1^^ On the Huilonian Theory of the Earth, 



In another experiment, ibid, he found this change to take place even before the glaf» 

 was in perfeft fufion. For while both ends of a fragment of this glafs were fupported 

 on refts of clay, it was found not to fink down between them until the heat was raifed to 

 30. In another experiment he found the confolidation, which he (improperly, as I think) 

 calls cryftalization, to take place even while the heat was gradually increafed, and the 

 fubftance dill fo vifcid as to retain the original (hape of the fragments. 



In another experiment, where the glafs was flowly cooled, its texture was found com- 

 pleatly to refemble that of whinftone, the fraBure was rough, Jiony, and cr^aline, with a 

 number of fliining facettes interfperfed through the mafs, and a few cryftals in the cavities 

 produced by air bubbles, p. 8. 



Thefe experiments may be confidered in two points of view; firft, with refpeft to 

 phenomena of confolidation in a heat either gradually increafed above, or gradually dimi- 

 niftved below the heat neceflary to foften the vitreous fubftance, the lofs of the vitreous 

 charadler, and the ftony appearance, affumed through flow refrigeration. 



And, in the fecond place, we may examine how far the phenomena here obferved tend 

 to countenance the Huttonian theory either of the formation of granite, trap, or bafalt, or 

 other ftony fubftances : in this refpe£t only it concerns me to examine thefe experiments, 

 yet I cannot forbear mentioning fome few refledlions on the fiift. 



It has been obferved by all thofe who have attended to the formation of common glafs 

 (and is, indeed, evident from the fumes that float over its furface) that from the inftant 

 it enters into fufion, it is in a conftant ftate of tl'icompofition, gradually becomes lefs 

 fufible, and increafes in denfity ; the fubftances that thus efcape are, in this cafe, the faline, 

 as Bofc D'Antic has fliewn, and Macquer alfo aflerts. See i Bofc D'Antic, 10, and 242, 

 213. and hence the-lofs of weight which gas thus fufFers, ibid. 220, and 4 Macquer 261. 

 Macqucr alfo obferved, that glafs kept too long in fufion loofes its tranfparency, and 

 becomes opaque, becaufe the flux evaporates. And he obferves, that glafles formed of 

 argill, lime, and gypfum, are particularly fubjeil to this accident. Lavoifier noticed the 

 fame plienomcnon 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 afterwards 

 found occafion to extend the fame remark to fteatites, and alfo to a mixture of equal parts 

 of quartz and calcareous fpar. This increafed in fufibility of certain fubftances by a gra- 

 dually increafed or continued heat, is not, therefore, a new difcovery having been already 

 noticed: but Sir James Hall has confiderably enlarged it, by fliewing that the ftones he 

 Operated upon had re-afTumed their ftony appearance, after having been in a vitreous ftate ; 

 this appearance, if I underftand him rightly, they have aflumed only in confequence of 

 flow cooling, and not merely by a heat either ftationary or gradually increafed ; confolida- 

 tion only being the effedl of fuch treatment. 



This confolidation Sir James calls cryjfalization, a term which feems to me highly im- 

 proper; for, according to every fenfe in which this term has ever been employed, whether 



that 

 3 



