Prince Cassano, a member of our Royal Society, produced an instance of the vio lent heat of the lava which issued from. Vesuvius, to that learned body, which seems more to the purpose than that of the Academicians : the torrent of lava al luded to, approached a convent of Car melites ; every combustible article was immediately consumed, even before the mass came into contact with it ; and the heat was so excessive, that the glasses standing upon a table in the refectory, were instantly reduced to shapeless pieces of that useful material : this ch..' cumstance produced an experiment, at tended with the same consequences, which was the fastening of a fragment of glass to the end of a long stick, and hold ing it near the lava, when, at the close of four minutes, it became a mere paste. A fact of the same nature is mentioned by Professor Itottis, in his account of the eruption of Vesuvius, in- the year 1667. Now, though this effect may he produc ed by suspending a piece of glass in the air of a glass furnace, it must be admitted, that this being in a state of full activity, and the heat excited to the utmost, by everrhuman means, is nut a just comps risen with a body of lava far removed from the spot where it acquired its heat, which must, without doubt, be at that spot ten times greater ; hence it appears decidedly clear, that the internal fires of Vesuvius far exceed that of our furnaces.
Bottis seems to have been one of the first naturalists who observed the rapidi ty with which the fire of Vesuviuscauses fusion ; that gentleman mentions, in description of.the eruption of J uly, 1779, that he saw a small hill, composed of po rous lava and scoria:, inclosing an iliconsi-• derable gulph, that produced a noise like that of oil or fat, in the act of boiling or simmering over a fire ; this appearance induced him to examine it, when he found it contained matter in fusion, which im mediately heated red-hot, and then melt ed fragments of.lava or scoria:, thrown in to it. As Spallanzani acknowledges that his efforts to melt similar substances required half an hour, we must admit, that this is another proof of the superior heat of volcanic fires. A still further evidence of their extreme heat is, the great length of time which lavas retain it. In the year 1737, some labourers were employed to remove the lava which had crossed a road, and although a month had elapsed from the period of the eruption, they were compelled to desist, as the heat softened their tools beyond the pos sibility of using them. Sir William Ha milton also found it very great, and, drop ping some pieces of wood into. the fis sures of a mass, situated four miles from the volcano, they immediately took fire ; but Spallanzani illustrated this fact more decidedly, by passing a body of lava near the upper crater of .7Etna, visibly red hot, even in full day-light, which had flowed from the mountain eleven months before.
It is supposed, that the volcanic fires of Iceland are very active and powerful, which is inferred from the incompetency of the blow-pipe to fuse the glass issuing from then'. Vail isneri, describing a new volcanic island, which rose from the sea in the year 1707, near Santorine, asserts, that the sea in its vicinity became so vio lently heated, that vast numbers of fish pe rished, and were actually boiled ; and it is well known that the same cause melted the pitch in the seams of ships' bottoms, and occasioned their leaking : this mOdern fact is corroborated on the authority of Strabo, who declares, that the sea was observed to boil for four days, between Thera and Therasia. The complete
fluidity of lava is another convincing proof of,the excessive heat,prevailing in the centre of vol canoes. M.11ottis produces two instances derived from Vesuvius in 1771 and 1776, which demonstrate that thisiliery mass assumes a state-of liquidity, almost equal to water: the Professor men tions four hills to have arisen suddealy. in the first case near the aperture whence the larva proceeded, and from three of those in the shape of cones, issued streams of the melted matter, exactly re sembling fountains of water. During the eruptions of the latter year, fresh rushing from the crater, fell upon that of 1771, and rebounding from it into the air, there congealed in various figures capri ciously ramified, and terminating in thin sharp points like needles. A circum stance observed by Sir William Hamilton, Count de Wilzeck, Cardinal Herzan, and the Archduke Maximilian of Austria, in the year 1775, seems to establish the fact, that the fluidity of the lava has been such at times, as to separate into portions, which, being thrown up from the crater, fell again near it, in a state so soft, that a guide who assisted in conducting these illustrious visitors, perceiving a fragment, passed his stick through it, and present ed it thus to the prince, who ordered both to be deposited in his private museum ; this, however, seldom occurs, at least the indefatigable Spallanzani never discover. ed these fragments flattened or indented, as if they hid fallen on some hard sub stance when in the consistence of paste.
With respect to the rapidity of its mo tion, this must greatly depend upon the quantity ejected, as well as the intensity of its heat : when an opportunity happens for attentive observation, the lava has been known to rise suddenly to the sum mit of the crater, and as suddenly over flowing its boundaries, rush down in va. rious rivulets of fire ; indeed Bottis com pares it to " a liquor which boils in a ves sel, and rises and overflows the edges of that vessel from the violence of the heat." The lava from Vesuvius, issuing in 1751, flowed over the space of twenty-eight palms in one minute ; in 1754, it pro ceeded in two branches, at the rate of thirty feet in forty-five seconds, and after wards uniting, at thirty-three feet in fifty seconds : to these facts may be added the testimony of Sir William Hamilton, who thought its velocity in 1765 equal to that of the Severn, at the passage near Bris tol. It may, however, be necessary to observe, that the fluidity of the matter does not always alone occasion its motion, which may be accelerated by a great de. scent, or the violent pressure of fresh la va constantly issuing from the source, particularly as lavas are known to harden when actually moving, so as to produce a sound when struck, and to bear stones thrown on their surface ; but to place this fact beyond a doubt, Sir William Hamilton informs us, that himself and others, following the example of Mr. Jai mineau, British consul at Naples, actually crossed a moving mass above fifty feet in breadth ; yet, even thus circumstanced, those dreadful rivers of fire have been known to reach the sea eighteen, twen ty, and even thirty miles from their com mencement.