Volcanoes, montes ignovomi (mountains that vomit fire). This is the name for mountains which at certain times belch forth smoke, flames, ash, stones, fiery torrents of melted and vitrified matter, sulphurs, salts, bitumen, and sometimes even water.

Volcanoes , like earthquakes, are caused by subterraneous fires stoked by air and whose force is increased by water. In speaking of earthquakes, I believe I have sufficiently explained the way in which these three agents operate, and the prodigious force they exert; it was shown in that article that the earth is filled with substances proper to kindle and aliment fire; it would therefore be useless to repeat here what has already been said elsewhere; it will suffice to refer the reader to that article. Volcanoes should be regarded as the earth's windows, or as the chimneys via which it rids itself of the substances which devour its bosom. These chimneys provide a free passage to the air and fire which have been set into expansion by the furnaces or fireplaces at their base; without this these agents would cause revolutions on our earth much more terrible than those which we see in earthquakes; they would always be accompanied by the complete subsidence of the countryside. Volcanoes are therefore a kindness of nature; they provide a free passage to fire and air; they stop them from pushing their ravages beyond certain limits, and from completely disrupting the face of our globe. In fact, all parts of the earth are agitated by tremors which are felt from time to time with varying violence. These contusions of the earth indicate immense masses of inflamed substances; it is therefore to give them passage that Providence provided volcanoes in order to ventilate the mine, so to speak. Thus we see that Providence placed volcanoes in all areas of the world: the hottest climates, being the most subject to earthquakes, have a great number of them. Today there are three principal volcanoes in Europe; these are Etna in Sicily, Mount Vesuvius in the kingdom of Naples, and Mount Hecla in Iceland; as each of these volcanoes are described in their own articles, we will only speak here of the general phenomena common to all volcanoes.

There are no phenomena in nature more astonishing than those which these fiery mountains present; whatever ill-informed travellers may say, there seems to be no proof that any mountains exist which perpetually vomit flames: sometimes after violent eruptions, the substances dry up and the volcano ceases to vomit, until a large enough quantity of substances have amassed to ignite a new eruption. Thus the fire will sometimes smoulder for a great many years in deep abysses inside the mountain, waiting until different circumstances put it into action.

The eruptions of volcanoes are normally announced by noises deep under the earth, similar to thunder, by terrible whistlings, by an interior tearing noise: the earth seems to shake to its very foundations: these phenomena last until the air dilated by fire has acquired enough force to vanquish the obstacles holding it in; and then there is an explosion more violent that that of the strongest volley of gunfire: the inflamed substances resembling fireworks are launched in all directions and to an incredible distance, and escape with suddenness from the summit of the mountain. Rock fragments of phenomenal size may be seen which, after having risen to a great height in the air, fall again and roll down the slope of the mountain; the surrounding fields are buried beneath prodigious heaps of ash, of burning sand, of pumice stones; often the sides of the mountain suddenly burst open to let out torrents of blazing liquid matter which then inundate the countryside, and which burn and destroy all trees, buildings and fields in their path.

History teaches us that in two eruptions of Vesuvius that volcano threw out such an amount of ashes that they flew as far as Egypt, Libya and Syria.

In 1600, at Arequipa in Peru, there was an eruption of a volcano which covered all the surrounding terrain for thirty or forty leagues with charred sand and with ash; some areas were covered with a layer two yards deep. The lava spewed forth by Mount Etna sometimes flowed in streams up to 18000 paces long; and the celebrated Borelli has calculated that this volcano , in an eruption in 1669, spewed forth enough matter to fill a space of 93838750 cubic paces. These examples suffice to judge of the phenomenal effects of volcanoes. See the article Lava.

Often volcanoes have been seen to emit from their bosom streams of boiling water, fish, shells and other marine bodies. During an eruption of Vesuvius in 1631 the sea was entirely drained; it seems to have been absorbed by this volcano, which shortly afterwards flooded the countryside with rivers of salty water.

Volcanic eruptions do not always occur with the same degree of violence; this depends on the profusion of inflamed substances, and on the different circumstances which increase or diminish the action of fire.

It has been observed that the majority of volcanoes are placed near to the sea; this position may even contribute to the violence of their eruptions. In fact, when water falls through the cracks in the mountain into the huge quantity of inflamed matter contained there, it cannot fail to produce very lively explosions, but the effects must become even greater when this water is bituminous and contains saline particles. A fairly trivial experiment serves to illustrate this truth: to make embers burn more strongly, cooks sometimes throw on a handful of salt, and the fire becomes much more fierce.

The summits of volcanoes commonly take the form of an inverted cone or funnel; when there is no eruption and when the ashes and rocks surrounding this part of the mountain allow a passage to the summit, a basin filled with sulphur can be seen, boiling in places, and giving off a very strong sulphurous odour and often a thick smoke. This part of the volcano is very subject to change in its appearance, and each eruption makes it present a different face to that which the summit had before; in fact, there are some portions of the mountain which collapse, and the abyss spews forth new substances to replace them. The paths leading to the summit of these mountains are also covered with sal ammoniac, bituminous matter, pumice stones, scoria of lava or of alum etc, and one may also find sources of hot, saline, sulphurous water, with an unbearable smell and taste. At moments preceding eruptions, the substances contained in the basin seem to boil; they sometimes swell to the point where they overflow its rim, and flow down the volcano 's slopes; this does not occur without a terrible din, and without whistlings and tearing noises enough to arouse great fear. It will be easily understood that these substances, in melting, must then form a crust which blocks the passage of air and fire, and thus which must cause an expansion which renews the violence of the eruptions.

Several physicians have held that some form of connection links the different volcanoes on the earth, and in their proximity renders this conjecture fairly probable in the case of Vesuvius and Etna which often inflict their ravages at the same time; in addition we have shown in the article Earthquakes that the conflagrations of the earth seem to propagate themselves over incredible distances by means of subterranean canals.

It sometimes happens that volcanoes, after having repeatedly erupted over many centuries together, finally cease; this results from the fact that the substances which ignite their fires are either at last completely spent, or have taken another route; in fact it has been seen that when certain volcanoes cease to throw out substances, other mountains become volcanoes , and begin to belch forth fire with as much and more fury than those whose place they have taken; thus a great many years ago mount Hecla in Iceland ceased to throw out flames, and another mountain on the same island became a volcano . Different areas of the world present a number of mountains to travellers which once served as chimneys to the earth's conflagrations, as can be judged by the abysses and precipices they display, by the pumice stones, burnt rocks, sulphur, ash, alum, and sal ammoniac with which the surrounding terrain is filled. It seems that some of these volcanoes exercised their ravages in a time of which history has conserved no memory, but an able observer will recognise their existence without difficulty by the presence of substances we have just mentioned, and above all by the layers of lava that the volcanoes expelled from their sides, and which flooded the countryside in their vicinity. See the article Lava. Several mountains of Europe were once volcanoes . Substances have been found in Auvergne which reveal without a doubt that in another age this province was riddled by subterranean fires. The region of Provence named the gorges of Olioule , on the road from Marseille to Toulon, bears marks which announce that there was once a volcano in this part of France. Several other countries would present the same signs, if they were examined more attentively. The description which the famous M. de Tournefort has given of mount Ararat in Armenia, may lead us to presume with great certainty that this mountain is a volcano whose fire has been extinguished; he says that there is an abyss here whose sides are completely vertical, and whose extremities are garnished with rocks of a blackness as if dirtied by smoke; one sees that this description conforms perfectly with the basin of a volcano .

Mountains are not always the seat of eruptions of subterranean fire; sometimes fires, blazing rocks, and pumice stones have been seen to emerge suddenly from the sea bed accompanied by great amounts of sand, ashes, and substances which then formed islands in places where shortly before there was only water; it is in this way that the famous island of Santorin was formed. A similar phenomenon occurred in 1720 near the island St. Michel, in the Azores; on the night of 7th December a prodigious quantity of stones, sand and blazing matter emerged suddenly from the depths of the sea and formed a completely new island next to the first, which this revolution had almost completely overturned. Urbani Hioerne.

The fires contained in the bosom of the earth do not always act with the same violence, often they burn silently, and smoulder, so to speak, under the earth; their presence is only noted by the sources of hot water which are seen to arrive at the surface of the earth, or by bituminous liquids such as petrol and naphtha which the heat causes to sweat out of the rocks and layers of the earth. Thus when digging in the vicinity of Modena one finds a phenomenal amount of petrol floating on the surface of the water.

Sometimes one finds on the surface of the earth areas which burn imperceptibly, so to speak; thus there may be seen in Dauphiné an area of ground which, while not blazing visibly, nevertheless ignites the straw and wood which is thrown onto it. A very similar, although much larger, area of ground is found in Persia, near Baku. See the article Naphtha. The area in Italy known as Solfatara should be put in the same class. See this article.