a name given to burning mountains, or to vents for subterraneous fires.

The number of volcanoes with which we are at present acquainted is very considerable, not much less than 100. In Europe there are Etna, Vesuvius, Hecla, Stromboli, Vulcano; in Asia, one in Mount Taurus, three in Kamtchatka, five in Japan, two in the Philippines, and a great number more scattered through the islands in the South Sea; in Africa, one in Fez, one in the island Bourbon, one in Fuego, one of the Cape Verde islands; and in America several in the Andes, Morne Garou in St Vincent, and two discovered by Captain Cook on the western coast of North America. There are others, but these are best known.

It is remarkable that all the volcanoes with which we are acquainted, four or five perhaps excepted, are situated at a small distance from the sea. Most of them have been burning from time immemorial; some few however have burnt out in our time. Volcanoes all occupy the tops of mountains, we find none of them in plains; some of them indeed, which are situated in the ocean, do not rise much above the surface; but even these volcanoes seem to be the apices of mountains, the greater part of which are covered by the sea. The substances ejected by volcanoes are fixed and inflammable air, water, ashes, pumice stones, stones that have undergone no fusion, and lava. The phenomena which take place during the eruptions of volcanoes have been so fully described already in the articles ETNA, HECLA, ICELAND, and VESUVIUS, that any repetition here would be unnecessary and improper. All that remains, therefore, is to explain the causes of volcanoes, or, to speak more properly, to mention the opinions of philosophers concerning the causes of volcanoes; for the real cause, we are afraid, after all that has been done, remains still unknown. The most elaborate theory that has yet appeared is that of M. Houel.

According to him, water is necessary for the formation of volcanoes. All volcanoes are near the sea; they are even extinguished when the sea retires from them, for we can still perceive the craters of volcanoes in several lofty inland mountains, which discover what they have been formerly. He supposes that a long series of ages was necessary for the formation of a volcano, and that they were all formed under the surface of the sea. The first explosion which laid open the foundations of the deep, would possibly be preceded by a vast globe of burning air, which would issue forth with a tremendous noise, opening at the same time a large and wide vent for the immense flame that was to follow; and which, as it issued from the bottom of the sea, would be spread over its surface by the first gusts of wind which followed. A fire which was to burn through thousands of years, could not be faint or feeble when it was first lighted up. Its first eruptions therefore have undoubtedly been very violent, and the ejected matter very copious. For a long series of ages it would continue to discharge torrents of lava from the bottom. bosom of its native earth; and its first crater would be composed of the fragments of the same earth.

Thus, according to our author, the foundations of the burning mountain would be laid in the bottom of the sea; and even then it would have an hollow cup or crater on the top similar to that which is to be found on all volcanoes at present. But the question now very naturally occurs, by what means was the internal fire preserved from extinction by the waters of the ocean, which must thus have been incumbent upon it? To this he replies, that "The fire, having dispelled the substances in fusion to make an eruption, next laid open the earth, and emitted as much matter as it could discharge, with force sufficient to overcome the resistance of the column of water which would oppose its ascent; but as the strength of the fire diminished, the matter discharged was no longer expelled beyond the mouth; but, by accumulating there, soon closed up the orifice. Thus only small orifices would be left sufficient for giving vent to the vapours of the volcano, and from which only small bubbles of air could ascend to the surface of the water, until new circumstances, such as originally gave occasion to the eruption of the volcano, again took place in the bowels of the earth, and produced new eruptions either through the same or other mouths. The appearance of the sea over the new formed volcano, in its state of tranquillity, would then be similar to what it is betwixt the islands of Basiluzzo and Pariaria. Columns of air-bubbles are there ascending at the depth of more than 30 feet, and burst on their arriving at the surface. This air would continue to disengage itself with little disturbance as long as it issues forth only in small quantity, until, at the very instant of explosion, when prodigious quantities, generated in the burning focus, would make their way all at once, and the same phenomena which originally took place would again make their appearance."

A volcano, while under water, cannot act precisely as it does in the open air. Its eruptions, though equally strong, cannot extend to so great a distance. The lava accumulates in greater quantity round the crater; the sands, ashes, and pozzolano are not carried away by the winds, but are deposited around its edges, and prevent the marine substances which are driven that way by the waters from entering. Thus they agglomerate with these bodies, and thus a pyramidal mount is formed of all the materials together.

In this manner Mr Houel supposes that the mountain was gradually raised out of the sea by the accumulation of lava, &c., at every eruption, and that the cavern of the volcano was gradually enlarged, being driven down into the bottom of the cavern by the continued action of the stones which the volcano is constantly throwing up; that it was there fused, and at last thrown out at the top of the mountain to accumulate on its sides. Mr Houel's opinion about the volcanic fire we shall give in his own words.

"We cannot form any idea of fire subsisting alone, without any pabulum, and unconnected with any other principle. We never behold it but in conjunction with some other body, which nourishes and is consumed by it. The matter in fusion, which issues from the focus, is but the incombustible part of that which nourishes the fire, and into the bosom of which that active principle penetrates in search of pabulum. But as the fire acts only in proportion to the facility with which it can dissolve and evaporate, I am of opinion, that it is only the bottom of the volcano on which it acts; and that its action extends no farther than to keep these substances which it has melted in a constant state of ebullition. That fusible matter being discharged from the mouth of the volcano, and hardening as it is gradually cooled by the action of the air, produces that species of stones which are distinguished by the name of lavas. This lava, even when in the focus, and in a state of fluidity, must also possess a certain degree of solidity, on account of the gravity and density of its particles. It therefore opposes the fire with a degree of resistance which irritates it, and requires, to put it into a state of ebullition, a power proportioned to the bulk of the mass.

"That quantity of matter, when dissolved by the action of the fire, must certainly resemble any other thick substance in a state of ebullition. Small explosions are produced in various parts over the surface of every such substance while in a state of ebullition; and, by the bursting of these bubbles, a great number of small particles are scattered around. This is the very process carried on in the focus of a volcano, though on a scale immensely more large; and the vast explosions there produced expel every body which lies in their way with the utmost violence; nor is there any piece of lava which falls down from the upper part of the arch of weight sufficient to relift this violent centripetal force.

"No estimate can be made of the power of these explosions, but by observing the obstacles they overcome, and the vast quantities of matter which are raised up and thrown to an immense height and distance. Such vast pieces of lava are to be seen on the top of Vesuvius and Lipari, that the projectile force by which they have been thrown out appears altogether incredible. No person can harbour the least suspicion of their having been laid there by any human power; and the appearance of them demonstrates that they have been ejected from the bottom of the volcano, not in a state of fusion, but coherent and solid. A piece of lava lies on the top of Etna of more than a cubic fathom in bulk, and whole weight therefore cannot be less than 16 tons. What an amazing force then must it have required, not only to raise this enormous mass from the volcanic focus, but to make it describe a parabola of about a league in diameter after it had come out of the crater?

"When we consider how much the volcanic focus is sunk below the base of the mountain, that the mountain itself is 10,000 feet high, and that consequently there must have been a power sufficient to raise such a mass 12,000 feet perpendicular, the boldest imagination must be lost in amazement.—This may serve to give us some idea of the nature of that power which operates in the foci of volcanoes; a power which is unknown and inconceivable, and may justly be reckoned among the mysteries of nature."

The pabulum by which the internal fire is supported, Mr Houel thinks to be substances contained in the mountain itself, together with bitumen, sulphur, and other inflammable materials which may from time to time flow into the focus of the volcano in a melted state through subterranean ducts, and the explosions he attributes to water making its way in the same manner. The water is converted into steam, which fills the cavern and pushes the melted lava out at the crater; this opinion is corroborated by the copious smoke which always precedes an eruption. But, combined with the water, there is always a quantity of other substances, whose effects precede, accompany, or follow the eruptions, and produce all the various phenomena which they display. The eruption of water from Etna in the year 1775 proceeded undoubtedly from this cause. The water from the sea, or some of the reservoirs in Etna or the adjacent mountains, by some means discharged a vast quantity of water into the focus of the volcano. That water was instantly resolved into vapour, which instantly filled the whole cavern, and issued from the mouth of the crater. As soon as it made its way into the open atmosphere, it was condensed again into water, which streamed down the sides of the mountain in a dreadful and destructive torrent. Thus we have given a view of Mr Houl's theory, according to which volcanoes originally began at the bottom of the sea; and not only the mountain, but all the adjoining country, was formed by successive eruptions. It is rather a theory of mountains raised by subterranean heat than of volcanoes, and does not attempt to explain the origin of the fire, which is the principal difficulty; neither does his theory account for the immense height to which matters are sometimes thrown during eruptions. This indeed it is impossible to account for, without supposing that the resistance of the air is diminished. The excessive opposition of the atmosphere to bodies moving with very great degrees of velocity has been taken notice of under the article Gunnery. If it has so much effect then upon solid and round globes of iron, what ought it to be on irregular masses of rock, or streams of liquid lava? Nevertheless, in the great eruption of Vesuvius in 1779, Sir William Hamilton informs us, that a vast stream of lava was projected to the height of at least 10,000 feet above the top of the mountain. Had the air resisted this liquid matter as it does a cannon-ball, it must have been dashed in pieces almost as soon as it issued from the crater. Either the extreme heat of the lava, therefore, or some other cause, must have contributed very much to diminish, or rather, in a manner to annihilate the resistance of the atmosphere at that time. As for the lighter materials, though they may be supposed to be carried to a vast distance by the wind, after being projected to a great height in the air, it is inconceivable how their motion was not suddenly checked, and they scattered all around the top of the volcano by the violence of the blast. Substances of this kind, when quickly carried up with smoke, will indeed fly to a great distance; for we are assured, that the ashes of the great fire at London in 1666 were carried by the wind to the distance of 16 miles. It is therefore the less incredible, that those of the great eruption of Vesuvius in 1779 should be carried to the distance of 100 miles, as we are informed was the case.

To account for the volcanic fire, Dr Woodward and others have had recourse to the hypothesis of a central fire, to which the volcanoes are only so many chimneys or spires. Dr Hutton, in his theory of the earth, adopts the same opinion; but as it did not immediately concern the subject of which he treated, he evades any question concerning its origin, by declaring himself satisfied of its existence without any inquiry into its origin.

Others, as Dr Lister, have had recourse to the well-known experiment of the fermentation of sulphur and iron, which will take fire when mixed in considerable quantity, and moistened with water. Pyrites, therefore, which are a natural mixture of these two substances, it is supposed, may naturally give rise to volcanoes. Instances are indeed adduced, which undeniably prove that these substances will spontaneously take fire when thrown together in large heaps. Of this we have a remarkable example in the following anecdote:

"A covetous copperas maker at Deptford having bought up all the pyrites he could find, in order to ruin the trade of his neighbours, collected a vast quantity below a shade in order to secure them from the rain. He was soon, however, punished for his avarice; for the pyrites began to smoke, glowed like red-hot coals, and melted into a kind of vitrified and partly metallic substance, grievously annoying the neighbourhood for a long time with the sulphurous steam they emitted." Beds of pyrites, therefore, taking fire in the earth by means of a fermentation occasioned by water, are now generally supposed to be the cause of volcanoes; and the observation, that volcanoes are generally near the sea, is thought to confirm this hypothesis.

When the matter is properly considered, however, it must be evident, that neither of these hypotheses can answer the purpose. The central fire of Dr Woodward and others is a cause too magnificent even for volcanoes. If any such fire volcanoes is supposed, we must imagine a burning globe in the centre of the earth, whose heat is sufficient to vitrify the most solid flint and refractory terrestrial substances. But of what dimensions are we to suppose this globe? Is it one, two, three, four, or more thousands of miles in diameter?—Very large indeed it must be; for we could scarce suppose that stones could be projected even from the depth of 500 miles into the air. But even this supposition is inadmissible; for as the fire of volcanoes is at times exceedingly augmented from some cause or other, were this cause general, as it must be in case of a burning central globe, the whole number of volcanoes existing on earth would be in a state of eruption at once. Besides, if we were to suppose a burning globe of 7000 miles in diameter to suffer the least dilatation throughout its vast bulk, which must be the undoubted consequence of an augmentation of heat from any unknown cause, all the volcanoes in the world would not be sufficient to give vent to it, though they should spout forth incessant cataclysms of lava for centuries together. A dilatation of the whole globe must therefore undoubtedly take place; and though we should lessen the diameter of our burning globe by 1000 miles, our difficulties will be as far from being removed as before.

Volcanic fire, therefore, cannot originate from any general collection of burning materials dispersed throughout the vast mass of solid earth which lies between the surface and the centre. All the volcanoes at present in an active state would not be such a vent for that fire as a tobacco-pipe would be to a glass bottle furnace. We must have recourse then to some operation by which we know that nature can kindle and extinguish fires occasionally; and if we can suppose such an operation to take place in the bowels of the earth, we may then reasonably conclude, that we have discovered a cause adequate to the production of volcanoes. Such a cause, however, cannot be pyrites, sulphur, or nitre, in any quantity under the surface of the earth. It is impossible that beds of pyrites can remain for fire by thousands of years under the same part of the surface of pyrites, for the earth, be occasionally inflamed and ejected, and afterwards undergo a renovation, in order to enable them to go through a similar operation. Nitre is never found in a fossil state; nor can it be inflamed in such a manner as to make any considerable explosion without a thorough mixture with sulphur and charcoal; neither would all the quantity which we can suppose to exist under the base of any mountain in the world be sufficient to give force to one of those dreadful volleys which are discharged by volcanoes a hundred times in a day. Besides, neither pyrites nor sulphur can be inflamed without access of air; which cannot take place in the bowels of the earth; for it must be remembered, that the first question is concerning the means by which the fire was originally kindled. Most writers, however, seem to overlook this difficulty, and to be solicitous only about the immediate cause of the explosive force, which is generally ascribed to steam of one kind or other. Mr Houl in general calls it the force of fire, or of steam; concerning which he does not enter very particularly into its nature; the cause Mr Whitehurst says, that it is the force of "fire and water, of the earth, which is the primary agent in all such operations of nature."

He also gives a figure, showing how, by means of confined steam, a jet, either of hot water or of liquid fire, may be produced. But this applies only to a particular case, which we cannot suppose always to happen; but volcanoes are constantly attended with explosions; nay, in great is the tendency of volcanic matters to this violent operation, that matters to many stones have been observed to burst in the air like bombs. Volcano bombs, after they were thrown out of the volcano; and Mr Houd even informs us, that such have burst three times during their flight. Water therefore cannot be always the cause of volcanic explosions. When thrown upon melted lead, felts, or especially copper, it explodes indeed with vast force. With the last mentioned metal it is peculiarly and incredibly violent; inasmuch, that it is said that furnaces have been burst, and buildings thrown down, by the mere circumstance of some of the workmen spitting among the melted metal; and Mr Whitehurst calculates the force of aqueous steam, when thus suddenly and violently heated, to be no less than 28 times stronger than inflamed gunpowder.

Many philosophers attempt to account for the origin and continuance of volcanoes by the agency of the electric fluid; but their theory is so ill supported by facts, that we think it would be improper at present to take up room with detailing it. It is certain that volcanoes exhibit many electrical appearances, and that great quantities of the electrical fluid are discharged at every eruption. But our knowledge of electricity is still too limited to draw any certain conclusion from these appearances.