VOLCANO. 



the fource of heat fituated ?— And, laftly, From what caufe 

 does it originate i Some philofophers contend, that vol- 

 canic heat greatly exceeds that of our common furnaces ; 

 whilft others aflert that it fcarcely exceeds that of a culinary 

 fire. 



The arguments in favour of the low degree of heat of 

 volcanic fire are founded on the experiments made upon lava 

 in a common furnace, which was obferved to vitrify them 

 more completely than volcanoes, and to melt many of the 

 imbedded cryftals, which were fuppofed to have been left 

 infufible by volcanic heat. M. Sage and Deluc firft fup. 

 ported the hypothefis of the low degree of volcanic heat. 

 M. Dolomieu endeavoured to prove it to be ftill lefs. His 

 principal argument is the following. " It cannot be too 

 frequently inculcated that lavas are not vitrifications ; their 

 fluidity is fimilar to that of metals reduced to fufion : when 

 they ceafe to flow, they refume, like metals, the grain, 

 texture, and all the charafters of their primitive bafe ; effefts 

 which we cannot produce on ftones in our furnaces, fince we 

 know not how to foften them by fire, without changing 

 the manner in which they are aggregated. The fire of vol- 

 canoes has not that intenfity which is fuppofed : the effeft 

 is produced rather by its extenfion and duration than by its 

 aftivity." We greatly refpeft the labours of this intelli- 

 gent obferver ; but we mult notice, that in the above ftate- 

 ment he has not appreciated the important difference between 

 the effefts of volcanic fire and that of a furnace, refulting 

 from the more rapid cooling of the materials in the latter 

 cafe. It has been proved by the important experiments of 

 fir James Hall, that vitrification depends not on the degree 

 of heat fo much as on the rapid coohng of ilone or lava in a 

 Hate of fufion ; and that lava, vitrified in the furnace, af- 

 fumes its ftony texture again, if it is remelted, and the heat 

 be very gradually diminifhed. It was proved alfo by the 

 interelling experiments of Mr. G. Watt, that if this procefs 

 of coohng be continued for a ftill longer time, a cryftalline 

 arrangement of the particles takes place. 



It was an opinion long entertained, that the cryftals ex- 

 ifting in lava, whether of fdfpar, aiigite, olivine, leuciti, 

 (fee thefe articles,) or other minerals, were original cryftals, 

 exifting in rocks which had been fubjefted to volcanic heat ; 

 and that this heat, though fufiicicnt to melt the rock itfclf, 

 was not powerful enough to melt the imbedded cr)'ftals. 

 It was fuppofed alfo, that fome of thefe cryftals, previoufly 

 exifting, were found detached by the lava in its courfe, and 

 buried in it. Thefe opinions, fo unphilofophical and impro- 

 bable, are giving place to a more correft and enlarged view 

 of thefe operations of nature. 



The cryftals in lava did not previoufly exift, but were 

 formed during the (low confolidation of the materi.ils, which 

 admitted the elementary particles to enter into different com- 

 binations, according to the laws of elcftive afiinity and 

 cryftalline arrangement, precifely in the fame way that dif- 

 ferent falls in the fame folution feparate from each other, 

 and cryftallize. In the flags from our furnaces we may 

 frequently obferve the fame procefs more or lefs perfeftly 

 completed ; and we have feen cryftals refenibling felfpar, 

 found in a mafs of coal-fliale or bituminous flate-clay, which 

 had been fufed and run down from the large ignited heaps, 

 in the vicinity of Newcaftle-upon-Tyne. The fafts ad- 

 duced to prove the low degree of heat in volcanic fires, 

 prove only its long continued aclion, and not its original 

 degree of intenfity. Dolomieu indeed admits, that a great 

 difference muft refult from the different periods of the con- 

 tinuance of heat. This was fubfequently demonftrated by 

 the experiments of Spallanzani. He took feveral ftones, 

 which had been found refraftory, when expofed to a certain 



degree of heat for two or three days, and placed them in a 



glafs furnace where the fame degree of heat, was continued 

 equally for more than fix weeks ; during which time they 

 were all more or lefs foftened by fire, and the vitrification, 

 which began on the furface, extended deeper and deeper into 

 the ftone, in proportion to the time. Hence, fays he, we may 

 learn, that a long continued heat of lefs ftrength is as effi- 

 cacious in the fufion of bodies as a ftronger heat of a fhorter 

 duration. Dolomieu further conjcftured, that the extreme 

 fluidity of fome lavas was occafioned by the prefence of 

 fulphur, which afted as a flux, in the fame manner as a bar 

 of iron, when brought nearly to a white heat, will inftantly 

 melt, if it be rubbed with fulphur ; but this opinion was not 

 confirmed by experiment. Spallanzani found that fulphur 

 produced no effeft, when mixed with ftone or lava, and ex- 

 pofed to heat ; nor did the lava melt fooner than in other 

 crucibles, in which it was expofed to the fame degree of 

 heat. 



The refult was the fame, whether he employed comiHOn 

 fulphur or iron pyrites. The fafts, jherefore, adduced for 

 the low degree of heat in volcanic fire, prove nothing ; and 

 it is only from the aftual ftate of the lava itfelf, that its 

 greater or leffer degree of intenfity can be afcertained. 



The extreme liquidity of lava flowing from the crater, in 

 fome inftances, has been fhewn in a former part of this ar- 

 ticle, where it is defcribed as fpouting up, and forming 

 curves like a fountain of water. Profeffor Bottis relates, 

 that on the loth of September, 1776, he obferved a fmall 

 hill on the fide of Vefuvius, formed of fcoriae, and fur- 

 rounded by lava recently ejefted. In this hill was a fmall 

 circular gulf, about three palms in diameter, and two in 

 depth. From this gulf proceeded a low noife, fimilar to 

 that of oil or any other fat fubftance fimmering over the 

 fire ; which found was doubtlefs produced, he fays, by fub- 

 ftances fufing within it. The fire was fo ftrong, that fome 

 fcorix being caft into it, immediately became red-hot, and 

 melted, producing the appearance of boihng pitch. Spal- 

 lanzani fays, that the fame kind of ftone required to be half 

 an hour in the furnace before it was foftened ; and in a re- 

 verberating furnace, it required a heat equal to the melting 

 of iron, to obtain a fpeedy fufion of thefe ftones. It is 

 likewife evident, that the heat in this fmall gulf, communi- 

 cating with the cold air above, muft be lefs intenfe than in 

 the internal part, fince this was only a fpiracle or vent to 

 the great mafs of lava which boiled in the deep receffes of 

 the mountain. 



Spallanzani alfo obferved, that when the lava, placed in a 

 common furnace, had been fufed feveral hours, and boiled 

 over the edge of the crucible, its tenacity was ftill fo great, 

 that he could fcarcely with all his force immerge a pointed 

 iron-wire to the bottom ; and when he took away the iron, 

 the impreffion remained fome minutes, though the crucible 

 ftill continued in the furnace. When the fame lavas were 

 expofed to the intenfe heat of the reverberating furnace, 

 they were more liquid, and might be penetrated with 

 greater eafe. 



From thefe experiments and obfervations we are war- 

 ranted in concluding, that the heat of volcanic fires fome- 

 times exceeds that of our moft violent furnaces, but that the 

 lava of different eruptions may poffefs different degrees of 

 fluidity and heat. It may alfo be worthy of notice, that the 

 lavas were confiderably reduced in weight by remaining long 

 in the furnace, the particles having been volatihzed and 

 fublimed. Another argument for the intenfity of volcanic 

 fire is derived from the long continued heat of certain cor- 

 rents of lava. Spallanzani fays, when he paffed a detached 

 current of lava near the fummit of Etna, which had flowed 



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