64 ART OF GLASSMAKINO. 



the glass, which was still fluid beneath the superficial stra- 

 tum, thut had beconie opake and more refractory, was car- 

 ried above »t by the motion of the crucible,iji taking it out of 

 the furnace. A comparison of it with another plate of the 

 same ghi^s, in which we hud only the two strata in thsir na- 

 tural order, appears to me to leave no doubt of the trqth of 

 this explanation, 



Obs. V. DetyUrtfication of glass hy the fire o/volcanoes* 



Supposition The hypothesis framed by the celebrated Dolomieu is welt 



d^fferfrom "" ^'^^^n : that the fires of volcanoes do not act like those of 



common fijes, our furnaces; that, though they produce prodigious effects, 



their activity is not great ; that the fluidity they occasion is 



not that of vitrifying matter ; and lastly, that even the most 



fusible substances, included in the body of rocks, might, 



have flowed in burning torrents, without having undergone 



any perceptible alteratioi*. 



Supposed He imagined he had found a proof of this in the state to 



proof of tl>!s ^yiiicj, pieces of glass liad been reduced at the time of the 



in piHss uuri€Ci i r* ■ 



iaiavaat » drcfidfu! eruption, that covered Torre del Greco, in 1794. 



Crecv^^' This glass, the shape of which was still distinguishable, 

 had become of an ©pake white. The alteration extended 

 sometimes throughout its whole thickness : sometimes it 

 left glass' still untouched, with its original colour and 

 transparency, between two opake crusts. Dolomieu laid 

 before the class several specimens of these glasses, found 

 in digging oii the spot* He was so good as to present me 

 •wit!) a lew specimens, some of which were authenticated by 



Similar effects volcanic scoriae still adhering to themf: and I promised him 



in our glass- jn return !5QVeral fragments, found in a furnace f four d 

 houses. , i, ^ 



(itendrr^, where, as is too frequently the case for the manu- 



* Jouin. de Phys. vol. XXXVII, p. 198- Joum. des Mines, No. 

 22, p. 55. 



f ^Tr. Bieislak mentions in his ToHrihrougb Campania, vol. I, p. 

 9B0, a piece of window glass ber.t indifferent dirtctions, tl»e surfaces 

 of which were coiiTfrted into Reaviomr's porcelain, while the interior 

 reliilned the state and appearance of giass. Ur. Thomson, in his cata- 

 logue of substances found in ihc la^a of 1794, had already described 

 fragmpnt-? 0I" jk-.s \h\\? modified, to vrhich he guvc the name of gl.iss- 

 ■touft. 



facturcr's 



