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cafe ihcy fhoiild affoiJ a tranfparent glafs whofe fradure would be 

 perfectly poliflicd wiih a flrong luflre, as we fee that of common 

 glafs, whereas in truth they melt only into an enatiiel nearly ap- 

 proaching to the perfed vitreous ftate ; even the bottles made of 

 them are nothing more ; and hence their fuperior hardnefs. Their 

 ingredients therefore are not uniformly difFufed through their whole 

 mafs, but lie in the fame order and pofition as before fufion, and in 

 efFefl they contain much more filex than can be completely vitrified 

 by the fmall proportion of lime and argil that enter into their 

 compofition, even though affifted by the foda ; and in the next 

 place we muft notice that the affinity of foda to filex diminishes 

 in the fame ratio as the heat diminifhes and confequently they 

 feparate, if the heat be not fo fuddenly diminifhed as to impede 

 all motion. This is evident by what happens to common glafs 

 when flowly cooled down to the temperature of the atmofphere, 

 as came to pafs in the glafs houfe at Leith, and conformably 

 to this inflance the faxification might take place even after a 

 perfed vitrifadion. That there are unions grounded on chymical 

 affinity which take place to a certain degree only at certain tem- 

 peratures and are in great meafure loofened at a lower tempera- 

 ture, appears in the common inflance of the folution of mofl 

 falts in water, fpirit of wine or other menflruum greater in a 

 high degree of heat than at a lower, and has alfo been noticed 



in 



