'»S8 Reinaridhle H-jhUmhs af Folcuntc PraJuSIs, 



tiiiphllofoplilcal opinions advanced with regard to volcanic heat, which has been dated as 

 po{refling very little intenfity, and as ailing by fome occult and inconceivable influence, or 

 with the help of fome invifiblc agent, fo as to produce liquidity v/ithout fufion, Thefe 

 fuppofitions, which have been maintained ferioufly by fome of the moll celebrated na- 

 turaliRs in Europe, have originated from the difFicuky of accounting for the ftony charadler 

 of lavas, when compared with that of glafs, which they aflume in confequence of fufion in 

 our furnaces. But now he hopes we may be relieved from the necefiity of fuch violent 

 efforts of imagination, fince the pha^noniena have been fully accounted for by the fimple 

 though unnoticed principle of refrigeration, and have been repeated again and again with 

 enfe and certainty in a fmall chamber furnace. 



T ABLE. 

 The fufibilities, according to Wedgwood's pyrometer, dated in the following table, were 

 afcertained by heating the fubffances in a muffle, in which they could be dillindlly ob- 

 ferved, while expofed to the a£lion of the heat. A fmall piece of the fubftance to be 

 examined being put into the muffle, a pyrometer-piece was placed clofe befide it, and the 

 heat raifed gradually. The fubifance was touched at intervals with a {lender iron rod ; 

 and when found fo foft as to yield eafily to flight preffurc, the pyrometer was withdrawn, 

 and meafured. 

 [ All the whins except No. -were taken from their oriijinal rocks in the neighbourhood of Edinl urgh. ] 



