NOME VI. OBSIDIAN. 



453 



when exposed to the fire, melts with difficulty, 

 without the addition of some other substance as 

 a flux. In this it differs from the present of Li- 

 pari, which soon begins to soften in the fur- 

 nace, and in a few hours undergoes a complete 

 fusion. 



" This kind of glass, however, is not the most 

 common to be met with on the Monte della 

 Castagna. It is found only in a few places, 

 scattered in large but solitary masses; nor can 

 I pretend to say whether these are remains of 

 currents, or whether they were thrown out by 

 the burning gulfs. 



" It happens to this glass as to the different Mixed, 

 kinds of precious stones, that is, the same 

 piece is not always throughout of equal purity 

 and value ; for on breaking some of these masses 

 we sometimes find one portion very pure glass, 

 such as has been already described, and the 

 other imperfect ; either because the fusion has 

 not been general, the substance containing bo- 

 dies foreign to the base, or because that base is 

 rather an enamel than vitreous. These bodies 

 are felspars, but of a new appearance. Nothing Felspars, 

 is more common than to find felspars in lavas, 

 and sometimes even in enamels and glasses; of 

 which we have frequent examples in this work, 

 as well as in the accounts of other writers : but 



