Vast showers of ashes, hov'ring in the smoke, But vents or orifices in the level surface of the country, sometimes give relief to the explosive forces, as the Geysers in Iceland, where steam is evolved with amazing force, and water thrown up as in artificial fountains, to a great height. In the case of elevated volcanos, the mass is most frequently formed by the gradual accumulation of ejected matter. Violent eruptions are generally attended by earthquakes, and the vast extent of the effect of these, proves that the exciting cause is very deeply seated in the interior of the earth. Thus the shock of the terrible earthquake which destroyed the City of Lisbon, in 1775, was felt throughout the whole of Europe, from Norway to the coast of Africa, in the Madeira isles, and the West Indies; and at the destruction of Lima, in Peru, in 1746, not only was the whole continent of America convulsed, but the shock was propagated across the Atlantic and sensibly perceived on the shores of Europe. Strabo, an ancient Greek geographer, first suggested that volcanos were "safety-valves" to regulate and restrain the action of the explosive forces in the earth's interior; and some geologists have asserted, that it is owing to the protection which these afford, that the earth is not now convulsed as it appears to have been in former periods; while astronomers reconcile us to the horrors and inconveniences of volcanic eruptions, by * Virgil, Æneid, lib. iii. assuring us, that without these " safety-valves," we should in all probability be blown into space, with the shattered fragments of the globe in which the explosive materials are generated: which when recovered from the shock might move in convoy, each in its own course, round the sun, like the little planets Ceres, Pallas, and Juno, whose origin they attribute to a similar catastrophe. It is certain, that where these safety-valves are absent, the surface of the earth has undergone the most considerable derangement; the accumulation of matter round the crater of a volcano, the result of repeated eruptions, is inconsiderable, compared with the mighty upheavings of continents and islands, which our own times and historical records, as well as geological evidence attest. In her efforts to be free, Nature has struggled with the greatest violence. Thus the bed of the ocean has been raised and rent, and islands thrown up high above its level in our own times. The latest instance on record is that of 1831, in the Mediterranean, between Pantellaria and the coast of Sicily, the progress of which was watched by scientific observers. In 1811, in the Azores, a similar creation, effected in the course of a few days-the formation of an island in the Atlantic a mile in circumference, and three hundred feet high-was witnessed from the shores of Saint Michael's. Numbers of such instances are on record in the Azores, the Grecian Archipelago, and the Ionian isles, in contemporary times, and Pliny enumerates several islands, which in his time were known to have arisen from the depths of the Mediterranean, at the birth of which Vulcan and Pluto had the credit of presiding, for in all cases their first appearance was attended by volcanic phenomena. The sudden swelling of an extensive plain in Mexico* and the formation of volcanic mountains, varying, from three to sixteen hundred feet in elevation, where no volcanos had before existed, seems scarcely credible; yet this has happened in modern times and is established on unequivocal testimony: while it is a fact in history that in 1538, a hill four hundred and forty feet high (Monte Nouvo, near Naples) was thrown up in twentyfour hours. The latter instance is near that active vent, Vesuvius; but in Peru, where only one volcano in the whole country is known, great changes have been effected in the surface by internal violence, and scarcely a week passes without an earthquake. Hot springs are seldom absent in volcanic countries, and in the regions of extinct volcanos they are generally abundant. Various hypotheses have been suggested to account for the phenomena of volcanos. Heat, it is well known, is merely an intense chemical action, which may be excited in matter in a variety of ways, independent of actual contact with ignited substances-by electricity, rapid oxidation, friction, and other processes. The brilliant discovery of Sir Humphry Davy of the metallic bases of the earths and alkalis, which has proved that the greater part of the substances constituting the earth, consist of certain delicate metals, united with the all-pervading principle oxygen, has been suggested as affording a clue to the process by which internal heat is generated. These metallic substances, when separated from their kindred element, are so eager to re-unite with it, that the moment oxygen is placed * Jorullo, in 1750. within their reach, they seize it, as it were, with incredible violence, and light and heat of the most intense description attend their embrace. * Now, although potassium, sodium, calcium, and the other metallic bases of the earths, are never found on the exterior of the globe uncombined with oxygen, it was supposed by Sir Humphry Davy, that at great depths in the interior, far removed from the action of the atmosphere, they might exist in a separate state: in which case, the accidental admission of oxygen by the percolation of water, would be attended with the production of that intense heat, whose expansive force we witness in the volcanic energy. He also suggested, that the proximity of volcanos to the sea, seemed to indicate that its waters were concerned in producing the effect: and a careful analysis of the vapours, the sublimations, and the other products of Vesuvius, seemed to confirm his conjecture. It is clear that water is present, from the immense quantity of aqueous vapour, or steam, constantly evolved, but the mere contact of water with the incandescent mass would be sufficient to account for this, without supposing that the heat is generated and maintained by its decomposition. Indeed, if the constant decomposition of water were going on in the manner supposed, water being composed of eight parts of oxygen united with one of hydrogen, when the oxygen is absorbed, an immense volume of hydrogen would be set free, and ought to be evolved either in flame or gas from the crater, in much greater quantities than it is found to be. That the vicinity of *It is this principle by which the intense light of the oxyhydrogen microscope is produced. the ocean is not essential, is proved: for although Ætna and Vesuvius, and Stromboli and Teneriffe, rise near the ocean, there are extensive volcanic regions, comprising an area of two thousand five hundred square miles, in the heart of Asia, at a distance of more than a thousand miles from the nearest sea; and in the central chain of the Andes, in South America, there are raging volcanos, far removed from the coast. To whatever cause, however, they may owe their origin—whether to chemical decomposition, or to a permanent central heat-—the fact is certain, that enormous furnaces, in a state of frightful ignition, exist at intervals, in every part of the earth, at no great distance from its surface, whose action is slowly, but constantly, altering the physical features of the globe; fusing and ejecting from beneath the hardest materials, upheaving mountains on the existing land, and raising islands from the depth of the ocean. Indeed, so extensive is the dominion of this powerful agent, that one is inclined to wonder with the elder Pliny a great Roman naturalist, who lost his life in watching the remarkable eruption of Vesuvius in the year 79-not at the frequency and magnitude of these awful phenomena, but that "a single day should pass without an explosion." Many of these furnaces have gone out at different periods of the earth's history. At an era, historically remote but geologically recent, Europe was illuminated from one end to the other with volcanic fires, compared with which, those of Etna and Vesuvius in modern times, are feeble coruscations. An examination of these deserted forges of Vulcan in Italy, the South of France, and Germany, has presented us with some of the most |