an Eruption of Mount Vefuvius. 81 
cheefe, which, when broken and gently feparated, fpins 
out tranfparent filaments from the little cells that con- 
tained the clammy liquor of which thofe filaments were 
compofed. The natural fpun-glafs, then, that fell at Ot- 
taiano during this eruption, as well as that which fell in 
the Ifle of Bourbon in the year 1766, muft have been 
formed, moil probably, by the operation of luch a fort 
of lava as has been juft defcribed, cracking and fepa- 
rating in the air at the time of its emiffion from the cra- 
ters of the volcanos, and by that means fpinning out the 
pure vitrified matter from its pores or cells, the wind at 
the fame time carrying off thofe filaments of glafs as fall 
as they were produced. 
I obferved, flicking to fome very large fragments 
of the new lava, which were of a clofe grain, fome 
pieces of a fubftance, whofe texture very much refem- 
bled that of a true pumice ftone ; and, upon a clofe exa- 
mination, and having feparated them from the lava, I 
perceived, that this fubftance had actually been forced 
out of the minute pores of the folid ftone itfelf, and was 
a collection of fine vitreous fibres or filaments, con- 
founded together at the time of their being prefled out 
by the contraction of the large fragments of lava in cool- 
ing, and which had bent downwards by their own 
weight. This curious fubftance has the lightnefs of a 
Vol. LXX. M pumice, 
