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It has been obferved by all thofe who have attended to the 

 formation of common glafs fand is indeed evident from the 

 fumes that float over its furface) that fr^m th; inftant it enters 

 into fufion, it is in a conftant Oate of decompofition, gradually 

 becomes lefs fufible and increafcs in denfity ; the fubflances that 

 thus efcape are, in this cafe the faline, as Bofc D'Antic has {hewn 

 and Maquer alio aflertF, (See i Bofc D'Antic, lo and 242, 213.) 

 and hence the lofs of weight whicl> glaf thus fuffcrs, ibid. 220. 

 and 4 Maquer, 261 ; Maquer alfo obferved that glafs kept too 

 long in fufion loofes its tranfparency and becomes opake, becaufe 

 the flux evaporates ; and he obferves that glaffes formed of 

 argil, lime and gypfum, are particularly fubjedl to this accident. 

 Lavoifier noticed the fame phcenomenon during the fufion of 

 felfpar even by oxygen air, namely, that the longer it was kept 

 in fufion the more infufible it became, Mem Par. 1783, p. 577, 

 which he imputed to the volatility of one of fome or other of 

 its ingredients ; and he aftei wards found occafion to extend the 

 fame remark to fteatites and alfo to a mixture of equal parts of 

 quartz and calcareous fpar ; this increafed infufibility of certain 

 fubftances by a gradually increafed or continued heat is not 

 therefore a new difcovery, having been already noticed ; but 

 Sir James Hall has confiderably inlarged it, by fhewing that the 

 ftones he operated upon had reaffumed their flony appearance, , 

 after having been in a vitreous ftate ; this appearance, if I under- 

 VoL. VIII. C fland. 



