drogene; and the lava of 1794 contained few traces of fulphur, and abounded in oxygene Though we have no means of determining the heat of a lava when it first iffues from its crater, perfectly liquid and in violent ebullition, the deftruction of Torre del Greco has provided us with an approximation to the heat it could communicate after it had been fix hours emitted, had traversed an extent of country three miles in length, and had been refrigerated by the contact of paved streets and houses. We find that, in the ruins of that unfortunate town, the window-glass near the lava was converted into porcelain jafper; that pretty large maffes of iron were oxidated to the heart; that copper was oxidated and foftened, and that filver was melted. Fine filver is faid to melt at 28° of Wedgwood's pyrometer, or at 4720° of Fahrenheit. The portions of lava which acted on these metals, must have been very confiderably cooled by the pavements and walls of the houses; and, befides, it was not in immediate contact with the metals. We must therefore aflign it a much higher temperature than that which was communicated to the fubftances affected. What that temperature was, we do not prefume to determine. Breiflac mentions one circumstance that indicates a tremendous heat. He fays augites were formed on the walls of the church by sublimation from the lava. In this particular, however, we cannot help thinking that his ufual accuracy must have failed him, as no other of the obferved effects appears at all proportioned to this. Admitting the lava to have been quite hot enough to have flowed with the ufual fluidity of glafs, it need not have been fo hot as to destroy the fubftances contained in it; for none of them will melt at a lower temperature than 120° of Wedgwood. The grand difficulty, however, ftill remains; for how does it happen that lavas are almoft univerfally found with a ftony fracture and texture, when a portion of the fame lava melted produces a glafs? Even for this enigma we are now provided with a folution. The converfion of glafs into a ftony fubftance, improperly called porcelain, was difcovered by Reaumur, and would have unveiled the whole mystery, had the circumstances in which it was C 3 operated The obfervations of Dolomieu are very ftrong contradictions to his theory. In his catalogue of the lavas of Etna, p. 370, he obferves, that the fublimation of fulphur is more abundant in half extinct volcanoes, like the Solfatara, than in those which have frequent eruptions. Etna only forms it in the principal crater, and in small quantity. operated been carefully obferved. This was the firft dawn of difcoveries of inconceivable importance and extent; and it feems more remarkable that their complete developement fhould have followed fo flowly, than that extenfive corollaries fhould now be deduced. Mr Keir, in 1776, directed the public attention to the cryftallizations formed in glats by cooling, and the ftony texture which glafs flowly cooled affumes. Thefe facts were not confiftently applied to account for the ftony appearances of lavas, till Sir James Hall, in 1790, projected, and partly performed fome experiments, the completion of which was referved to 1798. Dr Beddoes, in a paper contained in the Philofophical Tranfactions for 1791, amidst a chaos of inaccurate obfervations, diftinctly points out the change from the vitreous to the ftony texture produced by gradual cooling, and applies it to lavas, and illuftrates it by initancing Reaumur's porcelain and the cryftallization of flags. This doctrine received its full elucidation, when Dr Thomfon, in 1795, published his sketch of a claffification of volcanic products, in which he boldly and clearly affumes it as the bafis of his arrangement. He maintains all lavas to have been in a vitreous ftate, and to have become ftony by flow cooling. We find that Breiflac inclines to the fame opinion. Sir James Hall has fince fynthetically determined the point by the fatisfactory refults of his well imagined experiments. Indeed, it is wonderful how it fo long eluded obfervation, when the flag of every furnace exhibits it in the most Atriking manner. If it be inquired, how the known exiftence of volcanic glafs, fometimes in very large maffes, † is to be reconciled to this theory, it may be answered, that as the materials of lava appear to be conftantly varying, fome glaffes may be found lefs difpofed to cryftallize than others, and require a longer continuance in a regulated temperature. 2. That we know of no inftances of folid maffes of volcanic glass of great thickness; for, respecting thofe of Lipari, Spallanzani exprefsly ftates the facility with which they were divifible into thin flabs, which he attributes to afmall quantity of earth interpofed between each fab. This Itatement See Phil. Tranf. for 1776, Vol. LXVI. p. 530. Ten years af ter, M. Pagot de Charmes published fome obfervations in the Journal de Phyfique, Tom. XXXIII. Part II. p. 211. on the cryftals of glass; and M. D'Herminat afterwards added fome illuftrations. Thefe gentlemen, however, do not appear to have attributed the formation of the crystals to the gradual refrigeration of the glafs. Giornale Letterario di Napoli, Vol. XLI. p. 59: + Spallanzani Viaggio alle due Sicilie, statement leaves no doubt that these maffes were formed by the accumulation of fucceffive coats of very fluid lava, which, running over a large furface, and being in confequence very speed ily refrigerated, retained its vitreous texture. We may remark as an additional confirmation, that the eruption of Vesuvius in 1779, when the lava, was chiefly thrown up in a fountain from the crater, and was in confequence rapidly cooled, produced more vitrifications than all the other eruptions of Vefuvius taken collectively. * If the tony texture of lavas be confidered as accounted for, and it be admitted that they have all fuftained the igneous fuGon, and been in a vitreous ftate, all controverfy concerning their bafes may terminate. Dr Thomfon has obferved, that we can only judge of the basis of a lava, by the portions of unaltered ftones which are found in it. Even this is obviously an incorrect teft; for a lava may flow over and envelope ftones of all defcriptions. The bafes of lavas have been deduced from the fubftances contained in the lava, and fuppofed not to be generated in it. Thus, porphyry or granite furnished the feldfpars; augites were found occafionally in bafalt; but unfortunately no known rock contained the leucites which form fo abundant an ingredient in the lavas of Italy. + There feems no way of overcoming this difficulty, but by fuppofing either that the volcane had pierced through all the ftrata which appear on the surface of the globe, and had discovered fome unknown rock which ferved as its pabulum; or, more fimply, by holding that the leucites were generated in the lava. This opinion feems infinitely the • Abozzo d'una fciagrafia Volcanica, nel Giornale Letterario di Napoli, Vol. XLI. We believe this affertion to be correct. Many mistakes have arisen from confounding the zealythe dure cryftallized in 24drons (the analcime trapezoidal) with the leucite. It is readily diftinguishable by the great fulibility of the analcime. The leucites which Faujas St Fond imagined he found near Glasgow, were analcimes. Gioeni mentions leucites in limeftone ejected from Vefuvius; but as he does not feem av are of the approximation of form which the analcime is capable of afuming, there is reafon to doubt to which fpecies they belonged. Dolomieu in the Journal de Phyfique, Tom. 11. An 11. fays he poffeffes a fpecimen of gold ore from Mexico accompanied by minute leucites; and that Lelievre had found leucites in a granitic fubflançe, near Gaverne, in the Pyrenées. He probably affumed them to be leucites from their external form only, as no experiment is cited in confirmation. Even admitting the exiftence of thefe detached inflances, the general pofition is not invalidated. most rational, and is ftrengthened by numerous arguments derived from the confideration of thofe lavas in which leucites exist. Leucites are often found to contain a minute central nucleus, which not unfrequently appears to be a fpeck of lava. Glo bules, of a fubftance exactly fimilar to the enveloping bafis, are often found in the interior of leucites. They frequently contain augites, partly projecting into the bafis, partly imbedded in the leucite; and the leucites have been obferved to be elongated in the direction of the pores of the lava. * Lavas are often compofed almost entirely of leucites which abfolutely touch one another, and are adjufted fo as scarcely to afford any interstices for the bafis which connects them; and extremely minute leucites form not unfrequently a kind of basis for large cryftals of augite. Admitting the leucites to be generated in the lava, there can be no reafon for denying the fame origin to augites and feldfpars, and to other substances contained in lava, provided they are more difficult to fufe than the bafis in which they are engaged. After obferving the various infulated crystals that are formed in glaffes in cooling, the probability of fuch an origin cannot be denied. But it is equally clear, that all crystallized substances which are more fufible than the bafis, must be of pofterior formation. They never are constituents of the lava, and are found exifting ifolated in its cavities. Subftances generated in the lava, and those which have been afterwards introduced, have a striking diffimilitude in the manner of their connexion with it. The first are commonly clofely enveloped, the bafis of the lava applying itself to them in complete contact; or if it recedes, as it fometimes does, from leucites, it bears an impreffion of their fides, which shows that it merely retired in confequence of contraction; and the impref fion is fo sharp, as to prove how perfectly it had accommodated itfelf to the form of the leucite. When any of this clafs of fubftances appear in the cavities of the lava, we always find one end of the crystal entering the folid mafs; and it is evident that the apparent protrufion of the other part is merely in confequence of the cavity being formed by fome evolution of gas after the cryftal was formed; the gas forced afide the fluid bafis, and the cryftal remained projecting. The fubftances of fubfequent formation have no fuch connexion with the bafis of the lava, The line of their feparation is perfectly defined by the bounda pics of the cavity in which they are formed, and a very flight effort Sce Breiflac, Vol. 11. p. 9 effort detaches then entirely. Of this defcription are the zeo lytes, calcareous fpars, &c. which are frequently found in the cavities of the lavas of the Somma, and not unfrequently in thofe of more recent origin, particularly in the lava near Portici, called the Granitello. * Breiflac tells us, that even water is fometimes found in the interior cavities of lava, and endeavours to account for its being there by a rather mysterious application of the doctrine of infinite preffure. Admitting the preffure in the interior of the volcano to be fo great as to confine a globule of redhot water in lava, that preffure is removed the moment the lava iflues from the mountain, and the water muft inftantly force its way out. On the fame principle, zeolites containing water in a state capable of being cafily diffipated by heat, cannot be ge nerated in lava during its ignited state; and to account for their after exiftence in it, we fee no better mode than to recur to the theory of infiltration introduced by Dolomieu. This doctrine does not meet indeed with M. Breiflac's approbation; though we confess ourselves fomewhat at a lofs to perceive the force of his arguments, after confidering the facts he has himself prefented us with, refpecting the daily formation of filicious ftalactites, from hot humid vapours percolating through the cracks of lavas and other ftones, and even penetrating their apparently folid fubftance, and lining their cavities with filicious pearls. A fubject of much curious inquiry remains, respecting the minerals ejected unaltered by Vefuvius. The greater part of thefe confifts of varieties of carbonate of lime, fpathofe, fhiftofe, granular, compact, and fometimes containing thells. The doctrine of preffure has been applied to explain this phenomenon alfo; and we are farther told by the ingenious Dr Thomfon, whose opinion Breiflac feems inclined to adopt, that thefe fhiftofe or granular and apparently primitive limestones are nothing but the common fplintery limestone of the Appenines modified by heat and pref fure. He does not explain how the fpecimens containing petrifactions escaped change; and, befides, this explanation fails as the former one did; for if the internal heat was fufficient to change the texture of the limeftone, or the preffure great enough. to confine its carbonic acid, ftill, at the moment of its expulfion, it must have been intenfely hot, relieved from preffure, and expofed in open air. Why was it not reduced to quicklime? We think it more probable that these limeftones have never been acted on by the volcano at all. When Vefuvius made its firft Profeffor Playfair, in his Illuftrations of the Huttonian theory, 62, affirms that zeolite and calcareous fpar are never found in lavas, and applies this obfervation, in diftinguishing lava from what he terms whinflone, |