ART OF GLASSMAKIN*. 6l 



scratciied by rock crystal. The interior has remained of the 

 nature of a green glass, transparent, and forming a geode A gcode. 

 l^y the contraction of the matter, while adhering to the 

 fiTixst. This glass contained oxide of lead. 



No. 3 includes two artificial intaglios. Tliey are made of ^^.^Jji^^^m- 

 bottle glass, moulded first in a cupelling furnace on an ira- nglios of 

 pression taken with tripoli, and then devitrified at a heat of "^"^'^ S ^^s. 

 51* of Wedgwood. They are not electric by friction even 

 on the polished faces. Their specific gravity is 2*801 . 

 Corundum scarcely leaves a visible trace on them. This 

 hardness, which is such that it- allows them to be used as 

 dies, and the chastity with which impressions from them 

 represent antiques, have introduced the products of these 

 trials among the ornamental arts ; and this not only for 

 intaglios, but also for cameos, the figures of which are Cameo=. 

 QOW. executed of a different colour from the ground, by the 

 addition of coats of glass of a different composition, to imi- 

 tate onyxes; devitrification being afterward employed, to 

 give them that hardness, which is the principal characteristic 

 of precious stones. 1 shall suy no moce of this new art, the 

 processes of which, we may easily foresee, will be improved 

 by that industry, which will also extend its applications. 



No. 4 is a piece of a globe of the same glass, cut to be Specimen 4, 

 used as a capsule, and afterward devitrified in Reaumur's ^*P*" *^ 

 cement at 50* Wed g. The fragments with it, being from 

 a similar capsule, show the striated fracture. Corundum 

 'scarcely leaves a visible mark on them, and tUey are not 

 sensibly electric by friction. These fragments may be 

 heated red hot in the tire, and immediately thrown into 

 water, without losing any thing of their solidity. 1 have 

 kept them in sulphuric acid in the strongest fire, and they 

 have come out without the least alteration, or loss of 

 weight. 



No. 5 is remarkable for the differences it exhibits. It Specimen 5. 



has still the vitreous fracture: is evidently translucid ul the Effect pf slow 



> , « . . , . refrsgeratToti 



edges ; becomes electrical by friction ; and is scratched by merely. 



silex. Accordingly it differs from bottle glass only by the 



appearance of a grayish white porcelain or enamel, \vhi<;h it 



has acquired in losing its transparency. But these diffe- 



fences may be accounted for by the process employed in 



this ^ 



