On tht Huttonian Theory of the Earth, gg 



4thly, Various attempts have been made to fufe granites, in moft of which, as has 

 Already been faid, felfpar is the moft abundant ingredient ; but in almoft all the finely 

 pulverized, the quartz remained unfufed, and might be diftinguiflied by a lens ; fee 

 J Sauflure, §. 172, 173, and 174, i Gerh. Gefch. §. 51, and in the firft part of his new- 

 Mineral Syftem, publifhed in 1797, p. 412, and Hacgueft in i Crell Baytrage, p. 34, 

 35, &c. It is plain, then, that in all heats with which we are acquainted, the felfpar can- 

 not but in very rare cafes ferve as flux or a menftruum to the quartz with which it is 

 found in granites, the full proportion of quartz, which can be rendered fufible by its other 

 component earths, being already contained in the felfpar ; and, in fadt, there is no 

 analogy betwixt water a£ling as a menftruum on fait, and felfpar acting on quartz, for 

 water and fait are fubftances perfectly heterogeneous to each other, whereas felfpar and 

 quartz are both earthy fubftances, of which the former contains a large proportion of the 

 latter, as eflential to its compofition, and is fufible only by reafon of its compound nature ; 

 but if the quantity of the quartzy ingredient be increafed, the whole becomes infufible, as I 

 have experienced ; whereas if the proportion of fait in water be increafed, ftill the water 

 will be congealable if confiderably cooled ; moreover quartz frequently bears the impreflion 

 of ftones more fufible than itfelf, which could not happen in any poflible fuppofition, if 

 all had been in ftate of fufion. 



Again, Sir James obferved, that a quantity of green glafs, which had been allowed to 

 cool flowly, was found to have loft all its vitreous properties, being opaque, white, and 

 refraQory ; but being again melted by a blow pipe, and fuddenly cooled, it refumed its 

 former properties, and became glafs : hence he infers, that if the glafs produced by the 

 fufion of granite had been allowed to cool with fuflicient flownefs, it might have cryftalized, 

 producing a granite fimilar to the original, p. 1 1. 



The obfervation on glafs, here mentioned, is perfe£ily juft, and has been often re- 

 peated; but the analogy betwixt this cafe, and the formation of granite from a complete 

 fufion of its ingredients, is far from being accurate. Glafs confifts of a fimple earth, 

 namely, the filiceous, united to an alkali. To form this union, it is neceflary that the 

 integrant afiTmity of the filiceous particles to each other (hould yield to the -chemical 

 afilnity which the alkali bears to them, and this can happen only in fo high a degree of 

 heat, as confiderably leffens the affinity of the filiceous particles to each other. If, when 

 this union is effefted, the compound is confiderably and rapidly cooled, yet the union will 

 ftill continue, becaufe the alkaline menftruum being congealed, the filiceous particles can- 

 not move through it to reunite to each other, though their affinity to each other in a loiu 

 temperature be greater than their affinity to an alkali, and thus they continue in that ftate 

 •which we call glafs. Two experiments fet this explication beyond all doubt, the firft is, 

 tJiat if a folutlon of fait in water be fuddenly cooled from 140 degrees above to 6 degrees 

 ielow o of Fahrenheit, the whole will be- congealed, and no feparation of the fait will take 

 place, fee 8 Nov. Comment. Petropol. p. 346. . This cafe is perfeflly analogous to that of 

 glafs. The fecond experiment is that of Tromfdorf, 22 Ann. Chym. p. 115, where we 



O 3 find 



