[ s ] . 



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 ef- 

 fedled, the compound is confiderably and rapidly cooled, yet the 

 union will ftill continue, becaufe the alkaline menftruum being 

 congealed, the filiceous particles cannot move through it to re- 

 unite to each other, though their affinity to each other in a low 

 temperature be greater than their affinity to an alkali, and thus 

 they continue in that ftate w^hich we call glafs. Two experi- 

 ments fet this explication beyond all doubt, the firft is, that if 

 a folution of fait in water be fuddenly cooled from one hundred 

 and forty degrees above to fix degrees below 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 

 perfedly analagous to that of glafs. The fecond experiment is that 

 of Tromfdorf, 22 An. Chym. p. 115, where we find the filiceous 

 particles to have feparated by long ftanding (eight years) from 

 the alkaline in a folution of filicited alkali, and to have formed 

 perfe<5l cryftals hard enough to ftrike fire with fleel. 



That the glafs thus formed, being fuffered to cool flowly, 

 fhould be decompofed is very natural ; it is what happens when 

 certain falts, for inftance nitre, are diflblved in water to fanira- 

 tion, in a boiling heat ; if the water be flowly cooled mofl of 

 the nitre will cryftallize and feparate itfelf. That the filiceous 



earth 



