ART OF GLASSMAKING. ^^ 



facturers profit, the broken glasses are raised on the sides, 

 to remain till the working ceases, or till their accumulation 

 obliges it to be stopped for emptying the furnace; in which 

 he would find the same alterations, and the same progress 

 of devitrification. 



Dolomieu, having seen them in my collectipn, with prof, whkh con- 

 Pfaff, of Kiel, who was then in Paris, frankly confessed, n"f^ 

 that he had nothing to object agamst the identity of the ef- 

 fects of the glass-house fire and the fire that had acted on the 

 glasses found at Torre del Greco; and he selected a few 

 specimens for his collection. 



The fact, which authorizes us to compare th^ effects of Experiments 

 the fire of volcanoes and of our furnaces at equal intensities '" '^""^^f"*- 

 IS supported by experiments, communicated to me by Mr. d'Arcct. 

 d'Arcet, which are equally interesting for their practical 

 application, and the consequences deducible from them 

 respecting the formation of basaltes. 



It is well known, that basaltes fuses about 60* Wedg. : Sir James 

 gnd, as sir James Hall has very justly remarked, the Product ^^''''Jjj.Qgnjg 

 of this fusion is a glass, having all the characters and pro- 

 perties of volcanic glass*. I have obtained some myself, in 

 pretty considerable masses, from the basaltic prisms of the 

 extinct volcano of Drevin, which, after the operation, could 

 not be distinguished faom the glass produced by the fusion 

 of hornblende rock, touchstone, or vitreous obsidian lava. 



Mr. d'Arcet tried the processes of devitrification on vol- Devhrificatiaa 

 canic gla|s itself. He subjected to it pieces of 15 or ifiofrolcanic 

 cub. cent. [9 or 10 cub. in.], and of spec. grav. from 2*775 *'***' 

 to 2*784 ; and observed, that they were completely devitri- 

 fied in the fire of a cupelling furnace ; but, if the heat were 

 carried to 50* Wedg. only, part, that had been before devi- 

 tritied, returned to the state of glass. 



I need not remark the conformity of these results with The results 

 those, which sir James Hall obtained by the slow cooling of agreed witk 

 basaltes, which he had first converted into glass; and on ^" ^' "*"'*' 

 which principally he founded his opinion, that bajaltes had 

 been originally in a state of vitreous fusion. 



. * Journ. de Phys . jermiaal, aa 7, p. ^^17. [See Jaurnal, 4to «#» 

 XellV, pp. 8,56.] 



Vol. XXXI.—Jan. 1812. F The 



