NOME V. PUMICE. 435 



trification of the pumice itself; either because 

 the fire has there acted with somewhat more 

 force, or because the parts were there more 

 easily verifiable. 



" The pumices hitherto described, form one 

 of the species which the Liparese sell to foreign 

 traders. 



" None of these, so far as can be discerned by 

 the eye, or even with the assistance of the lens, 

 contain any extraneous bodies ; but were we too 

 hastily to conclude that they really do not, we 

 should commit an error, as their vitrification by 

 artificial means will prove. When kept in the 

 furnace during an hour, they become only more Effect* of heat, 

 friable and of a reddish yellow colour; but when 

 continued in the same heat for a longer time, 

 they condense into a vitreous and semitrans- 

 parent mass, within which appear a number of 

 small white felspar crystals, that were not visi- 

 ble in the pumice, because they were of the 

 same colour. These stones, however, are not 

 seen in every pumice thus fused ; either because 

 it did not contain them, or because they have 

 melted into one homogenous mass with the pu- 

 mice. This is one of the many important cases 

 in which we are able, by the means of common 

 fire, to discover the composition of volcanic 



