•286 Experimental Inve/ligat'ioti of the Caufes of 



This difficulty had formerly occurred to Sir James Hall; who had endeavoured to obviat*' 

 It by ftating, that the mafs during flow refrigeration in the bowels of the earth had under- 

 gone a change Cmilar to that of glafs into Reaumur's porcelain ; and that by cryftallization 

 it had loft the vitreous, and aflumed the ftony charafter. The truth of this explanation 

 has now been amply confirmed by thefe experiments, which comprehend feven different 

 fpccies of whinflone. Each of the original fubltances was reduced, by fufion and fubfe- 

 quent rapid cooling, to a ftate of perfeft glafs. This glafs, being again placed in the 

 furnace, was fubjefted to a fecond fufion. The heat, being then reduced to a temperature 

 ^encirally about 28° of Wedgwood, was maintained itationary for fome hours ; when the 

 crucible was either immediately removed, or allowed to cool with the furnace. The con- 

 fequence was, that in every cafe the fubftancc had loft the charadler of glafs, and by 

 cryftallization had aflumed in all refpeds that of an original whinftone. 



It muft be owned, that in moft cafes the new produdtion did not exaclly refemble the 

 particular original from which it was formed, but fome other original of the fame clafs j 

 owing to accidental varieties in the mode of refrigeration, and to chemical changes which 

 unavoidably took place during the procefs. In the cafe, however, of the rock of Edin- 

 burgh Caftle, and of that of the bafaltic columns of Staffa, the artificial fubftances bear a 

 complete refemblance to their originals, both in colour and texture. 



Sir James Hall conceives, that the objcftion to Dr. Button's fyftem, mentioned above, 

 is thus completely obviated ; fince the ftony character of whinftone is ftiown to be the 

 natural confequence of the flow refrigeration, which, according to that fyftem, muft have 

 taken place in the bowels of the earth, as the fubftancc of whinftone pafled from a liquid to 

 a folid ftate. 



Experiments were made with equal fuccefs on lava. This clafs of bodies is known to 

 pofl'efs the cryftalline and ftony chara£ler, in common with whinftone, and to refemble it 

 fo much, that in many cafes the two fubftances cannot be diftinguiflied by their external 

 appearance *. In others, however, certain circumftances occur by which they are charac- 

 terized. But in confequence of fufion thefe diftinguilliing marks dlfappear ; and the glafs 

 of a lava feems in no refpedt to differ from that of a whin. Sir James Hall therefore infers 

 that the two fubftances are intrinfically the fame, and afcribes their differences to the cir- 

 cumftances under which each clafs paffed from a liquid to a folid ftate ; the lavas having 

 cooled in the open air, and the whins, according to Dr. Hutton, in the bowels of the earth. 

 Six lavas, four of which f made part of undoubted currents of ^tna and Vefuvius, and 

 broken from thofe currents by Sir James himfelf, were fubmitted to experiment. The 

 glafs produced by fufion and rapid cooling, yielded, when treated like the glafs of whin, the 

 fame kind of cryftallized, ftony, or earthy maffes, completely refembling an original whin or 

 lava. 



In the courfe of thefe experiments, the fufibilities of the various original fubftances, and 

 of thofe artificially produced, were afcertained with much care by Wedgwood's pyrometer, 



• Thefe obfervations are applicable to lavas in general, comprehending all thofe of iEtna and Vefuvius ; but 

 do not ftriftly apply to fome of the lavas of the Lipari Ifles, and of Ifchla, which poflcfs fome curious and in- 

 terefting properties to be mentioned in a future communication, H. 



i Ko. S) 9) II anil '^ 0^ fhc tabic. 



an(i 



