DEVITRIFICATION OF GLASS. (j£ 



entirely changed. It no longer exhibits the tranfparency, but 

 is a mafs of cryftals, compofed of fmall needles, converging 

 towards common centers. It has no longer the appearance of 

 glafs. 



This fa<5l (hews with what facility bottle glafs is devitrified, 

 and always without the lead appearance of cementation. The 

 infiniie variety of fubftances which make up the compofition 

 of bottle glafs, greatly modifies the phenomena which take 

 place during their devitrification, and muft, no doubt, influ- 

 ence the form of the cryftals; but my opportunities of obferv- 

 ing this kind of glafs have not been numerous. 



Palling therefore to glafTes lefs earthy and compofed of a lefs Greenifh glafs of 

 number of fubftances, when I make the fame examination o( Al ^ tia is lefs 

 the bottoms of the furnaces of glafs tor windows called glafies and changes' 

 of Alfatia, or half clear glafles, into which is put a greater more flowl y« 

 portion of pure (and and of alkali, I obferve nearly the fame bllifh colour^ 

 phenomenon ; but as they are lefs hidden, they are more eafy 

 to diftinguifh and to feparate. At the firft inftant, and in the 

 mafles where the devitrification commences, we feem to ob- 

 ferve a fmall portion of blue colour diffufed through a greenifh 

 liquid. 



A very lingular fa6> here prefects itfelf, which I (hall here 

 .only point out, with the intention of more fully examining it 

 hereafter. This greenifh glafs mixed with blue, appears, in 

 facl, to become of an obfeure blue when confidered by re- 

 flected light ; but if placed fo as to tranfmit the light, it 

 always appears greenifh ; the blue being reflected and the 

 Ijreen refradied, each fingly. 



By continuing to obferve the devitrification of this glafs, Second change, 

 it is foon perceived that the blue precipitation is followed by ^ whlte °f a " 

 another more abundant, which affords a dirty white, and is 

 very diftindt from the former. The colour of this glafs becomes 

 deeper and deeper, and at length it refembles grey horn. 



In thefe different tranfitions the pafte of the glafs appears Third change, 

 conftantly to remain, diftincuimable in its polifh, its frafluie, f eedled "jriiab 



J ° ' in group**. 



and all its other properties, except its tranfparency. But in 

 the midft of (his pafte refembling horn, very diftincl cryftalli- 

 zations are formed, confifting of nodules compofed of fmall 

 needles, all converging towards the center. In this ftate it is 

 no longer glafs, but a cryftal poflefling all the phyfical pro- 

 perties of mineral fubftances l«ft to themfelves. 



An 



