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 ICELAND, No. 2, &c. Ætna, Vesuvius, Volcano, &c.

The lava, at its first discharge, is in a state of prodigious ignition, greatly superior to any thing 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 from 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 alkalis, 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 3rd of March 1766, were perfectly transparent." 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 scarce 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 hath 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, the lava 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 subterranean 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 to or 12 feet high, which rolling from the top soon formed another wall, and so on.—While a lava is in this state, Sir William 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 Ætna 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. It hath been also observed of the lavas of Ætna, that they do not constantly fall down to the lowest places, but will sometimes ascend in such a manner as to make the valleys lowest places 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."

The composition of the lavas of different volcanoes, and even of different parts of those of the same volcano, varies; and it 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 impressions of time. I have often observed on mount... 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."—No person, however, hath yet accurately analysed any lava; neither is it an easy task to do so. Mr Bergman hath indeed made some observations upon the Icelandic lavas, which throw a good deal of light upon this subject. One kind of this lava, he tells us, 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 10 to 12 pounds of iron in every hundred weight. It does not dissolve in the least with sal soda, and very difficultly with borax, and hardly visible 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, however, soluble with some difficulty by borax and 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, tourmalin, and sirtl, 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 Veronese 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, cryolites, and pietre obidiane," 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, (chalcedoni opali embryi), are dug out of the volcanic cineritous hills near Vicenza. One might consider these flints as being torn and dragged from the scaglia, and thence to have been by floods heaped together with ashes and lavas; because it is a fact, that innumerable quantities of flints, jaspers, and agates, are found in the china and potters clay-hills near S. Ulderico nel Tretto, (exactly as similar flints are found in the Saxonian and other china clays.) But how did they come into these volcanic hills, which, like those of St Rocco near S. Ulderico, never contain any clay whatever? Supposing their having been by subterraneous fire separated from veins pre-existing in or near the very bottom of the ancient volcanoes; this explains pretty well how they came into their lava and the china clay, when in an aqueous diffusion or mixture it was vomited, since fragments of quartz-crystallization, marble, and other pre-existing stones, are likewise found in these argillaceous beds, &c.—All these circumstances agree in support of Mr Arduini's assertion, that the beforementioned flint-horn stones found among volcanic materials are owing to subterraneous fire and its meltings. Knowing that by vitreous compositions and chemical fire even the hardest precious stones can be nearly imitated, why should we deny the same power to nature and its greater subterraneous furnaces?"

On this passage Mr Raspe has the following note. Mr Raspe: "To prevent mistakes, and the charge of inconsequent writing or reasoning, the author should have explained himself with more propriety; and with more justice to nature, and perhaps to Mr Arduini. Therefore the translator, who has examined several volcanoes, and studied nature in her own manufactories or officines, endeavours to set him right. His observations are so far agreeing with Messrs Ferber's and Arduini's, that he considers the chalcedonies as volcanic productions, but in a quite different sense from that in which the ashes and lavas, with their various inclosed shell-crystallizations, chrysolite or hyacinth-like vitrifications, and pietre obidiane, are called so. These are undoubtedly immediate productions of the fire, and violent violent melting; the former being but paraffitical stones of volcanic matrices; that is to say, but mediate productions of the fire, as being visibly produced by water, either soaking through and into the holes of volcanic stones, and depositing therein the flint-like fire-striking sediment of chalcedony; or, if properly qualified and heated by natural fire or fermentation, precipitating the same under other circumstances. The former appears to conviction by the Vicentine and Iceland chalcedonies; the latter by a singular phenomenon, which I shall take notice of. The Vicentine chalcedonies found in volcanic tufa, contain now and then, inclosed in their middle, drops of the water which produced them; and the Iceland-chalcedonies bear likewise undoubted marks of an aqueous origin. The translator knows, by good authority, that they have been discovered but of late; and ocular inspection has convinced him, not only that these Iceland-chalcedonies are equal in grain and colour to the Oriental ones, but remarkably superior to them on account of their bigness. He had large pieces sent him from Copenhagen above a foot square; and, what is more to the subject, inclosed in a brownish tufa, in which they appeared to have been stratified, or successively deposited by water; consisting more or less of white transparent beds, about an inch thick, and sticking as close together as the similar strata of the coloured agates or onyx. Mr. Banks's late voyage to Iceland brings us still a step further. He examined there the marvellous intermittent spouting hot wells, called the Geysers, at Laugafell, which in the middle of a folfatara, or ancient volcano, by their accumulated sediments have produced or raised a wide sloping hill of white lebes or pot-stone. I forbear to draw from this singular phenomenon the many consequences which it offers for natural history; observing only what is more to the purpose, that in some harder pieces of white lebes, kept in Mr. Banks's Iceland collection, there appears a stratified white chalcedony which cannot be considered as adventitious; and undoubtedly is produced either by a finer sediment, or by its greater saturation; proving, that the substantial earth of chalcedony and lebes are the same, and that both are nearly related to the lapis nephriticus, the serpentine, the bacon-stone, the amiant, and the tale, which are found in many volcanic places; and, according to Mr. Marggraf's experiments, have been by many mineralogists wrongly placed among the argillaceous stones. Similar operations of properly qualified hot wells, so common and various in volcanic countries, might very well answer for the jaspers, agates, and other flints in the china clays and boles. But whether they have in fact produced them, must be left to future proper inquiries in the volcanic countries where they are so very common.

The same author is also of opinion, that the basaltes itself, or those pillars of which the Giants Causeway in Ireland and the island of Staffa in the Hebrides consist, is no other than a species of lava, which has taken upon itself these regular figures during the time of its cooling, as Glauber salts and some others likewise take upon them a kind of columnar figure during the time of their crystallization. Of this crystallization of lavas Mr Ferber tells us he had an instance in the black kind formerly mentioned. At the time he went from Rome to Ostia, they were paving the road with this sort of lava. In some of the broken pieces he observed little empty holes of the bigness of a walnut, incrusted all around their inner sides by white or amethystine semi-pellucid, pointed, or truncated pyramidal crystallizations, entirely resembling the agate nodules or geodes, which commonly are filled with quartz-crystallizations. There was no crack or fissure in the ambient compact lava; the crystal sherds were pretty hard, and might rather be called quartz. In the rest of the holes was some fine brownish dust, impalpable and light as ashes. In another place also, he tells us, that in the greatest part of the Vicentine, Veronese, and Padoan lavas, is to be found an infinite quantity of white polygonal shell-crystallizations, whose figure is as regular, and still more polygonal than the basaltes; and can only be supposed to have been formed in the lava while in a fluid state. It is indeed very improbable that such substances should be thrown up along with the lava unchanged, seeing the intense heat must have been sufficient to melt the most refractory substance we can imagine, and we have no evidence of such numbers of these substances existing at the bottoms of the burning mountains. Still, however, neither Mr Ferber, nor Mr Rafse, nor any other advocate for the volcanic origin of basaltes, hath been able to find a basaltic column either in the lavas of Vesuvius, Aetna, or the Iceland volcanoes; so that this fact must be in some measure dubious, though the analogical reasoning should be ever so strong. Mr Rafse indeed attempts to solve this difficulty by another. "The question," (says he), "Why do not all lavas crystallize into prismatical basaltes, or why do not the Vesuvian lavas show that form?" is the same as asking, Why does not every quartz appear in crystallizations?" But the insufficiency of such answers is evident.—On the production of basaltes, &c., from the lava of volcanoes, Mr Bergman gives his opinion in the following words.

"As it is not uncommon, even in the professors of Mr Bergmorality, to pass from one wrong step to another, so man's opinions are not without examples of this kind in those who make nature their study. Ten years ago it was a general opinion that the surface of the earth, together with the mountains upon it, had been produced by moisture. It is true, some declared the fire to be the first original cause; but the greater number paid little attention to this opinion. Now, on the contrary, that a subterraneous fire had been the principal agent, gains ground daily: every thing is supposed to have been melted, even to the granite. My own opinion with regard to it is this, That both the fire and water have contributed their share in this operation, though in such a proportion, that the force of the former extends much farther than the latter; and, on the contrary, that the fire has only worked in some parts of the surface of the earth.

"Of all the mountains hitherto known, there are without doubt none more remarkable than those that are composed of angular pillars. A few years ago only one or two of this kind were known; but new ones are daily discovered; which is a plain proof how much our attention requires being roused, to prevent it from slumbering, even on the most important occasions.

"It cannot much be doubted, that there has been some connection between these pillars and the effects of subterraneous fire, as they are found in places where the signs of fire are yet visible, and as they are even found mixed with lava, tophus, and other substances produced by fire.

"The cause of the regular form of these pillars is a problem which we have hitherto been unable to solve satisfactorily. This difficulty has appeared so insur- mountable to some, that they have thought it impos- sible for them to be the effects of nature, and have considered them as works made by human hands: this idea betrays the utmost ignorance in regard to the true nature of these mountains of pillars, and does not even deserve a refutation.

"As far as we know, nature makes use of three methods to produce regular forms in the mineral king- dom: 1. That of crystallization, or precipitation; 2. The cooling or settling of the external surface of a liquid mass while it is cooling; and, 3. The bursting of a moist substance while it is drying.

"The first method is the most common; but, to all appearance, nature has not made use of this in the pre- sent case. Crystals are seldom or never found in any considerable quantity running in the same direction, but either inclining from one another, or, what is still more common, placed towards one another, in sloping directions. They are also generally separated a little from one another when they are regular; the nature of the thing requires this, because the several particles of which the crystals are composed must have the li- berty of following that power which affects their reg- ular disposition.

"The basaltic columns, on the contrary, whose height is frequently from 30 to 40 feet, are placed parallel to one another in considerable numbers, and so close together, that the point of a knife can hardly be introduced between them. Besides, in most places, each pillar is divided into several parts or joints, which seem to be placed upon one another; and indeed, it is not uncommon for crystals to be formed above one another in different layers, when the solvent has been visibly diminished at different times; but then the upper crystals never fit so exactly upon the lower ones as to produce connected prisms of the same length and depth as all the strata taken together; but each stra- tum separately taken, forms its own crystals.

"How then can the Giant's Causeway in the county of Antrim, Fingal's Cave at Staffa, and all other assemblages of pillars of the same kind, be con- sidered as crystallizations? Precipitation, both in the wet and dry manner, requires that the particles should be free enough to fix themselves in a certain order; and as this is not practicable in a large melted mass, no crystallizations appear in it, except on its surface or in its cavities.

"Add to this, that the basaltic in a fresh fracture do not show a plain smooth surface under the micro- scope; but appear sometimes like grains of different magnitude, and at other times resemble fine rays run- ning in different directions, which does not correspond with the internal structure of the crystals.

"From what I have hitherto mentioned, the opi- nion that the basalts have been produced by crys- tallization becomes at least less probable, whether we admit the wet or the dry method. But I must not omit, that the spars exhibit a kind of crystallization,

which at first sight resembles a heap of basalts; but, upon a closer examination, a very great difference is observed. The form of the spar is everywhere alike, but the basalts differ from one another in point of size and number of sides: the former when broken consists of many small unequal cubes, but the basalt does not separate in regular parts, &c. &c.

"Nature's second method to produce regular forms is that of cooling the outer-surface of a melted mass. By a sudden refrigeration, nature, to effect this purpose, makes use of polyhedrons and irregular forms. If we suppose a considerable bed which is become fluid by fire, and spread over a plain, it evidently appears that the surface must first of all lose the degree of heat re- quisite for melting, and begin to congeal; but the cold requisite for this purpose, likewise contracts the uppermost congealed stratum into a narrower space, and consequently causes it to separate from the re- maining liquid mass, as the side exposed to the air is already too stiff to give way. In this manner a stra- tum is produced running in a parallel direction with the whole mass; others still are produced by the same cause, in proportion as the refrigeration penetrates deeper.

"Hence we may, in my opinion, very plainly see how a bed may be divided into strata. In the same manner the refrigeration advances on the sides, which consequently divides the strata into polyhedron pil- lars, which can hardly ever be exactly square, as the strongest refrigeration into the inner-parts of the mass advances almost in a diagonal line from the corners. If we add to this, that a large mass cannot be equal through its composition, nor every-where liquid in the same degree; it will be easy to discover the cause of several irregularities. If the depth of the bed is very considerable in proportion to its breadth, prismatic pillars, without cross divisions, are pro- duced, at least lengthways, from the uppermost surface downwards.

"The third way is perfectly similar to the prece- ding in respect to the effect; but is different from it by the mass being soaked with water, and by the bursting of it, being the effect of the con- traction whilst it is drying. If we suppose such a bed to be spread over a level space, the drying ad- vances in the same manner as the refrigeration in the former case.

"This separation into strata properly happens when a considerable quantity of clay enters into the whole composition, because the clay decreases more than any other kind of earth in drying."

Our author now proceeds to consider the manner in which he thinks the basalts most probably may be produced; and having rejected the hypothesis of those who derive them from a crystallization of melted lava, he gives his own opinion as follows. "It seems more credible to me, that they have been produced out of their substance whilst it was yet soft, or at least not too hard to be softened by exhalations. If we therefore suppose that a bed is spread over a place where a volcano begins to work, it is evident that a great quantity of the water, always present on these occasions, is driven upwards in exhalations or vapours; there it is well known possess a penetrating softening power, by means of which they also produce their their first effect; but when they are increased to a sufficient quantity, they force this tough moist substance upwards, which then gradually falls, and during this time bursts in the manner described above.

"My reasons for this opinion are these: First, we do not find the internal grain of the basalts melted or vitrified, which however soon happens by fusion, and for which purpose only a very small degree of fire is requisite. It consequently is very hard to explain how this substance could have been so fluid, that no traces of bubbles appear in it (at least I have not been able to discover any, after the nicest examination into the Scotch and Icelandic basalts), and yet when broken appear dull and uneven. I know very well, that lava is seldom vitrified within; but the great number of bubbles and pores which are found in the whole mass, are more than sufficient proofs that it has not been perfectly melted to its smallest parts, but has only been brought to be near fluid. Secondly, the basalts so much resemble the finer trap, both in respect to their grain and original composition, that they can hardly be distinguished in small fragments."

These are the principal arguments on both sides of the question with regard to basalts, which yet seem far from being decisive on either.

The quantity of matter thrown out from volcanoes under the name of lava is prodigious. After 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.

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 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 noxious vapours shut up with a door; and this cavern is made use of to cool liquors and fruit, which it does in a degree by old 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; though 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.

As to the nature and quality of lavas, Sir William Hamilton informs us, that those 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.