VOLCANO. 



The range of volcanoes in the Andes, to the fouth of 

 Quito, extends in a right line nearly two tlioufand miles ; 

 and if thefe have originally rifen from one vaft chafm, like 

 the volcanic ranges of whoffe origin we have authentic re- 

 cords, it would not appear extravagant to fuppofe that this 

 chafm may deCcend to the very centre of the globe. Some 

 philofophers, indeed, contend for the exiftence of central 

 heat in our planet, which gives rife to all the different phe- 

 nomena of earthquakes and volcanoes. (See Systems of 

 Geology.) We (hall advert to this opinion in treating of 

 the various explanations which have been given of the ori- 

 gin and fupport of volcanic fires. 



In contemplating the impreflive phenomena of volcanoes, 

 and the great changes they have produced on the furface of 

 the globe, we cannot be furprifed that philofophers, ancient 

 and modern, (hould have been anxious to difcover the origin 

 of thefe fires, and the means by which they are fupported, 

 but from the nature of the fubjeft, their theories can be en- 

 titled to little more than tlie appellation of probable con- 

 jeftures. 



In all inquiries of this kind, it is important to bear in 

 mind the elTential diftinftion between the caufe of any 

 natural phenomenon, and the mode in which that caufe 

 operates. With the latter we may become well acquainted 

 by attentive obfervation, while we remain profoundly ig- 

 norant of the former. Thus, when in volcanic operations 

 we obferve the expanfive effefts of heat, forcing a vent for 

 the difchargeof aeriform, fluid, or fohd matter, we may infer 

 that thefe effefts do not differ in kind, but in degree only, 

 from the fame effefts of heat when fubjefted to the controul 

 of human agency ; but we can draw no certain inference 

 from hence refpefting the origin of volcanic fire, or the 

 fubftamces by which they are kept burning for thoufands of 

 years with increafed or diminifhed intenfity. 



The opinion formerly moft prevalent refpefting the origin 

 of volcanic fire, was that it proceeded from the fubterranean 

 Jermentation of certain materials which were difpofed to 

 inflame and explode fpontaneoufly. When the decompo- 

 fitiou of iron pyrites by water, and the ipontaneous inflam- 

 mation attending it was firfl obferved, and particularly when 

 tlie experiment of Lemery was known, where inflammation 

 is produced by a mixture of iron-fiUngs, fulphur, and water, 

 it was imagined that a fatisfaftory explanation of the caufe 

 of volcanic fire was difcovered. 



In this experiment he mixed twenty-five pounds of pow- 

 dered fulphur with an equal weight of iron-filings : and 

 having made with water a pafte of the mixture, he put it 

 into an iron pot, covered it with a cloth, and buried it a 

 foot under ground. In about eight or nine hours time 

 the earth fwelled, became warm, and cracked : hot ful- 

 phureous vapours were perceived ; a flame which dilated 

 the cracks was obferved ; the fuperincumbent earth was 

 covered with a yellow and black powder ; and, in fliort, 

 a fubterraneous fire, producing a volcano in miniature, 

 was fpontaneoufly lighted up from the reciprocal aftions of 

 fulphur, iron, and water. See Artificial Eartiiquakks. 



The above experiment has been often repeated ; and it 

 has been obferved, that large quantities of the materials 

 arc not requifile to make the experiment fucceed, pro- 

 vided there be a due proportion of water : half a pound 

 of fteel-fiLngs, half a pound of flower of brimflonc, 

 and fourteen ounces of water, will, when well mixed, ac- 

 quire heat enough to make the mafs take fire. But it 

 was known long before the time of Lcmcry, that natu- 

 ral mixtures of fulphur and iron would fpontaneoufly 

 take fire. Thefe fubilanccs, it is well known, are fupphcd 

 by the pyrites ; a fmall quantity of which is fufficient to 

 kindle a firo ; a proper portion of water (for too great a 

 Vol. XXXVII. 



quantity would extinguifli the fubterraneous fire) may be 

 derived either from tifTures and channels communicating 

 with the fea, or from fources in the earth, wherein it is 

 known to abound ; and air, if it fliould be thought abfo- 

 lutely neceffary to the fpontaneous firing of the pyrites, 

 may be conceived eitlier to accompany the wt>t';r, or to 

 defcend into the innermoft parts of the earth through the 

 fifl"ure3 which are found on its furface. Or, if we fuppofe the 

 heated pyrites to have been in contaft with the oxyd of manga- 

 nefe and petrol, the flame may arife, as it is produced by art, 

 from the deficcation of that fubilance, and its mixture with 

 the mineral oil. That ore when heated affords oxygen gas, 

 of which a very fmall quantity is fufficient to produce flame ; 

 and the flame, when once produced, may be fupported by 

 pure air from other ores, as Dr. Prieftley has fliewn (Obf. 

 on Air, vol. iv. p. 210, &c. ) ; and tlie inflammable matter, ac- 

 cording to his fyfl:em, may be fupplied by pyrites, bituminous 

 fchiftus, bitumen, and coal. After the eruption in any place, 

 the volcanoes themfelves ferve for fpiracles or air-holes, by 

 which the fubterranean fire may receive neceffary fupplies ; fo 

 that thefe may ferve to keep the magazines of internal fire iu 

 a due ftate, as well as to difcharge the fmoke and other 

 matters with which it would otherwife be choaked up and 

 extinguifhed. 



Many of the regular ftrata are impregnated with iron and 

 fulphur in the form of pyrites, and it was only neceffary to 

 provide for the accefs of water and air, to produce fpon- 

 taneous inflammation. Thus the cliffs near Charmoutli, m 

 Dorfetfhire, abound in pyrites, and after a very hot fummer 

 and heavy rains, they took fire, and continued burning 

 flowly for a long time. Thefe cliffs are principally cora- 

 pofed of pyritous clay, forming part of the great llratum, 

 called lias, in the well of England. See Strata. 



Tlie abundant evolution of fulphuretted hydrogen gas from 

 the decompofition of pyrites, tended further to confirm the 

 opinion that afcribed to this caufe the origin of volcanic fire. 

 We conceive, however, that this theory is quite inadequate to 

 explain volcanic phenomena on a great fcale, fuch as the con- 

 neftion which difl;ant volcanoes have with each other, the 

 long continuance of the fire, and its breaking forth again in 

 the fame place, after it has ccafed to burn for ages. Some 

 phenomena, however, which are nearly allied to volcanic, 

 and appear to be local, may be produced by pyritous de- 

 compofition. The eruptions of mud in the Crimea, and at 

 Maccaluba in Sicily, may derive their origin from this caufe, 

 particularly as the matter thrown out is obferved to contain 

 particles of pyrites, whereas they have rarely, if ever, been 

 obferved in the matter erupted from nre volcanoes. 



The inflammation of fulphur and bitumen has been fup- 

 pofed by fome philofophers to occafion the various phe- 

 nomena of volcanoes, but where do thefe fubflances derive 

 the oxygen neceffary to fupport their combuflion ? Spal- 

 lanzani lias conjectured that this may be obtained from va- 

 rious faliiie ingredients which yield their oxygen to heat, or 

 it may be derived from the decompofition of water ; but 

 here we meet again with the fame difficulty as before ; iiow 

 are the combullible materials renewed for ages in the lame 

 place ? This would feem to require currents of liquid ful- 

 phur and bitumen to circulate through the interior of the 

 globe, a circumllaiice which the theory of Spallanzani has 

 not provided for, but which docs not appear to us very im- 

 probable. This induflnous obferver could not deleft the 

 flightefl fmell of bitumen in the volcanic fmoke of Strom- 

 boli, but according to Dolomieu and Humboldt, it is very 

 perceptible in Veluvius, and bitumen is even found in the 

 recently erupted lava of that mountain. Sulphur, in its 

 different combinations, is a conftant produft of all volcanoes. 



According to the opinion of Spallanzani and others, the 

 3 M lava 



