed the crater; while geologists have erroneously thought they recognised in them either sea-water or muddy products of the volcano, “Eruptions boueuses,” or, in the language of some old French systematists, products of an igneo-aqueous liquefaction.

Where, as is generally the case in the Andes, the summit of the volcano rises into the region of perpetual snow, (even attaining, in some cases, an elevation twice as great as that of Etna), the melting of the snows renders such inundations as have been described far more abundant and disastrous. The phenomena in question are meteorologically connected with the eruptions of volcanos, and are variously modified by the height of the mountain, the dimensions of that part of it which is always covered with snow, and the extent and degree to which the sides of the cone of cinders become heated; but they are not to be regarded as volcanic phenomena properly so called. Vast cavities also often exist on the slope or at the foot of volcanos which, communicating through many channels with the mountain torrents, form large subterranean lakes or reservoirs of water. When earthquake shocks, which, in the Andes, usually precede all igneous eruptions, convulse the entire mass of the volcano, these subterranean reservoirs are opened, and there issue from them water, fishes, and tufaceous mud. This is the singular phenomenon which brings to light an otherwise unknown fish, the Pimelodes Cyclopum, called by the inhabitants of the highlands of Quito “Preñadilla,” and which I described soon after my return. When, on the night of the 19th of June, 1698, the summit of a mountain situated to the north of Chimborazo, the Carguairazo, above 19000 English feet high, fell in, the country for nearly thirty English geographical square miles round was covered with mud and fishes; and seven years earlier a putrid fever, in the town of Ibarra, was ascribed to a similar eruption of fish from the volcano of Imbaburu.

I recall these facts, because they throw some light on the difference between the eruption of dry ashes and miry inundations of tufa and trass, carrying with them wood, charcoal, and shells. The quantity of ashes emitted by Vesuvius in the recent eruption, like every thing connected with volcanos and other great natural phenomena of a character to excite terror, has been exceedingly exaggerated in the public papers; and two Neapolitan chemists, Vicenzo Pepe and Giuseppe di Nobili, notwithstanding the statements of Monticelli and Covelli to the contrary, even describe the ashes as containing silver and gold. According to the results of my researches and inquiries, the thickness of the bed of ashes formed by the twelve days’ shower was but little above three feet, towards Bosche Tre Case, on the slope of the cone where rapilli were mingled with them; and in the plain, from 15½ to 19 inches at the utmost. Such measurements ought not to be taken in places where the ashes have been heaped up by the action of wind, like drifted snow or sand, or have accumulated from being carried thither by water. The times are passed for seeking only the marvellous in volcanic phenomena, in the manner of the ancients among whom Ctesias made the ashes of Etna to be conveyed as far as the Indian peninsula. There are in Mexico veins of gold and silver in trachytic porphyry; but in the ashes of Vesuvius which I brought back with me, and which an excellent chemist, Heinrich Rose, has examined at my request, no traces of either gold or silver have been discovered.

Although the above mentioned results, which are quite in accordance with the exact observations of Monticelli, differ much from the accounts which have been current during the short interval which has elapsed, it is nevertheless true that the eruption of ashes from Vesuvius from the 24th to the 28th of last October (1822) is the most memorable of any of which we possess an authentic account, since that which occasioned the death of the elder Pliny. The quantity of ashes is, perhaps, three times as great as has ever been seen to fall since volcanic phenomena have been attentively observed in Italy. A stratum of ashes, from 16 to 19 inches thick, appears at first sight insignificant compared with the mass which we find covering Pompeii; but, not to speak of the increase which that mass has probably received by the effects of heavy rains and other causes during the centuries which have since elapsed, and without renewing the animated debate respecting the causes of the destruction of the Campanian towns, and which, on the other side of the Alps, has been carried on with a considerable degree of scepticism, it should here be recalled to recollection that the eruptions of a volcano, at widely separated epochs, do not well admit of comparison, as respects their intensity. All inferences derived from analogy are inadequate where quantitative relations are concerned; as the quantity of lava and ashes, the height of the column of smoke, and the loudness or intensity of the detonations.

From the geographical description of Strabo, and from an opinion given by Vitruvius respecting the volcanic origin of pumice, we perceive that, up to the year of the death of Vespasian, i. e. previous to the eruption which overwhelmed Pompeii, Vesuvius had more the appearance of an extinct volcano than of a Solfatara. When, after long repose, the subterranean forces suddenly opened for themselves new channels, and again broke through the beds of primitive and trachytic rocks, effects must have been produced for which subsequent ones do not furnish a standard. From the well-known letter in which the younger Pliny informs Tacitus of his uncle’s death, it may be clearly seen that the renewal of volcanic outbursts, or what might be called the revival of the slumbering volcano, began with an eruption of ashes. The same thing was observed at Jorullo when, in September 1759, the new volcano, breaking through beds of syenite and trachyte, rose suddenly in the plain. The country-people took flight on finding their huts strewed with ashes which had been emitted from the everywhere opening ground. In the ordinary periodical manifestations of volcanic activity, on the contrary, the shower of ashes marks the termination of each particular eruption. There is a passage in the letter of the younger Pliny which shews clearly that, at a very early stage of the eruption, the dry ashes which had fallen had reached a thickness of four or five feet, without accumulation from drift or other extraneous cause. He writes, in the course of his narrative, “the court which had to be crossed to reach the room in which Pliny was taking his noon-day repose was so filled with ashes and pumice, that, if he had longer delayed coming forth, he would have found the passage stopped.” In an enclosed space like a court, the action of wind in drifting the ashes can scarcely have been very considerable.

I have interrupted my general comparative view of volcanos by a notice of particular observations made on Vesuvius, partly on account of the great interest excited by the recent eruption, and partly on account of those recollections of the catastrophes of Pompeii and Herculaneum, which are almost involuntarily recalled to our minds by the occurrence of any considerable shower of ashes. I have recorded in a note the measurements of height made by myself and others on Vesuvius and in its vicinity.

We have hitherto been considering the structure and mode of action of those volcanos which have a permanent communication with the interior of the Earth by craters. The summits of such volcanos consist of masses of trachyte and lava upheaved by elastic forces and traversed by veins. The permanency of their action gives us reason to infer great complexity of structure. They have, so to speak, an individual character which remains unaltered for long periods of time. Neighbouring mountains often present the greatest differences in their products: leucitic and feldspathic lavas, obsidian with pumice, and masses of basalt containing olivine. They belong to the most recent terrestrial phænomena, breaking through almost all the sedimentary strata, and their products and lava currents are of later origin than our valleys. Their life, if I may permit myself to employ this figurative mode of expression, depends on the manner and permanence of their communications with the interior of the Earth. They often continue for centuries in a state of repose, are then suddenly rekindled, and end by becoming Solfataras, emitting aqueous vapours, gases, and acids; sometimes, however, as in the case of the Peak of Teneriffe, we find that their summit has already become a laboratory of regenerated sulphur; while from the sides of the mountain there still issue large torrents of lava, basaltic in the lower part, but towards the upper part, where the pressure is less,[39] presenting the form of obsidian with pumice.