QQ DEVITRIFICATION OF GLASS. 



Thefe accidents, their different caufes and remedies, are 

 treated at length in the firft part of ray work, where I fpeak 

 of the fonding of glafs ; but though the explanation of thefe 

 facts entirely belongs to the theory of devitrification, it will 

 be fufficient in this place to mention the phenomena which 

 arife from the prefence of the different earths. 



Cryftailization Colourlefs glaffes contain more or lefs of lime, for the rea- 



ot lime in glafs, f ons w hj c h I have mentioned in fpeaking of the different com- 

 pofitions of glafs. This lime, when in excefs, gives cryftals 

 very well, as Loyfel remarks: they are eafily known, and fo 

 abundant, that they abfolutely impair the tranfparency : they 

 are prifms, which feem to float in the midft of the pafte of 

 glafs, and tend to unite in ftars differently figured : thefe 

 prifms are all nearly of the fame thicknefs, and two or three 

 millimetres in length (about one-tenth of an inch). 



in large mr'fTes. When this cr) ftallization takes place fpontaneoufly upon 

 x large mafles at the bottom of the furnace, the colour of the 

 glafs becomes darker, and inclines to black, by the mixture 

 of a certain quantity of the allies. The ftrialed ftars here 

 fpoken of, become more numerous, the more remote they are 

 from the fide in contact with the fire. Cryftallizations at firft 

 infulatcd, are loon fucceeded by a mafs entirely cryftallized, 

 in which the vitreous character is no longer diftinguifhable. 



Modified cryftal- Thefe are the mod common characters of cry flail ization ; 



iizauonsin glafs. ^ ut others are frequently feen, which are certainly owing to 

 accident, and well deferve notice on account of their va- 

 rieties*. I have fome pieces of glafs in which cryftals are 

 difcernable, fo fine, that they can fcareely be feen with a mag- 

 nifier. They are prifms divergent from the fame center, and 

 forming ftars, which frequently-do not exceed one millimetre 

 (or half a tenth of an inch) in diameter: their union refembles 

 a (light -mi ft in the pafteof the glafs. 



Some pieces have the afpeel of a faline cruft applied to a 

 foreign body, againft which the glafs was in contact. In 

 fome of thefe this cruft being compofed of ftriated protube- 

 rances, appears to advance more and more into the glafs 

 ilfelf. 



Laftly, there is another variety perhaps more curious than 

 all the reft, in which centers, or cryftallizations refembling 



* Mr. Sage poffeffes a piece of glafs cryftallized in fix-fided ba- 

 faltic prifms, totally devitrified. 



peas, 



