4t)0 DOMAIN XII. VOLCANIC. 



into contact with the cold air, as it flows down 

 an inclined plane. 



M fuml n c? e " Every one of these eight kinds of glasses 

 and enamels may be completely remelted in the 

 furnace. When speaking of the compact glass 

 of the rock of the castle of Lipari, I remarked 

 its extraordinary inflation in the furnace, and 

 said that this tumefaction usually accompanies 

 a refusion, in our fires, of solid glasses and vol- 

 canic enamels. I then had in view those of the 

 Monte della Castagna, five of which, though 

 compact and solid, in the furnace swelled high 

 above the edges ; notwithstanding that, before 

 their refusion, they only filled a third part of it." 



These ample descriptions may serve to show 

 the precise nature of volcanic glasses, which 

 some have confounded with the aqueous pro- 

 ductions. 



The obsidians, or volcanic glasses, and amels, 

 may be arranged in the following order. 



HYPONOME I. VITREOUS. 



Diversities. This can scarcely be distinguished from glass. 

 The general colour is black, whence it forms ex- 

 cellent mirrors for landscapes : it sometimes pre- 

 sents white spots, which are decayed crystals of 

 felspar, whence the base is supposed to be a vitri- 



