DEVITRIFICATION OF GLASS. 61 



the talk I have undertaken, and in the prefent memoir on de- 

 vitrification, I (hall give the refults of my firft experiments. 

 As my fituation gives me the ufeof a fire of extreme violence 

 and continued for years together, it may have been in my power 

 to make obfervations not within the reach of every one. The 

 fa£is I (hall report do partly explain themfelves; they are the 

 refult of laws to which all bodies are fubjeaed. All the merit 

 of the obfervation confifts in having feen them in fubftances, 

 and on occafions where it was not known that thefe laws do 

 take place. 



The bottom of the furnaces for fonding or fufing the glafs ^J^J j£ n . q 

 ufually prefents large cavities excavated by the aftion of the g!a f s f urnace s. 

 fire, and of the corrofive fubftances which often flow out of 

 the melting pots. Thefe cavities are filled with a kind of glafs 

 called picadil, which is compofed of afhes which become vi- 

 trified, ftones of the furnace which become fufed, and more 

 particularly of glafs which falls from the pots. Care is taken 

 to remove this at each font or making of glafs. When the fur- 

 nace is nearly worn out, the cavities having become large, can- 

 not be entirely clearfed, but fome of the picadil remains. 

 When the furnace is extinguilhed, this picadil undergoes an 

 extremely flow cooling, becaufe it is (unrounded with mafonry 

 of feveral cubic fathoms, which is itfelf penetrated with heat 

 that has continued for upwards of a year. I have always re- Cryftals in glafs. 

 marked that it was in the glafs of thefe bottoms of the furnace 

 that I found cryftallizations through the mafs of glafs, the reft 

 of which was very tranfparent and pure. Thefe cryftalliza- 

 tions, which were always confiderably regular and numerous, 

 excited my curiouty, as they had done that of other glafs- 

 makers before me. I collected many fpecimens, taking care 

 to chufe thofe which appeared the moft curious, and prefented 

 the moft extrordinary characters. 



Soon afterwards, from comparing the pieces themfelves, and Examination of 

 the circumftances under which they had been produced; and 1 e ac s " 

 by the combinations of remarks, trials, and experiments to 

 imitate thefe cryftallizations at pleafure, I at laft fucceeded 

 in diftinguithing various claffes, all of which are produced by 

 the nature of the different circumftances which accompany the 

 formation of the glafs. I (hall give a rapid outline of thefe, 

 without entering into any remarks on the devitrification which 

 almoft conftantly takes place in the fcoria of forge furnaces, 



This 



