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"when in fufion and the particles of each chymically united, 

 they require a higher degree of heat to keep them in fufion, 

 their eledive affinities promoting fufion before the union, and 

 impeding it after the union is formed ; it is thus that iron and 

 platina, metals feparately highly infufible, contribute to each 

 others fufion ; but when fufed, become ftill more infufible, 

 as appears by Rinm. § 135; fulphur and lead are feparately and 

 eafily fufible, but when united their fufion becomes much more 

 difiicult. 



Again, Dr. Kennedy has difcovered that all thefe whins con- 

 tain ten per cent, of foda, and Vauquelin has lately difcovered 

 tartarin in felfpar; in the high heats to which thefe ftones are 

 expofed in order to vitrify them, may it not be fuppofed that 

 thefe falts are in fome meafure volatilized and the compound 

 thus rend'ered lefs fufible ? Though in an high heat rapidly 

 produced, they may flill be fufible, as a fmaller proportion of 

 foda will in that circumftance fuffice to that effedt. 



The next circumftance to be accounted for is the faxification 

 or. Jlony appearance aflTumed by the vitrified ftones when flowly 

 cooled, by far the moft curious fad, for which we are indebted 

 to the ingenuity of Sir James. To account for this change it is 

 proper to remark that though whins are faid to be vitrified in 

 a high degree of heat, yet this is not rigoroufly true, for in that 



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