434 DOMAIN XII. VOLCANIC. 



however, does hot prevent their swimming on 

 the water. 



" Others are full of pores and vacuities of a 

 larger size, usually of a round figure ; and their 

 texture is formed by filaments and streaks, in 

 general parallel to each other, of a shining silver 

 whiteness ; and which, at first view, might seem 

 to be silken, did they not present to the touch 

 the usual roughness of the pumice. 



" These varieties are not only observable in 

 different globes of pumice, but frequently in the 

 same : it is therefore indubitable that these dif- 

 ferences are not intrinsical and essential to the 

 nature of pumices ; but accidental, and arising 

 from the action of aeriform fluids, which dilating 

 them in many places, when they were in a state 

 effusion, have produced that multitude of pores, 

 and those filaments and subtile streaks that de- 

 note a separation of the parts; whereas the 

 other pumices, which have not been acted on by 

 these gases, have preserved that compactness 

 which results from the force of aggregation. 



" The fractures of the compact pumices are, 

 in some places, shaded with a blackish but at 

 the same time shining tinge ; which, when care- 

 fully examined, is found to be caused by a 

 greater, though still a very slight, degree of vi- 



