ART OP QLASSMAKtNG. . 59 



real cause of this phenomenon, which was too long consi- ^^ ^Qj^gj^,,^^, 

 dered as the product of a cementation accordinj; to Reau- nued heat, 

 mur's process*. Certain facts, which I noticed long ago, 

 may furnish some particulars illustrative of the explanation 

 he has given. 



In 1782, Mr. Ciffle, a porcelain manufacture at Luneville, Specimens, by 

 sent me several specimens of glass of different qualities, 

 rendered opake by the long-continued action of heat, with- 

 out having been Burrounded by a mixture of sand and 

 gypsu^ in Reaumur's mode. The five pieces, which I lay 

 before the clasSj made part of these specimens, and have the 

 original labels still fastened to them. 



No. 1 is a piece of common window glass, 13 cent, by 10 No. 1. 

 [5 in. by 3'9t]» which, by exposure to the strong heat of m 

 porcelain furnace, without any cement, has become abso- 

 lutely opake, and very white, without any alteration of its 

 shape ; and has acquired much greater hardness and soli- 

 dity. 



No. 2, a piece of the same winri^sw glass, exposed in the No. 2. 

 same furnace, and touched by the flame, has also become 

 opake, and of a fine white in the fracture ; but the surface 

 has a yellowish tinge. 



No. 3 is a piece of bottle glass, kept in the fire in char- No. 8. 

 coal dust, equally become opake, of a fine white internally, 

 with u uniform shining coat of brown black over all its 

 surface. 



No. 4 is a piece of a bottle, which has undergone the No. 4. 

 heat of a porcelain furnace surrounded with powdered soot. 

 ]t has acquired a coat of a deep bistre-colour, but within is 

 completely devitrified, and equally white. 



No. 5 is the bottom of a bottle, which was exposed to the No. 5, 

 most violent heat without being surrounded with any thing, 

 and has become white and opake throughout its whole 

 thickness. 



At the time of these experiments by Mr. CifHe, and in- GlatsAeyitri- 

 deed some years before, Mr, James Keir had announced, ^^^ without 

 that glass might be rendered opake by long continued an- Mr Keir. 



* Mem. de PAcad. des Sciences, 1739. 

 t It was longer when it came to my hands, but I reduced it to these 

 dimensions, in order to make experiments with some pieces of it. 



nealing. 



