225 



James Hall. These experiments prove, that a 

 stone, which is fusible only at thirty-eight de- 

 grees of Wedgwood's pyrometer, yields a glass, 

 that softens at fourteen degrees ; and that this 

 glass, melted again and unverified (glastenise), 

 is fusible again only at thirty-five degrees of the 

 same pyrometer. I applied the blowpipe to some 

 black pumice stones from the volcano of the 

 Isle of Bourbon, which, on the slightest contact 

 of the flame, whitened and melted into an 

 enamel. 



But whether obsidians be primitive rocks, 

 which have undergone the action of volcanic fire, 

 or lavas repeatedly melted within the crater, the 

 origin of the pumice stones which they envelope 

 at the Peak of TenerifFe is not less problematic, 

 This subject is the more worthy of being inves- 

 tigated, since it is generally interesting to the 

 geology of volcanoes ; and since an excellent 

 mineralogist *, after having visited Italy and the 

 adjacent islands with great attention, affirms, 

 that it is highly improbable, that pumice stone 

 owes it's origin to the swelling of obsidian. 



On recurring to the observations, which I have 

 had the means of making in Europe, in the Ca- 

 nary islands, and in America, I conclude, that 

 the term pumice stone does not denote a simple 

 fossil, like the word calcedony, opal, or pyroxene, 



* M. Fleuriau de Belleyue, Journ. de Phy, t. lx, p. 451 

 et 461. 



VOL. r. Q 



