^2 ART OF OLASSMAKING* 



thh experiment, the object of which was, to learti what 

 would be the result of slow refrigeration alone. It is evi- 

 dent, that the heat was not carried to a sufficient height, 

 or that it was not continued long enough, to complete the 

 devitrification. 

 Specimens 6, No. 6, 7> and 9^ are the results of trials to devitrify pieces 

 7 and 9. (jf Stained glass from church-windows, coloured red by 

 oxide of gold, and blue by oxide of cobalt. The first two, 

 ^^^ in losing their transparency, have acquired a purple tinge : 



but one of them, which had lead in its composition, had 

 but little consistency, and was interiorly full of blebs, and 

 ^ as it were spongy ; while in the other the devitrification had 

 pursued its usual and regular course from tiietwo sur£»ecs, 

 leaving in tHe middle a remainder of vitreous matter, which 

 would have disappeared on a longer continuance of the fire. 

 This exhibits some signs of electricity by friction; and 

 both are scratched by rock crystal. 

 Blue. The fragment stained by cobalt announced by the aspec*^ 



of its fracture, which was stiil a little vitreous, that the devi- 

 trification was not far advanced, It had l6st all its transpa- 

 rency however : and its blue colour, though weakened in 

 the mass, was much more intense at the surface. It wa» 

 still a weak insulator. Its hardness was such, that corun- 

 dum scarcely made a perceptible impression on it. 

 Specimens. ^^* ^ ^^ remarkable as a fresh proof, that devitrification 



Farther proof, always commences at the surfaces, and proceeds gradually 

 tion b^y^^n's ^^" *° *^^ ^^"*''^^' when the heat is continued long enough, 

 at ibe surface. This piece resembles a small geode, the crust of which, 

 completely devitrified, includes a portion of the substancft 

 still in the state of perfect glass. We shall find, that these 

 accidents equally occur in the devitrifications by the fire of 

 volcanoes. 

 specimen 10. No. JO exhibits a result still njore interesting. It is aft 



Attempt to attempt to form an intaglio, not bj' raouldiug it in the 



form an intag- ^ .... 



Ijo, after the state of glass, and devitrifying it afterward ; but by. devi- 



lOTi'rifie^d trifying it previous to its being placed on the mould, to re- 

 ceive the impression. The fusion has produced a very 

 honiogeneous mass, of a dull gray colour, which exhibited, 

 though im{)erfectly, an impression of the figure in relief on 

 which it \iad been cast ; while its conxpletely vitreous frac- 

 ture, 



