NOME V. PUMICE. 441 



thickens the sides of the vitreous vesicles, of the 

 second species, and'diminishes the internal pores. 

 A longer continued heat entirely annihilates the 

 pores, and changes the pumice into a fixed, ob- 

 scure, homogenous, and hard glass, which gives 

 sparks plentifully with steel. 



" The third species is likewise an object of Another kind, 

 traffic with the natives of the island, who dig it 

 in the same places where they find the second ; 

 and, in like manner, shape it into parallelopi- 

 pedons. This is likewise an aggregate of bub- 

 bles, but differing from those of the former in 

 several respects. Those, as we have seen, are 

 conglutinated together in some points, while 

 they are separated in others, so that we can fre- 

 quently detach them without breaking; while 

 these, on the contrary, are so incorporated by 

 different solid points, that if we attempt the se- 

 paration of one, we break the others that are 

 contiguous. Here the elastic gases, investing 

 the pumiceous substance in several points, have 

 expanded it in every part into tumours and 

 cavities, nearly as we see in raised and baked 

 paste. It is worthy remark, that frequently 

 when we break one vesicle, we meet with an- 

 other within it, and concentrical. There is like- 

 wise another difference between these two pu- 

 mices. The vesicles of the second species 



