On the Devitrification of Glass. 26J 



rather the appearance of earthen-ware, or strongly baked 

 g!:iv. 



Cooling, continued for an hour or two, is often suflicient 

 to efR'Ct an entire devitrification of the glass of bottles. I 

 have pieces about eight centimetres in thickness, which I 

 collected in tlie glasshouse of M. Saget, at La Gare. Ou 

 taking from the furnace a pot intended to be renewed, the 

 glass which remained at the bottom was preserved from 

 cooling during the time employed by the pot in cooling : 

 and the nature of this glass was entirely changed ; it was 

 nothing but a mass of crystals composed of su;all needles 

 converging towards common centres. There was no longer 

 Anv appearance of vitrification. 



This fact shows with what facility the glass of bottles 

 becomes vitrified, and always without the least appearance 

 of cementation. 



The infinite variety of the substances emploved in makinf 

 bottles produces a great change in the phaenomena which 

 take place during their devitrification, and no doubt must 

 have an influence on the form of the crystals ; but I have 

 not had much opportunity of observing this kind of glass. 



Proceeding then to glass less earthy, and composed of 

 fewer substances, — if I examine also the botton) of furnace? 

 for fusing that kind of glass called glass of Alsace, or half 

 white glass, in which there is more pure silex, and more 

 alkali to fuse it, I observe nearly the same pha?nomena, 

 but, being less abrupt, they are more easily remarked and 

 separated. At first, and in pieces where devitrification is 

 commencing, people might almost say that they saw a 

 blue colour diflused throuirhout a trrecnish liquid. 



I shall here briefly mention a verv singular fact, whicli 

 I intend to examine at more length in another memoir. 

 This greenish glass mixed with blue seems, indeed, to jiave 

 become of a dirty blue when looked at opposite to the light; 

 but if it be placed between the light and the eve, it seems 

 always greenish, so that it reflects the blue, and only refracts 

 the greenish colour. Continuing to observe the devitrifi- 

 cation of (lemi-white glass, it is seen that the blue precipi- 

 tation is followed bv another more abundant, which gives 

 the dirty white, and is very distinct from the former. The 

 latter becomes still darker and darker, till it at length as- 

 sumes the colour of gray horn. 



In these different transition-s the paste of the glass seems 



always to exist. One may distinguish its polish, its frac'- 



turc, and all its other properties except transparency : but 



\n. the midst ofjhis paste similar to horn, very distinct crv- 



li 4 stallization** 



