( i87 } 



which it exerts Its power ; though I grant 

 that the petrofiiex and feltfpar are more 

 adapted to this vitrification than other 

 flones. To produce a lava, a certain de- 

 gree of heat Is neceffary ; and a ftill greater 

 to convert it into pumice. The lavas, at 

 leaft thofe of the compad; fpecies, ufually 

 preferve the grain, hardnefs, and fometimes 

 the weight and colour of the primordial 

 rock ; but the greater part of thefe external 

 qualities difappear in the pumice, from 

 the ftronger adlion of the fire. This muft 

 operate ilill more in glafs, in which the ho- 

 mogeneity and finenefs of the pafte efface 

 every trace of its primitive texture. 



Thefe gradual tranfitions of lava into 

 pumice, and pumice into glafs, I have feve- 

 ral times obferved, and indeed have already 

 defcribed, in the fame volcanic piece. It 

 has alfo been obferved, that a lava frequent- 

 ly paffes immediately into perfed; glafs, 

 which muft be caufed by a fudden heat, 

 greater than that required to change it into 

 pumice. This theory fatisfadorily explains 



why 



