DEVITRIFICATION OV GLASS. QJ 



peas, and almoft fimilar to grain, are feen in the midft of the 

 pafte of glafs. They are fmall fpheres flattened at each end 

 with an indentation in the middle of each depreffion. The 

 fides have facetts, like the grain of Indian nafturtium, and 

 thefe fafletts are always fix in number. 



I intend, when I (hall have procured a fufficient number of 

 this lingular fpecies of cryfial, to analyze it, in order to de- 

 termine which of the earths it is that effects fo extraordinary a 

 form. 



I have thus given a fhort account of the principal fa<ts 

 which characterize the precipitation and cryftallization of glafs. 

 It is evident that they are of the fame nature as thofe produced 

 by the cementation pointed out by Reaumur ; and that tin's 

 cementation is always a more or lefs abfolute devitrification of 

 the glafs. 



When the glafs is devitrified, it has no longer a vitreous Recapitulation. 

 but a granulated fracture ; it has no tranfparency, ar.d perfectly 

 refembles a (tone ; it becomes a lefs bad conductor of heal and 

 of electricity ; laftly, it is no longer fufceptible of fufion at the 

 fame degree of heat; and in order to reftore it more eafily to Revivification. 

 the vitreous ftate, it muft firft be pulverized, to bripo- into 

 contact thofe fubftances which, during the cryftallization, had 

 become feparated from each other, and could no longer ferve 

 as fluxes to each other. 



I hope the novelty of many of the facts I have here indicated, General condu- 

 and the confequences I have drawn from them, may be fuffi- f i on and a PP lica - 

 ciently interefting to philosophers to have fome claim to their 

 attention. I have no doubt but that other general inductions 

 will prefent themfelves to them which may have efcaped me, 

 or which could not be introduced in a fhort memoir. The 

 refemblance of my fpecimens of devitrified giafs with thofe of 

 certain lavas ; the poffibility that other lavas may have under- 

 gone a more complete devitrification by a much flower cooling 

 beneath the volcanic currents which are known to liaveilowed, 

 or to have remained fluid for whole years beneath congealed 

 crufts ; — every fact leads me to think, that thefe phenomena 

 may afford a folulion of many geological events, concerning 

 which no general opinion has yet been adopted, becaufe there 

 was no reafon to believe that ftones may have formerly pof- 

 jTefled the vitreous ftate. 



Letter 



