VOLCANO. 



fubftances emitted during an aftive ftate of the volcano, we 

 can only infer the exiilence from the fmell or from their being 

 found in combination with the fohd produfts of volcanoes ; 

 the principal of thefe are ammoniacal gas, muriatic acid 

 gas, and fulphureous acid gas. Probably almoft every mineral 

 tubftance which can be rendered volatile by heat, may be 

 emitted in an aeriform ftate during violent eruptions ; even the 

 earthy matter of lavas is volatilized at a high temperature, 

 as was proved in the experiments of Dr. Prieftley and 

 Spallanzani. 



Volcanic fubftances fluid at a heat below 2 1 2° Fahren- 

 heit are water, which is fometimes thrown out in tor- 

 rents : and fulphuric acid, found in fome volcanic water and 

 bitumen, which has been obferved exuding from lavas at 

 Vefuvius. Sulphuric acid, that abounds in fome water near 

 volcanic mountains, is probably formed during a quiefcent 

 ftate of a volcano, from the combuftion of fulphur in the 

 crater, or in the upper receffes of the mountain. It is not 

 difficult to conceive how the acid may become diffufed in 

 the rain-water, or in that from melted fnow, which may 

 permeate the porous rocks, and delcend in ftreams from a 

 lofty volcano. 



The folid fubftances ejefted from volcanoes, or formed 

 by chemical combination in the crater, comprife inflam- 

 mable, fahne, metallic,, and earthy minerals. The three 

 former are by far the leaft confiderable in bulk. 



Sulphur is found in abundance in the craters of dormant 

 volcanoes : its formation is attributed to the gradual decom- 

 pofition of fulphuretted hydrogen gas, exhaled copioufly 

 through fiffures from below. See Solfaterua and SouF- 



FFRERE. 



Phofphorus is too inflammable to be found fohd among apa- 

 tite volcanic produfts ; it is only from the white colour of the 

 fmoke, from its pecuUar fmell, and from its combination 

 with lime in the mineral called apatite, found near fome vol- 

 canoes, that we can infer its exiftence as a volcanic fubftance. 

 See Apatite. 



SoUd carbon has only been found in fmall quantities, in 

 concrete bitumen in fome volcanic produfts. Carbonized 

 wood and vegetable matter have been found occafionally in 

 lava or tufa ; in all probabihty they were enveloped during 

 an eruption, and cannot therefore be regarded as volcanic 

 fubftances. Carbon, in the ftate of mineral coal, has been 

 fuppofed by M. Werner and his followers to be the prin- 

 cipal fupport and caufe of volcanic fires ; but this opinion is 

 deftitute of all proof, and is at variance with all geological 

 analogies. 



The faline fubftances found in the craters of volcanoes, or 

 formed by volcanic fire, are numerous, though not very 

 abundant. Muriate of ammonia (fal ammoniac) forms an 

 incruftation on many lavas foon after they cool. Muriate 

 of foda (common fait) is found in fome volcanoes in confi- 

 derable quantities, even entire beds of rock-falt are found in 

 volcanic craters, as at Pofa, near Burgos. Mount Cologero, 

 near Sciacca, in Sicily, appears to be a volcanic mountain, 

 impregnated throughout with common fait. Muriate of 

 copper and of iron are found in fome volcanoes, as that 

 of Vefuvius. Sulphate of iron and fulphate of copper, 

 or green and blue vitriol, alum, gypfum, and fulphate of 

 niagnefia may alfo be enumerated among the faline fubftances 

 found in volcanoes. See Sulphate o/" /ron, &c. 



The metallic fubftances found in volcanoes, or among their 

 produfts, are antimony, copper, gold, manganefe, mercury, 

 iron, tellurium, and titanium. 

 . Antimony is found combined with fulphur. 



Copper is found native, and combined with fulphur, with 



7 



iron, and with the muriatic and fulphuric acids, as before 

 ftated. 



Gold is faid to be found in fome volcanic products, 

 and the gold-mine of Nagyag is ftated by Breizlak to be 

 placed in the crater of a volcano. There is a gold-mine in 

 the ifland of Ifchia, which is entirely volcanic. 



Manganefe exilts in a fmall proportion combined with 

 iron in obfidian and lava. 



Mercury is found at Guanca Velua in great quantities^ 

 and it is faid the mine is fituated in the crater of a volcano, 

 M. Patrin fuppofes that fome of the Cinnabar mines in Afia 

 have a finiilar fituation. 



Iron is abundantly diffufed through all volcanic rocks, 

 which have a dark-brown, a black, or red colour. It forme 

 one-eighth part of the fubftance of moft lavas. Iron exifts 

 alfo in craters in the form of fpecular iron ore. 



Tellurium is found with gold in the mines of Nagyag. 

 See Tellurium Mines. 



Titanium, combined writh iron, appears, from the obferva- 

 tions of Cordier, to be a conftituent part of almoft all dark- 

 coloured volcanic rocks. 



The earthy produfts of volcanoes confift principally of 

 lava, obfidian, pumice, volcanic flags or fcorise, vWth vol- 

 canic fand, tufa, and we may alfo enumerate the earthy tufa 

 formed of the indurated mud thrown out of the Americar 

 volcanoes. Many geologills enumerate bafalt and wacke 

 among volcanic produfts, which they refemble both in ap- 

 pearance and in the nature of their conftituent parts. Va- 

 rious cryftallized minerals are found imbedded in lava, par- 

 ticularly augite, cryfolite, or olivine, felfpar, leucite, V'e- 

 fuvian, and zeolite. (See Augite, &c.) Under the articles 

 Lava, Obftd'mn, &c. are given fome account of thefe 

 minerals. 



The ftones firft thrown out of volcanoes are frequently 

 pieces of granite or other primitive rocks, either untouched 

 or only partially changed by fire. This circumftance proves 

 that the feat of volcanic fire is far below thefe rocks. 

 Scorix or volcanic flags are generally thrown out before 

 the eruption of lava. Thefe flags are more or lefs vitrified ; 

 they fometimes take a globofe form in the air, and become 

 confolidated before they cool. Thefe havebeen called volcanic 

 bombs. Immenfe black clouds, confifting of pieces of fcorise 

 and minute fragments and particles, fimilar to the fcoriae, are 

 thrown out with it. Some volcanic eruptions confift entirely 

 of thefe powders or fand, which are driven to vaft diftancee, 

 and have been carried by currents of air more than five hun- 

 dred miles from the volcano. 



Vefuvius threw out fcorije and powders without any lava, 

 for many centuries after the eruption in 79 A.D. ■ 



Lava. — Currents of melted ftone or lava, of twenty or 

 thirty miles in length, from two to four miles in breadth, 

 and from twenty to forty feet in depth, are found in vol- 

 canic dittrifts, equalling in fize fome of the regular ftrata 

 of the globe. The upper furface of thefe lavas is gene- 

 rally more or lefs veficular and fcoriaceous ; and it is only 

 where the beds liave been broken or cut through, that the 

 compaft ftony fubftance of the lava can be ieen. From 

 this circumftance alone many philofophers have been led to 

 doubt the volcanic origin of more compaft rocks ; but, as 

 M. Cordier obferves, in a paper recently publiftied, " to 

 judge of the fubftance of a current of lava, from what ap- 

 pears on the furface, would be like judging of a vat of wine 

 from the froth with which it was covered." The cryftals 

 imbedded in lavas were fuppofed by many geologitts to have 

 exifted previoufly in the rocks which formed the lava, but 

 were too infufible to be melted by the volcanic fire. On 

 this erroneous fuppofition, they concluded that volcanic fire 



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