VOLCANO. 



lava and earthy produAs of volcanoea are formed of the in- 

 ternal beds of rock, which are melted by the inflammation 

 of fulphur or bitumen, and thrown up by the violent pref- 

 fure of elaftic vapour, either from fteam or more per- 

 manently elaftic fluids. From feme ingenious experiments, 

 he afcertained that even the lava itfelf, at a certain tempera- 

 ture, partly afl"umes an aeriform (late, and may then further 

 contribute to the violence of thefe explofions, by which it is 

 ejefted from the crater. 



It was the opinion of bifhop Berkeley, that a vacuum 

 was made within the body of the earth by a vaft body of 

 inflammable matter taking fire, and that the water, by com- 

 munication with the fea, rufhed in, and was converted into 

 fleam. However this be, it is certain, that by the procefs 

 above explained, a vapour would be produced, whofe elafl:ic 

 force is known to be feveral times greater than that of gun- 

 powder ; and, therefore, if the fuperincumbent weight were 

 not too great, it might caufe earthquakes ; and it would 

 propel the matter melted by the fubterraneous fire laterally 

 towards the mouth of the volcano, where meeting with leait 

 refiftance, it would expel it, together with all the unmelted 

 ftony mafles which it found in its paffage. It is eafy to 

 conceive, that before the denfe matter is ejefted, the dilated 

 air of the volcano will be firft forced out, and carry with 

 it the alhes and loofer ftones adhering to the fides and crater 

 of the volcano, in the manner obferved and defcribed by fir 

 William Hamilton. 



That fteam is one of the mod important agents in lifting 

 up torrents of lava to fuch prodigious heights, has been ge- 

 nerally fuppofed : it is even aflerted that the fea has been 

 obferved to retire in the bav of Naples previoufly to erup- 

 tions from Vefuvius ; but this may, with more probability, 

 be afcribed to the upheaving of the ground, than to the 

 fudden abforption of water. Were the water to be ab- 

 forbed ever fo rapidly, other water would inftantly flow on 

 to fupply its place, fo that the apparent level of the fea 

 from this caufe could not perceptibly vary, except for a few 

 minutes. But if the ground were foftened and raifed up by 

 fubterranean heat, the eff^eft might continue for a longer 

 time ; and rtiould even a fmall quantity of water find accefs 

 through fiffures to the deep recefles of melted lava, this, by 

 its rapid expanfion, might force up part of the lava to the 

 fammit of the volcano, and produce the moft; tremendous 

 commotions. 



According to the experiments of Spallanzani, water 

 poured on the furface of melted lava, produced little effect, 

 but when introduced under the furface, it occafioned a moft 

 violent explofion. Similar effefts are often feen in founde- 

 ries ; for if the moulds contain the leaft raoifture when the 

 melted metal is poured in, it is driven back with a loud re- 

 port, and is violently difperfed in every direftion. Thefe 

 experiments, and the reafonings founded upon them, apply 

 rather to the mode in which volcanic fires operate, than to 

 the caufe of thefe fires. It feems exceedingly probable, 

 that the fudden accefs of water, and the generation of im- 

 menfe volumes of elaftic vapour, may be the immediate caufe 

 of moft volcanic eruptions. An explanation of the eruption 

 of Etna, nearly fimilar to this, is given by the poet 

 Lucretius. 



" Praeterea, magna ex parte mare montis ad ejus 

 Radices frangit fluftus, asftumque refolvit. 

 Ex hoc ufque mare fpeluncae montis ad altas 

 Perveniunt fubter fauces : hac ire, fatendum eft, 

 Et penetrare, mari, penitus res cogit, aperto : 

 Atque ecflare foras ; ideoque extollere flammas, 

 Saxaque fubjeftare, et arena toUere nimbos." 



Lib. vi, 1. 694, &c. 



We have ftill however to feck for the origin of the fire 

 itfelf, which this illuftration does not explain. 



The caufe of volcanic fire muft probably be fought in the 

 chemical combination of the elementary matter, of which 

 mineral fubftances are compofed, and not in the combuftioii 

 of any inflammable materials like thofe which exift on the 

 earth's furface. The folid produfts ejefted from volcanoes 

 are compofed of the different earths and alk.ilies ; thefe are 

 not fimple fubftances, but confiil of metalline bafes and oxy- 

 gen. Some of thefe metalline bafes, or metalloids, (as 

 they have been called,) inftantly inflame on contaft with 

 water, and abforb the oxygen from it, whereby they are con- 

 verted into earths or alkalies, having all the properties which 

 the fame bodies poffefs in their natural ftate. ( See Potas- 

 sium. ) This important difcovery of fir H. Davy has been 

 applied to explain the origin of volcanic fires. It has been 

 fuppofed that the furface of the globe, formed of the dif- 

 ferent earths, may be regarded as its oxyded cruft, but that 

 the internal parts are prmcipally compofed of the metalhne 

 bafes of thefe earths ; and whenever water finds accefs to 

 them, they oxydate rapidly, and inflame, and are thrown 

 up in the form of e.irthy lavas, &c. giving rife to all the 

 various phenomena attending volcanic eruptions. This hy- 

 potheCs, though fimple and ingenious, is not free from va- 

 rious objeftions. It is exceedingly difficult to conceive how 

 fubftances fo inflammable and oxydable could remain for 

 ages in a metallic ftate, protefted from the accefs of moif- 

 ture. Perhaps the difficulty we feel in admitting this may 

 arife from our having obferved in the inflammation of potaf- 

 fium by water, that the whole was almoft inftantly burned 

 and diffolved ; but were we to fuppofe a compaft mafs of 

 this fubftance to exill in the earth, of a mile or more in 

 thicknefs, on the accefs of a limited quantity of water, the 

 furface would inflame, and be reduced to an alkali, and form 

 a cruft, which would proteft the internal part from inflamma- 

 tion. Another current of water might diffolve this cruft, 

 and again inflame the potaffium. By a fucceflion of fuch 

 currents, the metalline beds in the earth may be fuppofed to 

 be repeatedly inflamed, until the whole mafs was oxydized, 

 when the volcanic fires would there be for ever extinft ; 

 unlefs we can conceive a procefs of deoxydation to take 

 place, and reduce the earths and alkalies once more to a me- 

 tallic ftate. The currents of eleftric light at the north and 

 fouth poles may lead us to fufpeft that eleftric agency is 

 operative in the interior of the globe, and it would not ap- 

 pear contrary to analogies, were we to fuppofe that it may 

 perform an important part in the procefs of deoxydation, and 

 other chemical changes, which produce metallic veins, vol- 

 canic eruptions, and other geological phenomena. When 

 the attention of philofophers was ftrongly drawn to the 

 phenomena of eleftricity by the difcovery of the Leyden 

 phial, in the middle of the laft century, it was fuppofed 

 that this powerful and myfterious agent was the principal 

 caufe of the phenomena of earthquakes. Ingenious and 

 plaufible theories were framed, to explain its mode of opera- 

 tion, and its agency was extended to account for volcanic 

 fires alfo. The quantity of eleftric matter evolved from 

 volcanic fmoke in the thunders and lightnings which accom- 

 panied eruptions, were fuppofed to indicate that the difen- 

 gagement of eleftric matter gave rife to all the phenomena 

 of volcanoes. It may be obferved that the data on which 

 thefe theories were formed was defeftive : the eleftric matter 

 evolved from the fmoke and vapour of volcanoes was the 

 neceffary effeft of the fudden formation and expanfion of 

 aeriform fluids : this is rendered fenfible when a fingle drop 

 of water is converted into fteam, and. muft be moft powerful 

 when immenfe volumes of vapour are inftantly generated. 



In our fpeculations refpefting the origin of volcanic fires, 



it 



