the Stony and the Fitrecus Char alters in Lavas. all 



and are dated In the annexed table, which may be depended upon, in every example, to 

 Mpithin two, or at mod three degrees. The artificially cryllallized fubftances have been 

 denoted, at Dr. Hope's fuggeftion, by the name of cryjiallite. 



The various fufibilities ftated in the table afford fome conclufions of confequence. The 

 whins fiibmitted to trial are more refracSlory than the lavas j though their excefs in this re- 

 fpc£l is not great, fince the mofl: fuftble of the former clafs equal the moft refradory of 

 the latter. The glafles are all incomparably more fufible than the original ftones. This 

 laft circumllance has long been known as a fail •, but Sir James propofes to fugged the 

 theory of it, and of all the peculiarities which occurred in thofe experinieots, in a fecond 

 communication which he means foon to lay before the Society. 



It is obfervable, that the lava No. 12 is fufible at i8 ; that is, it refembles in this pro- 

 perty the moft fufible glafles. And it is in fadt a glafs ; for, being lifted in a foft date from 

 n flowing lava of Vefuvius by Sir James Hall, it quickly cooled, and has of confequence 

 affumcd the vitreous charafter in every refpedl : for, befides ks eafy fufibility, it pofleflbii 

 the fliining furface and fradlure of glafs. This fubftance, being treated like the artificial 

 glafles of whin and lava, crydallized like thefc, and aflumed the chara6ter of a dony laVa, 

 both in texture and in difficult fufibility, fince it foftencd only at 35°. Here then is a 

 proof beyond difpute, that the dony chara£ter of a lava isoccafioned folely by flow cooling. 

 Although the internal drudure of lava was thus accounted fo_r, yet Sir James was em- 

 barrafled with the date of its external furface ; which, though cooled in conta6l with the 

 open air, is feldom or ever vitreous, holding an intermediate dation between glafs and 

 done ; — bqt thi? difficulty was removed by a circumdance which took place in the courfe of 

 thefe experiments. It was found that a fniall piece of glafs of any of the lavas, or of feveral 

 of the whins, being introduced into a muffle, the temperature of which was at any point 

 between the 20th and the 22d degree of Wedgwood's fcale, the glafs became quite loft ia- 

 the fpace of one minute ; but, being allowed to remain till the end of a fecond minute, it was 

 found to have become hard throughout in confequence of a rapid crydallization, to have 

 lodits character of glafs, and to have become by 12 or 14 degrees more infufible, being. 

 iraafFefled by any heat under 30, though the glafs had been fufible at 18° or at 16. This, 

 accounted for the fcoria on the furface of lavas ; for the fubdance even at the furface, being 

 in contact with the flowing dream and furrounded with heated air, could not cool with 

 exceffive rapidity; and the experiment fliows, that diould any part of the mafs, in defcend- 

 ing heat, employ more than one or two minutes in cooling from 22 to 20, it would itii 

 fallibly lofe its vitreous chara£ler. , .! 



The internal phaenomena of volcanos being thus explained by the properties of commoa 

 fire, and the refemblancc, or:identity, of many lavas and of whindone being thus edablifh- 

 ed. Sir James Hall conceives, that the powerful arguments advanced by Dr. Hutton to 

 prove the igneous origin of whindone and other mineral bodies, are very drongly corrobo- 

 rated ; fince thefe experiments fhow that thefe fubdances may have been formed by a fimple 

 extenfion of the fame caufes which continue at this day to agitate various parts of our 

 globe. 



Independently however of any allufion to fyftem or to general theory, Sir James Hall 

 flatlets himfelf that thefe experiments may be of fome importance by fimplifying the hiftory 

 ^f volcanos } and above all by fujcrfeding fome very extraordinary, and-, he conceives,, 



P P ? Mnphilofophical 



