DEVITRIFICATION OK CLASS. 59 



d<enfed heat foftens and renders them fluid, and they more or 

 ids continue to exhibit, when cooled, the tranfparency and 

 fome other of the well known phyfical properties of glafs. 



But in the examination and defcrjption of that fufion to It ha folution 

 which vitreous compounds are fiibjected in order to fit them for b ? heaU 

 the purpofes of life, it is neceffary to obferve that the effect 

 arifes from two phenomena. It is not fimply the refult of ac- 

 cumulated heat, but it is jointly produced by the affinities of 

 the fubftances which enter into the mixture. Thefe fubftances 

 which tend to combine and mutually to penetrate each other, 

 act according to the laws of their affinities as foon as ever the 

 temperature is fufficiently elevated. Thus it is that a mixture 

 of feveral earths becomes fufed at a temperature which could 

 not have rendered any one of them fluid. 



The common vitrification is therefore the refult of a com- Nature of the 

 bination effected at an elevated temperature between different c j*np° und i or 

 and heterogenous fubftances; and the produd is a compound 

 perfectly homogenous more or lefs tranfparent, elaftic, and 

 breaking in a peculiar manner, from which the term vitreous 

 fradure is derived. This body is a remarkably bad conductor 

 of heat and of electricity; it is capable of becoming foftat a 

 temperature inferior to that at which it was fufed, fo that it 

 may be rendered patty, ductile, &c. 



The phenomenon during which all thefe properties difap- Devitrification. 

 pear is what I have called devitrification; an expreffion which 

 may at firft feem extraordinary, but which the fads will (hew 

 to be perfectly accurate. 



Several philofophers have before noticed this effect; fome Ic is ^ e confe- 

 have even made obfervations upon it, and have noted various ft a iij zat i on . 

 circumftances fcarcely connected with each other; but I am 

 not aware that any one has yet pubiifhed a feries of refearches 

 proper to explain the effects, fo as to fhew that it is one of the 

 known properties of all natural bodies, being in fact nothing 

 more than the product of a cryftallization. 



Reaumur fjrft obferved that a glafs, more efpecially if it be The porcelain 0/ 

 compofed of different earths, as bottle glafs is in general, eaumur * 

 may be decompofed, and lofe its tranfparency and other vi- 

 treous properties. Fully occupied in his refearches upon 

 porcelains, he was defirous of applying this difcovery to the 

 fabrication of potteries, and attributed the phenomenon to the 

 fubftances in which he had cemented his glafs. The fad was 

 2 denomi* 



