a stream of melted minerals which runs out of the mouths, or bursts out through the sides of burning mountains during the time of an eruption. See **ETNA**, **VESUVIUS**, **HECLA**, **VOLCANO**, &c.

The lava at its first discharge is in a state of prodigious ignition, greatly superior to anything we can have an idea of from the small artificial furnaces made by us. Sir William Hamilton informs us, that the lava of Vesuvius, at the place whence it issued (in the year 1767), "had the appearance of a river of red-hot and liquid metal, such as we see in the glass-houses, on which were large floating cinders half lighted, and rolling over one another with great precipitation down the side of the mountain, forming on the whole a most beautiful and uncommon cascade." Now, if we consider the materials of which the lava consists, which undoubtedly are the common matters to be found everywhere in the earth, namely, stones, metallic ores, clay, sand, &c. we shall find that our hottest furnaces would by no means be able to bring them into any degree of fusion; since the materials for glass cannot be melted without a great quantity of very fusible salts, such as alkalies, nitre, &c. mixed along with them. The heat of a volcano must therefore be immense: and besides its heat, it is sometimes attended with a very uncommon circumstance; for Sir William Hamilton informs us, that "the red-hot stones thrown up by Vesuvius on the 31st of March, 1766 were perfectly transparent," and the like remark he makes on the vast stream of lava which issued from this volcano in 1779: (See **VESUVIUS**). This we cannot look upon to be the mere effect of heat: for mere heat with us will not make a solid body transparent; and these stones, we are sure, were not in a state of fusion, or the resistance of the air would have broken them all to pieces, even supposing them, which is very improbable, to have been in that state detached from the rest of the lava. For the transparency, therefore, we must have recourse to electricity; which in some of our experiments hath the property of rendering opaque bodies transparent*. Indeed it is scarcely possible but the lava and every other matter thrown out of a volcano must be in the highest degree electrical, seeing the fire itself most probably takes its rise from electricity, as is shown under the article **VOLCANO**.

The lava, after having once broke out, does not constantly continue running from the same vent, but often has interruptions, after which it will burst out sometimes at the same place, and sometimes at another. No real flame ever appears to come from the lava. In the daytime its progress is marked by a thick white smoke, from which the light of the red hot matter being reflected in the night-time, makes it appear like flame. But if, during its progress, it meets with trees or other combustible substances, which it frequently does, a bright flame immediately issues from its surface, as hath also been remarked by Sir William Hamilton.—This liquid substance, after having run pure for about 100 yards (more or less, no doubt, according to different circumstances), begins to collect cinders, stones, and a scum is formed on the surface. Our author informs us, that the lava which he observed, with its scum, had the appearance of the river Thames, as he had seen it after a hard frost and a great fall of snow, when beginning to thaw, carrying down vast masses of snow and ice. In some places it totally disappeared, and ran in a subterraneous passage formed by the scum for several paces; after which it came out pure, having left the scum behind, though a new one was quickly formed. This lava at the farthest extremity from its source did not appear liquid, but like a heap of red-hot coals, forming a wall in some places 10 or 12 feet high, which rolling from the top soon formed another wall, and so on.—This was the appearance also put on by the lava which issued in the great eruption of 1783 in Iceland; with this difference, that the wall was at one time 210 feet high, and the general thickness of it was more than 100: (See **HECLA**). While a lava is in this state, Sir William liam is of opinion, that it is very practicable to divert it into another channel, in a manner somewhat similar to what is practised with rivers. This he was afterwards told had been done with success during the great eruption of Etna in 1669; that the lava was directing its course towards the walls of Catania, and advancing very slowly, when they prepared a channel for it round the walls of the town, and turned it into the sea. A succession of men, covered with sheep skins wetted, were employed to cut through the tough flanks of lava, till they made a passage for that in the centre, which was in perfect fusion, to disgorge itself into the channel prepared for it. But this, it is evident, can only take place in small streams of this burning matter; with that above mentioned it would have been impossible. It hath been also observed of the lavas of Etna, that they do not constantly fall down to the lowest places, but will sometimes ascend in such a manner as to make the valleys rise into hills. On this Sir William Hamilton has the following note: "Having heard the same remark with regard to the lavas of Vesuvius, I determined, during an eruption of that volcano, to watch the progress of a current of lava, and I was soon enabled to comprehend this seeming phenomenon, though it is, I fear, very difficult to explain. Certain it is, that the lavas, while in their most fluid state, follow always the laws of other fluids; but when at a great distance from their source, and consequently encumbered with scoriae and cinders, the air likewise having rendered their outward coat tough, they will sometimes (as I have seen) be forced up a small ascent, the fresh matter pushing forward that which went before it, and the exterior parts of the lava acting always as conductors (or pipes, if I may be allowed the expression) for the interior parts, that have retained their fluidity from not being exposed to the air."

From the year 1767 to 1779, this gentleman made many curious observations on the lavas of Vesuvius. He found, that they constantly formed channels in the mountain as regular as if they had been made by art; and that, whilst in a state of perfect fusion, they continued their course in those channels, which were sometimes full to the brim, and at others more or less so according to the quantity of matter thrown out. These channels, after small eruptions, were generally from two to five or six feet wide, and seven or eight in depth. They were often hid from the sight by a quantity of scoriae that had formed a crust over them, and the lava, having been conveyed in a covered way for some yards, came out again fresh into an open channel. Our author informs us, that he had walked in some of these subterranean galleries, which were exceedingly curious, the sides, top, and bottom, being exceedingly smooth and even; others were incrusted with what he calls very extraordinary scoriae, beautifully ramified white fangs in the form of dropping stalactites, &c.

On viewing a stream of lava while in its fluid state in the month of May 1779, he perceived the operation of it in the channels above described in great perfection. After quitting them, it spread itself in the valley, and ran gently like a river that had been frozen, and had masses of ice floating upon it. The wind happening then to shift, our traveller was so inclosed by the smoke, that the guide proposed to cross it, which was instantly put in execution without any other inconvenience than the violent heat with which the legs and feet were affected. The crust was so tough, that their weight made no impression upon it, and the motion so slow that they were in no danger of falling. This circumstance, according to Sir William, points out a method of escape should any person happen to be inclosed between two lavas, but ought never to be tried except in cases of real necessity; and indeed, if the current of melted matter was very broad, must undoubtedly be attended with extreme danger, both from the heat of the upper crust and the chance of its breaking and falling down with the passenger into the burning liquid below. That which Sir William Hamilton crossed was about 50 or 60 feet broad.

Having passed this burning stream, our travellers walked up along the side of it to its very source. Here they saw it boiling and bubbling violently up out of the ground, with a hissing and crackling noise like that which attends the playing off an artificial fire-work. An hillock of about 15 feet high was formed by the continual splashing up and cooling of the vitrified matter. Under this was an arched hollow, red-hot within, like an heated oven; the lava which ran from it being received into a regular channel raised upon a sort of wall of scoriae and cinders, almost perpendicularly, of about the height of 8 or 10 feet, and much resembling an ancient aqueduct. On quitting this fountain of lava, they went quite up to the crater, where as usual they found a little mountain throwing up stones and red-hot scoriae with loud explosions; but the smoke and smell of sulphur was so intolerable, that they were obliged to quit the place with precipitation.

By the great eruption in August 1779, the curious channels above mentioned were entirely destroyed, the cone of the mountain was covered with a stratum of lava full of deep cracks, from whence continually issued a fulphurous smoke that tinged the scoriae and cinders with a deep yellow, or sometimes white tint. The lava of this eruption appeared to be more perfectly vitrified than that of any former one he had observed. The pores of the fresh lava were generally full of a perfect vitrification, and the scoriae themselves, viewed through a magnifying glass, appeared like a confused heap of filaments of a foul vitrification. When a piece of the solid lava had been cracked in its fall, without separating entirely, fibres of perfect glass were always observed reaching from side to side within the cracks. The natural spun-glass which fell in some places along with the ashes of this eruption, and which has likewise been observed in other places, he is of opinion must have proceeded from an operation of the kind just mentioned; the lava cracking and separating in the air at the time of its emission from the crater, and by that means spinning out the pure vitrified matter from its pores or cells; the wind at the same time carrying off the filaments of glass as fast as they were produced.

Our author observed a kind of pumice stone sticking to some very large fragments of the new lava. On close inspection, however, he found that this substance had been forced out of the minute pores of the solid lava itself; and was a collection of fine vitreous fibres or filaments confounded together at the time of their being pressed out by the contraction of the large fragments of lava in cooling, and which had been bent downwards by their own weight. "This curious substance (says he) has the lightness of a pumice, and resembles it in every respect, except that it is of a darker colour."

When the pores of this lava were large, and filled with pure vitrified matter, the latter was sometimes found blown into bubbles on the surface; probably by the air which had been forced out at the time the lava contracted itself in cooling; and from these thin bubbles it appeared, that this kind of volcanic glass has much the same transparency with our common glass bottles, and like them is of a dirty yellow colour; but when large pieces of it were broken off with a hammer, they appeared perfectly black and opaque.

In the lava of this eruption it was observed, that many detached pieces were in the shape of a barley-corn or plum-stone, small at each end, and thick in the middle. Some of these did not weigh above an ounce; but others could not be less than 60 pounds. Our author took them to be drops from the liquid fountain of fire, which might naturally acquire such a form in their fall. There were also many other curious vitrifications, different from any he had seen before, mixed with this huge shower of scoriae and masses of lava.

In treating of Mount Etna, M. Houel makes mention of a piece of lava which, after having been once ejected by the volcano, was swallowed up, and thrown out a second time. The intense heat to which it was then subjected, had such an effect upon it, that it appeared all full of chinks to a considerable depth, and which run at right angles to one another. He had also an opportunity of observing to great advantage some of the hollow channels formed by the lavas of Etna similar to those described by Sir William Hamilton, but on a much larger scale. Here the great eruption of water in 1755 had overturned, in a vertical direction, an huge tube of this kind for the length of half a mile. The tube itself appeared to be composed of enormous masses, somewhat resembling planks; each two feet thick and twelve or fifteen in breadth, continued in a straight line through the whole of that space. At the same time by the action of the lava a kind of walls had been formed, from ten to sixteen feet in height, and curved at the top. Some of these walls appear rolled together like paper; and M. Houel is of opinion, that these various appearances on the surface of the lava when cooled must have arisen from particles heterogeneous to the real lava; and which detach themselves from it, rising to the surface under a variety of forms proportioned to the spaces of time taken up in cooling. These crusts are formed of different kinds of scoriae and dirty lava, mixed with sand or ashes. At the same place are found also great numbers of small pieces like those of ice heaped upon one another after having floated for some time on a river. Beneath these the pure lava is met with, and which has evidently been in a state of perfect fusion. This is extremely dense; and by looking narrowly into its chinks, the composition of the whole appears to be merely homogeneous. "It is curious (says he) to observe, so near one species of lava which is very pure, another which has likewise arrived at the same place in a fluid state, and has there undergone so great a change as scarce to retain an appearance of its original state. It is, however, like iron drofs, in grains of unequal sizes. We find it also at various distances, such as one, two, or more hundred fathoms. It is sometimes found in large pieces like tables, covered over with sharp points, some longer and others shorter. All these pieces are quite detached from one another, as if they had been brought thither and scattered from a tumbril. The matter of which the crust of the lava is formed, seems to have issued from it in the same manner in which froth rises upon solution of soap in water. It appears afterwards to have swelled, burst, and assumed its present form, presenting to the view various spaces filled with small loose stones. A great number of new lavas were likewise observed, all of them putting forth various kinds of efflorescences in great quantity.

The hardness, density, and solidity, of lavas, no doubt proceed from the degree of heat to which they have been exposed, and which seems to be greater or less according to their quantity. Hence the Icelandic volcanoes, which pour forth the greatest quantities of lava, produce it also in the greatest degree of liquefaction, and Dr Van Troil observes, that what he saw must have been liquefied to an extreme degree.

The composition of the lavas of different volcanoes, Observations on the is extremely different. Sir William Hamilton is of different opinion that this difference in composition contributes not a little to the facility or difficulty with which they afterwards receive earth capable of vegetation. "Some W. Hamilton (says he) have been in a more perfect state of vitrification than others, and are consequently less liable to the impreparations of time. I have often observed on Mount Vesuvius, when I have been close to a mouth from whence the lava was disgorging itself, that the quality of it varied greatly from time to time. I have seen it as fluid and coherent as glass when in fusion; and I have seen it farinaceous, the particles separating as they forced their way out, just like meal coming from under the grindstones. A stream of lava of this sort being less compact, and containing more earthy particles, would certainly be much sooner fit for vegetation than one composed of the more perfect vitrified matter." Mr Bergman, who has accurately analysed some Icelandic lavas, informs us, that one kind is very coarse, heavy, and hard, full of bladders, almost black, intermixed with white grains resembling quartz, which in some places have a figure not very unlike a square. This black matter is not attracted by the magnet; but if a piece of it is held against a compass, the needle visibly moves. When tried in the crucible, it yields from ten to twelve pounds of iron in every hundred weight. It does not dissolve in the leach with sal soda, and very difficultly with borax, and scarce at all with urinous salt. It seems to contain a great deal of clay in its composition, which may be extracted by all acid solvents. This last he is likewise, from experiments, assured is the case with the lava of Solfaterra in Italy.

The white lava, which possesses more or less of those transparent grains or rays with which lavas are generally chequered, does not seem to be of the nature of quartz, as it cannot be attacked by sal soda; it is how. Lava, however, soluble with some difficulty by borax or fusible urinous salt, or microcosmic acid. These effects are perfectly similar to those produced upon the diamond, ruby, sapphire, topaz, and hyacinth. The chrysolite, garnet, tourmaline, and sapphire, can neither be dissolved by sal soda, though they are somewhat attacked by it when reduced to a fine powder; and upon the two last mentioned ones it produces a slight effervescence; on which account, says Mr Bergman, it is possible that the precious stones found upon Mount Vesuvius, which are sold at Naples, are nearer related to the real precious stones than is generally imagined. He found no such grains in a finer kind of lava, quite porous within, and entirely burnt out, and considerably lighter than the former ones.

The Iceland agate is of a black or blackish brown colour, a little transparent at the thin edges like glass, and gives fire with steel. It cannot easily be melted by itself; but becomes white, and flies in pieces. It can hardly be dissolved in the fire by fusible urinous salt; but it succeeds a little better with borax, though with some difficulty. With sal soda it dissolves very little; though in the first moments some ebullition is perceived, and the whole mass is afterwards reduced to powder. Hence Mr Bergman concludes, that this agate hath been produced by an excessive fire out of the black lava formerly mentioned.

In the Iceland pumice-stone, quartz and crystals are often found, particularly in the black and reddish-brown kind. The stones thrown out of the volcano, whether grey, or burnt brown, seemed to consist of a hardened clay, mixed with a siliceous earth. They were sprinkled with rays and grains resembling quartz, and some few flakes of mica. They fused with great difficulty in the fire; with sal soda they showed some effervescence at first, but which ceased in a short time. The parts resembling quartz produced no motion at all; from whence Mr Bergman concludes, that the black lava already mentioned proceeds principally from this mass. Several other stones which were sent him from Iceland, Mr Bergman supposed to have no connection with the eruptions, but to have been produced in some other way.

In Mr Ferber's travels through Italy, we are informed, that he has seen a species of lava so exactly resembling blue iron flags, that it was not to be distinguished from them but with great difficulty. The same author tells us likewise, that "the Vicentine and Venetian lavas and volcanic ashes contain inclosed several sorts of fire-striking and flint-horn stones, of a red, black, white, green, and variegated colour, such as jaspers and agates; that hyacinths, chrysolites, and pietre obfaliane, described by Mr Arduini in his Giornale d'Italia, are found at Leonedo; and that chalcedony or opal pebbles, and noduli with inclosed water-drops, (chalcedonii opali enhydri), are dug out of the volcanic cineritious hills near Vicenza.

M. Dolomieu considers the chemical analysis of lava as but of little account. When subjected to the force of fire a second time, they are all of them reducible to the same kind of glass; from which it has been concluded, that all volcanic products have been formed of the same kind of materials, and that the subterranean fire has always acted on and variously modified the same kind of stone. But an analysis by fire, he justly observes, is of all others the most fallacious. The substances are all fusible, and we have no proper methods of measuring the intensity of our fires; so that the same substance which today may come out of our furnaces untouched, may tomorrow be found completely altered, even though the fire employed should not appear to us to be any more violent than the former. Analyses by different methods have not been more successful. Mr Bergman has indeed analysed some lavas with acids, and gives with astonishing precision the following result, viz. that an hundred parts of lava contain 49 of siliceous earth, 35 of argillaceous earth, four of calcareous earth, and 12 of iron. These experiments, however, our author observes, give us no information with regard to lavas in general. They only show the composition of the particular specimens that he tried; and even after the descriptions that he has given, we are a good deal at a loss to discover the species of lava which he subjected to analysis. "It would be as ridiculous (says M. Dolomieu) to apply this analysis to every volcanic product, as it would be to believe that the component parts of a flinty rock were the same with those of every rock composed of laminae or thin strata." For these reasons he is of opinion, that, in order to understand the nature of lavas, we should consider not only that of volcanoes themselves, but of the bases on which they rest. Had this been done, we would have found that the volcanic fires generally exist in beds of argillaceous schistus and hornstone; frequently in a species of porphyry, the gluten of which is intermediate between volcanic twixt hornstone and petrolium; containing a large quantity of schorl, feldspar, and greenish quartz or chrysolite, in little rounded nodules. These substances, he tells us, would have been found in those mountains which are called primitive, and in strata buried under beds of calcareous stone; and, among other things, would have convinced us, that the fluidity of lavas does not make them lose the distinctive characters of their bases. In the mountains called Primitive, those rocks which are assigned as the bases of the more common lavas are found intermixed with micaceous ones, with gneiss, granite, &c. and they generally rest on masses of granite. Hence lavas must consist of all these matters, and the fire must act upon them all whenever it meets with them. Our author has constantly observed, that volcanoes situated at the greatest distance from the centre of the chain, or group of mountains on which they are established, produce lavas of a more homogeneous composition, and less varied, and which contain most iron and argillaceous earth. Those, on the contrary, placed nearer the centre, are more diversified in their products; containing substances of an infinite variety of different kinds. The seat of the fire, however, he observes, does not long continue among the granites, the inflammation being either extinguished, or returning to the centre of the schistus rocks in its neighbourhood.

From this knowledge of the materials of which lavas are composed, we acquire also a considerable knowledge of the matters that are found in greatest quantity in the bowels of the earth. The excavations made by mines, &c. on the surface of the earth, are mere scratches in comparison of the depths of volcanic fires; and as he considers the mountains themselves as the fires, productions of those fires, it thence follows, that by attentively examining the materials of which they are composed, we may thence determine what kind of substances are most common at these great depths in the earth.

Thus our author thinks it probable, that schorls and porphyries, though rare on the surface, are very common in the internal parts of the earth. As an instance of the truth of his observations, our author informs us, that he was convinced, from no other circumstance but merely inspecting the lavas of Mount Etna, that, in some parts of the island of Sicily, there existed granites, porphyries, with schistus and argillaceous hornstones. In this opinion he persisted, notwithstanding the generally opposite sentiments of the inhabitants themselves. He searched in vain three-fourths of the island; and at last found that all the mountains, forming the point of Sicily, called Pelorus, contain rocks of the kind above mentioned. He then saw that the base of these mountains was produced under Mount Etna on one side, and under the Lipari islands on the other. "We must, therefore, (says he) believe, that these mountains have furnished the materials on which the volcanoes have, for thousands of years, exerted their power."

By travelling among those elevations called the Neptunian Mountains, or Mons Pelorus, he was enabled to discover the reason why the products of Etna and the Lipari islands differ from one another. This, he says, is the unequal distribution of the granite and schistus rocks among them. The islands rest almost immediately on the granite, or are separated from it by a very thin stratum of argillaceous rock which contains porphyry; but the Sicilian volcano is situated on the prolongation of the schistous rock, which it must pierce before it reaches the granite; and accordingly very little of its lava seems to have granite for its basis. If the seat of the fire was still more distant from the centre of the mountains, their lavas would be more homogeneous, because the schist, which succeeds to the horn-stone, is less various, and hardly includes any bodies foreign to its own substance. Thus the lavas, in the extinguished volcanoes of the Val di Noto, which lie 15 leagues to the south-east of Etna, contain neither granite nor porphyry; but have for their bases simple rocks, with particles of chrysolite and some schorls.

To the granites which extend to Metazzo, opposite to Lipari, he ascribes the formation of pumice; as they contain an immense quantity of scaly and micaeous rocks, black and white, with fossil granites or gneiss, the basis of which is a very fusible feldspar; and these he supposes to be the proper materials of the pumice, having found pieces of them almost untouched in pumice-stones. There are beds of almost pure feldspar; to the semivitrification of which he ascribes an opaque enamel like lava mentioned in other parts of his works. Few porphyries, however, he acknowledges, are to be met with among the Neptunian mountains, though these stones abound in the lavas of Etna. "They are not distant (says he) from the granites; and those I have found have neither the hardness nor perfection of those pieces which I gathered in the gullies, and which had been apparently washed out of the anterior parts of the mountain by water.

But though the porphyries I saw here bear no proportion to those in the products of Etna, I was sufficiently convinced of their existence, and their analogy with those of volcanoes, by discovering that the centre of these mountains contains a great number of them. Porphyries, in general, are very rare on the surface of the earth. Nature generally conceals them from us by burying them under calcareous strata, or by inclosing them in schistus rocks with which they are almost always mixed: but we are indebted to the labour of volcanoes for informing us that they are among the most common substances in the bowels of the earth; and they are never so much disguised by the subterranean fire as to be mistaken in the lavas of which they form the basis."

In Cronstedt's Mineralogy we find all the volcanic products clasped under the general name of Slags; of which he enumerates the following species:

1. The Achates Islandicus Niger, or Iceland Agate. It is black, solid, and of a glairy texture; but in thin pieces: it is greenish, and semitransparent, like bottle-glass which contains much iron. It is found in Iceland and in the island of Ascension. The jewellers employ it as an agate, though it is too soft to resist the wear. "The most remarkable thing concerning this (says he) is, that such large solid masses are found of it, that there is no possibility of producing the like in any glass-house. In Magellan's notes on this subject, we find the Iceland agate clasped among the transparent basaltes. To the same class belong the Lapis Olydianus of Pliny, and the Lapis Gallinaceus of Peru, which by its beautiful blackness approaches to the colour of a large black-bird of the crow kind, in that country called the Gallinago.

2. Lapis molaris Rhenanus, Rhenish Millstone, is blackish-grey, porous, and perfectly resembling a sort of flint produced by Mount Vesuvius.

3. Pumex, the pumice-stone. See Pumex.

4. The Pearl-Slag is compounded of white and greenish glass particles, which seem to have been conglutinated while yet soft or in fusion. It is found in the island of Ascension.

5. Slag-sand, or ashes, thrown out by volcanoes in larger or smaller grains. "This (says Cronstedt) may perhaps be the principle of the Terra Puzzolana, because such an earth is said at this time to cover the ruins of Herculaneum near Naples, which was destroyed by Vesuvius." In the notes, we are informed, that if the ashes of a volcano be plentifully moistened, they produce that kind of tufa or tophi, treas, and pori, all of which are nearly of the same kind. Great heaps of tufa or tophi are found in Italy, forming various hills, and covering large tracts of land; from whence it is cut, and carried, for making the walls, vaults, and upper ceilings of houses. It is a very soft kind of stone, extremely advantageous for these purposes, on account of its little weight, and being easily cut into any form. The inhabitants of Umbria and other parts of Italy dig with very little labour various subterranean excavations for the keeping of wines and provisions of different kinds.

Mr Kirwan is of opinion, that the lavas ought to be distinguished from the other volcanic productions. All lavas, according to him, are magnetic, give fire with steel, are generally of a granular texture, and fusible They may be reduced to three varieties, viz. the Cellular, the Compact, and the Vitreous. The cellular appear to have undergone only the first degree of fusion, being just mollified and heated sufficiently to expel the fixed air contained in the argillaceous particles. Hence they abound in small cavities arising from the expansion of that air after it had recovered its elastic state; and thus they are often so light as to float upon water, and have been mistaken for pumice-stones. They are of black, grey, brown, or reddish colours; and their cavities are even filled with crystallizations. Of this kind is Cronstedt's second species, the millstone of the Rhine. These contain from 45 to 50 per cent. of siliceous earth; from 15 to 20 of iron; four or five of pure calcareous earth; the remainder being argillaceous.

The compact lavas have undergone a more perfect degree of fusion, though even these are not destitute of cavities. They contain finer crystals, or such as are more completely vitrified than the former; they have a black or brown colour; but still their fracture is obscure and not glassy. Their constituent parts are the same with the preceding ones; the usual fluxes attack them with difficulty, and the fusible salt of urine has scarce any power over them.

The vitreous lava has been more completely melted, and forms vitrifications of different colours, generally black or ash-coloured, but rarely blue or greenish. A species of this was analysed by Mr. Bergman, as has been already mentioned, and afforded 49 per cent. of filex, 35 of argillaceous, 4 of calcareous earth, and 12 of iron. Another specimen from the Lipari islands afforded 69 parts of filex, 20 of argillaceous earth, and 9 of iron. This kind of lava melts by itself with great difficulty. The black agate of Iceland belongs to this species, as does also the harder sort of pitch stone, which gives fire with steel. This stone is of various colours, grey, green, black, red, or brown; has a glassy appearance, being composed of semifluid substances, and melts easily per se. It contains 65 per cent. of filex, 16 of argillaceous earth, and four of iron; 14 parts were dissipated in the analysis made by Wiegbleb, as Mr. Kirwan asserts.

The beds of lava are deepest and narrowest near the crater, and broader and shallower as they advance, unless some valley intervenes. Pumice-stones lie at a still greater distance; and from these observations, says Mr. Kirwan, extinguished volcanoes may be traced.

Cronstedt conjectured that there might be a kind of circulation among the different earths, from the vegetable mould, which he supposed to occupy one extreme, to the flags or volcanic productions, which might be reckoned to occupy another, and back again from the flags to the vegetable mould. "It is obvious (says he) how the old heaps of flags from the iron furnaces decay, and at last produce vegetables, which cannot be ascribed solely to a black mould carried thither by the wind. The same may perhaps happen with the natural flags in the open air." Other naturalists have verified this conjecture. All lavas are found to be decomposable by long exposure to the air, sooner or later according to the quantity of iron and calcareous earth they contain, and according as their fusion was more or less complete. Sir William Hamilton has concluded that they gain only one or two feet mould in 1000 years; from which, and Roupero's calculations, extravagant ideas have been formed of the duration of the world; but all these are found, when properly examined, to be built on a false foundation. See the article Earth, n° 176, 177.

The quantity of matter thrown out from volcanoes under the name of lava is prodigious. After quantities of lava were thrown out during the great eruption of Etna in 1669, Borelli went from Pisa to Sicily to observe the effects of it. The matter thrown out at that time amounted to 93,830,750 cubical paces; so that, had it been extended in length upon the surface of the earth, it would have reached more than four times round the whole earth. All this matter, however, was not lava, but consisted also of sand, stone, gravel, &c. The lava he computed at 6,300,000 paces, which formed a river, according to our author, sometimes two miles broad; but according to others it was six or seven miles broad, and sometimes 20 or 30 yards in depth. Sir William Hamilton informs us, that the lavas of Etna are very commonly 15 or 20 miles in length, six or seven in breadth, and 50 feet deep. The most considerable is scarce less than 30 miles long and 15 broad. The most considerable lavas of Vesuvius do not exceed seven miles in length. The same author, however, tells us, that the lava which issued from Vesuvius in 1767, was six miles long, two in breadth, and in most places 60 or 70 feet deep. In one place it had run along a hollow-way made by currents of rain not less than 200 feet deep and 100 wide; and this vast hollow it had in one place filled up. He says, he could not have believed that so great a quantity of matter could have been thrown out in such a short time, if he had not examined the whole course of it himself. Even this quantity, however, great as it is, appears very trifling in comparison of that thrown out in Iceland in the year 1783, which covered a space of ground 90 miles in length and 42 in breadth, to the depth of more than 100 feet. Dr. Van Troil, in his Letters on Iceland, tells us, that he and his companions travelled over a tract of lava upwards of 300 miles in length; and in 1728, we are told that an eruption of lava took place, which continued for two years to run into a great lake, which it almost filled up.

As the lavas are thrown out from the volcanoes in the highest degree of ignition, it may easily be supposed that such vast bodies will retain their heat to cool for a long time. It would indeed be well worth observing, what length of time is required to cool a lava perfectly; as from thence we might in some measure judge how far those philosophers are in the right, who argue concerning the length of time required to cool an ignited globe of the size of our earth or larger. Sir William Hamilton tells us, that in the month of April 1771, he thrust sticks into some of the crevices of the lava which had issued from Vesuvius in October 1767, and they immediately took fire. On Mount Etna, in 1769, he observed the lava that had been disgorged three years before to smoke in many parts. No particular observation, however, hath been made in what proportion the heat of lavas is gradually lost. Sir William Hamilton informs us of a curious fact relating to a lava in the island called Lacco. Here is a cavern shut up with a door; and this cavern is made use of to cool liquors and fruit, which it does in a short time as effectually as ice. Before the door was opened, he felt the cold on his legs very sensibly; but when it was opened, the cold rushed out so as to give him pain; and within the grotto it was intolerable. He was not sensible of wind attending this cold; tho' upon Mount Etna and Vesuvius, where there are caverns of this kind, the cold is evidently occasioned by a subterraneous wind; the natives call such places ventaroli. From old lavas there also frequently happens an eruption of noxious vapours called mofetes. These likewise break out from wells and subterraneous places in the neighbourhood of a volcano before an eruption. Our author tells us, that the vapour affects the nostrils, throat, and stomach, just as the spirit of hartshorn or any strong volatile salt; and would soon prove fatal if you did not immediately withdraw from it. These mofetes, he says, are at all times to be met with under the ancient lavas of Vesuvius, particularly the great eruption of 1631.

Sir William Hamilton informs us, that the lavas of Etna and Vesuvius are much the same, but those of Etna rather blacker and more porous than those of Vesuvius. Some kinds of lava take a fine polish, and are frequently manufactured into boxes, tables, &c. In Naples, the inhabitants commonly make use of it for paving the streets, and even the subterraneous cities of Pompeii and Herculaneum have been paved with the same substance. A fine large cubic piece of lava is preserved in the hall of the British Museum.