We wish the attention of our readers to be particularly fixed on fome of these recent and well authenticated facts, as they are of much importance in explaining the general operations of volcanoes. They should particularly obferve the rapidity with which the lava moved the heat that it communicated to fubftances at Torre del Greco-the fcarcity of fulphur, proved by the lava converting the metallic bodies it approached into oxides, inflead of fulphurets or fulphates-the formation of filicious ftalactites, by the hot, humid vapours-and the inundations of mud caused by the mixture of athes and rain. Thefe facts appear not eafily reconciled with the affertious of many able naturalifts refpecting the imperfect fluidity of lavas, their low temperature, and the abundance of fulphur they contain, which has been regarded as the vechicle of their particles, and the pabulum of their inflammation. The rain and afhes forming a pafte, and overflowing the country, feem to account for the formation of tufas and imperfectly confolidated volcanic bodies, without having recourfe to an eruption of mud; and the formation of filicious ftalactites opens a wide field to curious inveftigation. In order to appreciate the full importance of thefe remarks, it is neceffary to confider fome of the opinions on the most important questions fuggefted by inquiries into the conftitution of volcanoes, which have been fupported by the greatcft ingenuity, and fanctioned by the most accurate obfervations. The most ancient and the moft fimple mode of accounting for volcanoes, is that which attributes them to the eructations of a central fire occupying the interior of the earth. To this theory it may be objected, 1. That it is founded on an entirely gratuitous affumption; 2. That it is extremely improbable; and, 3. That it is inadequate to explain the phenomena. The two firit. propofitions require no proof; on the third it may be remarked, that admitting the centre of the earth to be melted matter, it muft, from the duration of the fufion, have obtained perfect homogeneity. There can be no grounds for fuppofing that it was not originally conftituted homogeneous; but even if it was originally heterogenecus, its long continued fluidity must have produced a complete and chemical mixture. A fluid, in fuch a ftate, must be completely quiefcent; and its tranquil existence in the centre of the earth will not avail in accounting for vol canoes. We Sir William Ha.nilton obferves, that the mud formed by rain and afhes became in a few days fo hard as to require a pick-axe to break it. See his Account of the late eruption of Veluvius,' in the Philofoph. Tranfact. for 1794, P. 73. We are indeed told by the ableft advocate of this fyftem, that, in the mineral regions, the only effects of heat are fufion and expanfion. How is this expanfion produced? It cannot result from the continuance of the fame degree of heat. There are no methods we can devife, by which a homogeneous fluid can be expanded by heat, but by increafing the temperature till the fluid itfelf be rarefied, or by introducing fome new fubftance whose folution may produce an evolution of gas. But what is this fubftance to be, and whence is it to come? It will require a new, affumption to provide the leaven which is to fet the bowels of the earth in fermentation. The expanfion by increase of heat cannot take place, because the theorifts themselves have affigned its limits, by depriving the central fire of all pabulum. Increase being impoffible, it must, in conformity with the laws of heat, diminish, by equalizing the temperature of the furrounding bodies, and therefore cannot produce an expanfion. It is in vain that water is prefumed to trickle on it from above. It is equally in vain that the fea is fuppofed to be introduced. This might produce earthquakes, with furious emiffions of gafes and fteam, but no lava. The water must find its way into the interior of the melted mafs, before it could produce the expulfion of a lava; and fuch an introduction is effectually prevented by the inferiority of its fpecific gravity. Pour water on melted iron, and there is no explosion; pour melted iron into water, and ftill there is no explosion; enclofe a drop of water in the heated metal, and no known power can controul it. Befides, admitting the homogeneiety of the melted mafs, which we think cannot be denied, whence come the diversities of lavas ? Why have we bafalt, which is a lava, according to this fyftem, in one place; and glafs in another; pumice in a third, and the earthy lavas in a fourth? Why have we fometimes fulphureous vapours, fometimes muriatic acid, and fometimes hydrogene gas? In thort, whence arife the perpetual variations of volcanic productions? The fpecific gravity of the earth, taken collectively, is found to be nearly double the average gravity of the rocks which compofe its furface. The central fluid muft therefore be of at leaft double the average gravity of rocks. How comes it that lavas and volcanic glafs are generally under the medium gravity of rocks, and that bafalts are very little above it? We have difcuffed this ingenious theory at fome length, becaufe it has been adopted by men of talents, and becaufe, at firft fight, it appears completely to overcome every difficulty, by affuming all that is required VOL. IV. NO. 7. C See Illuftrations of the Huttonian Theory, by Profeffor Playfair, $89. required to be proved. But it appears to us, that, granting this unwarrantable poftulatum in its utmoft extent, it is infufficient to provide the elucidation required. Werner, who had ftudied the extraordinary appearances produced on fuperincumbent rocks by the combustion of beds of coal, applied thefe facts to the explanation of volcanic fires; and fuppofed lavas were formed by the fufion of bafalt. This opinion has fome plaufibility; but it is wholly incapable of accounting for the duration of volcanoes, for their intermittence, or the extent of their operations. Still lefs probable were the opinions of the philofophers who recurred to petroleum and to fulphurets of iron. Breiflac, who, like most men of very extenfive obfervation, is little addicted to theorizing, has been rather unfortunate where he has attempted it. He finds nothing incongruous in the joint action of coal, pyrites and petroleum. He difcoters a bed of coal a foot thick near Beneventum, which he regards with much exultation; though he might as well think of feeding a furnace with a sheet of paper, as of ftimulating a volcano by fuch a fupply. By decompofing his pyrites, he diftils petroleum from the limeftone of the Appenines; it carries with it fome phosphoric matter, (created exprefsly we prefame), and finds its way to commodious refervoirs under Vefuvius. There, water faturated with common falt waits to receive it, and their anion is cemented by the Hymeneal torch of electric flame. The ufual confequences of matrimony, difcord, fury, and uproar, enfue; and the unnatural parents turn out of doors the lava they engender between them." Theorists who thus endeavoured to account for the inflammation of Vefuvius, were much embarraffed to obtain the neceffary fupplies of oxygene. Dr Thomson, whofe refidence at Naples afforded him ample opportunities of observation, and whofe acute genius has in feveral inftances thrown light on volcanic operations, has devifed an explication of this difficulty, more remarkable for its boldness than its probability. He fuppofes that, at certain degrees of heat, the oxygene contained in the carbonic acid of the limestone of the Appenines, may be inclined to enter into new combinations; and he illuftrates this doctrine by the beautiful and well known experiment of Tennant, who operated the decompofition of carbonic acid by means of phofphorus. On this theory, it may be obferved, that it commences by fuppofing the previous exiftence of a heat of great intenfity, without providing any means for its production: 2. It fuppofes the application of fome unspecified bafe to the carbonic acid, to attract the oxygene; Giornale Letterario di Napoli, vol. 106, p. 3. gene; he cannot poffibly fuppofe the phofphorefcent limestone to. contain phosphorus enough for this purpose: 3. It affords no employment for the charcoal of the carbonic acid, which is left. to crystallize into diamonds, plumbago, or what it likes best: 4. There is no way of difpofing of the immenfe quantity of quicklime which this procefs would produce; part of it may be incorporated with the lavas, but the whole cannot be employed in this way, without rendering their bafis almost entirely lime, which is notoriously not the cafe. But the palm of fuperior originality, in this conteft of theoretic invention, must be accorded to the genius of M. Patrin, who has long been advantageously known to the world by his travels in Siberia, and his fplendid collection of Siberian minerals. In an etlay read at the Institute, and afterwards published in a separate form, he procures muriatic acid from common falt by a rather arbitrary procefs, and decompofes pyrites by its means +. He fuppofes fulphur to be concrete electricity, and then identifies it with phosphorus . He manufactures calcareous earth from thunder and lightning §; and he discovers a metalliferous fluid, which is at once the bafe of the muriatic acid, and the generator of metallic veins. It affifts phosphorus in fixing oxygene under an earthy form *; and, with the united aid of the other fubftances we have enumerated, he very fuccessfully accounts for every existing phenomenon. On this theory, we do not prefume to offer any obfervations. Refearches into the original caufes of volcanic inflammation may well admit of diverfity of opinion, where the operations, from which our information fhould be derived, are fo profound ly concealed. The products only are fubmitted to examination and though they are prodigiously abundant, and the obfervers proportionably numerous, there is a woful fcarcity of confiftent evidence. Yet there are fome points on which all agree; and perhaps it may be poffible to arrange the principal facts, so that they may not appear contradictory. Much difputation has arisen refpecting the intensity of the volcanic heat. Those who derived it from the inexhauftible magazine of central fire, were lavifh of it to a degree which very ill fuited the parfimony with which thofe were obliged to husband their fuel, who trufted to coal and petroleum for a fupply. They . contended, C 2 contended, that the heat of volcanoes was extremely small, because it was incapable of altering the forms of the leucites, augites, and feldfpars, which lava fo abundantly contained. Even the illuftrious Dolomieu, the father of correct obfervations on volcanocs, was fwayed by this confideration fo much, as to adopt a very improbable mode of explaining the fufion of lavas, at a low temperature, by means of fulphur. Obferving the fimilitude of lavas to primitive rocks, he concluded that igneous fufion was not produced, but that the heat expanded the fubftance, and allowed its particles to flide on one another. Even this operation was confined to the bafis; for he fuppofes the feldfpars, augites and leucites, to be wholly unchanged. Though he appears to attribute very myfterious effects to the long continuance of heat, he was fo confcious of the improbability of his theory, that he endeavoured to render it more reconcileable to the known laws of nature, by fuppofing that this. ftrange fufion was operated by introducing between the particles an intermediary fubftance in which they were to be fufpended, and which was to be the vehicle of their apparent fluidity. When this fubftance was removed, they approached, and were reunited into a rock refembling that which they had formed previous to the operation. This convenient agent was fulphur; and Dolomieu attempted to establish an analogy between its fuppofed action in rendering rocks easily fufible, and the action of phosphorus in facilitating the fufion of platina *. No analogy, however, exifts between thefe operations. Phofphorus chemically combines with platina, but fulphur does not enter into any fuch combination with lavas; and Spallanzani determined, by direct experiment, that the addition of fulphur nowife affifted their fufion. Even fuppofing that the particles of lava were thus fufpended, it is obvious that, the moment the vehicle was taken away, as Dolomieu fuppofes the fulphur to have been by combuftion, the particles, instead of confolidating, would be left difunited like fand, unless the heat was fufficient to produce their agglutination by igneous fufion; and if it was fo great, the fulphur would be only an unneceffary incumbrance. It may be farther observed, that this theory affumes the exiftence of an immenfe quantity of fulphur, and fuppofes its lavish combuftion in every eruption. But the vapours of Vefuvius contain very fmall quantities of fulphureous fumes. They confift principally of muriatic acid, or hydrogene; Vide Dolomieu, Lipari, p. 95. Id. Journal de Phyfique, an 2. tome 1. p. 118-120 |