Obfervations on the Flints of Chalk-beds. 307 
iffues from the iron with very little force, even 
when the heat is confiderable. Hence it is extri- 
cated from the furface only of the bar; and this 
alone is converted into malleable iron. During this 
converfion, the furface is heaved and feparated from 
the internal parts; and fome fpace within is afforded 
for the extrication of air: And if the bar fhould be 
cooled while the particles are difunited, in con- 
fequence of this extrication, it will be foynd to 
contain a powder. 
The duft and afhes, ejected in fuch abundance by 
volcanoes, muft be produced by very nearly the 
fame mechanifm. Let us fuppofe a fubftance in 
fufion, from which, or from below which, air or 
fteam is rapidly and copioufly evolved—a very 
common occurrence at the time of an eruption, 
Thefe elaftic fluids iffue with fuch prodigious vio- 
lence as to diffipate the matter in fufion, and bear 
it forward, as duft is elevated by a ftrong wind, 
On its arrival in the atmofphere, or before, it is 
cooled, becomes concrete, and defcends like fnow 
upon the ground. 
IV. The glaffy texture and fracture of flints, in 
the firft place, leads me to believe that they have 
been fufed. I have fometimes, within the hollow 
fpecimens, feen filaments paffing acrofs from fide to 
fide —another analogy, as I apprehend the fact, with 
glafs ; thefe filaments having been drawn out in the 
manner of fpun gla{s, as the flint bubble was blown 
.up by elaftic fluids formed within, Jn the fame 
light 
