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juft mentioned ; the lava cracking and feparating in the 
air at the time of its emiflion from the crater, and by that 
means fpinning out the pure vitrified matter from its 
pores or cells; the wind at the fame time carrying off the 
filaments of glafs as fall as they were produced. 
Our author obferved a kind of pumice-ftone (licking to 
fome very large fragments of the new lava. On clofe in- 
fpeflion, however, lie found that this fubftance had been 
forced out of the minute pores of the folid lava itfelf; and 
was a collection of fme 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 cooling, 
and which had been bent down wards by their own weight. 
“ This curious fubftance (fays he) has the lightnefs of a 
pumice, and refembles it in every refpect, except that it 
is -of a darker colour.” 
When the pores of this lava were large, and filled with 
pure vitrified matter, the latter was fometimes found 
blown into bubbles on the furface ; probably by the air 
which had been forced out at the time the lava contrafled 
itfelf in cooling; and from thel’e thin bubbles it appeared, 
that this kind of volcanic glafs has much the lame tranf- 
parency with our common glafs bottles, and like them is 
of a dirty yellow colour ; but, when large pieces of it 
were broken off 7 with a hammer, they appeared perfectly 
black and opaque. In the lava of this eruption it was 
obferved, that many detached pieces were in the lhape of 
a barley-corn or plum-ltone, fmall at each end, and thick 
in the middle. Some of thefe did not weigh above an 
ounce; but others could not be lefs than fixty pounds. 
Our author took them to be drops from the liquid foun¬ 
tain of fire, which might naturally acquire fuch a form in 
their fall. There were alfo many other curious vitrifica¬ 
tions, different from any he had feen before, mixed with 
this huge (bower of fcorfie and mafl'es of lava. 
In treating of Mount Etna, M. Houel makes mention 
of a piece of lava which, after having been once ejected 
by the volcano, was fwallowed up, and thrown out a fe- 
cond time. The intenfe heat to which it was then 1 'ub- 
jeded, had fuch an effeft upon it, that it appeared all full 
of chinks to a coniiderable .depth, and running at right 
angles to one another. He had alfo an opportunity of 
oblerving to great advantage fome of the hollow channel 
formed by the lavas of Etna, fimilar to thofe del'cribed by 
fir William Hamilton, but on a much larger fcale. Here 
the great eruption of water in 1755 had overturned, in a 
vertical direction, a huge tube of this kind for the length 
of half a mile. The tube itfelf appeared to be compoi’ed 
of enormous mafl'es, fomewhat refembling planks; each 
two feet thick and twelve or fifteen in breadth, continued 
in a ftraight line through the whole of that (pace. At 
the fame time, by the action of the lava, a kind of walls 
had been formed, from ten to fixteen feet in height, and 
curved at the top. Some of thefe walls appear rolled to¬ 
gether like paper; and M. Houel is of opinion, that thefe 
various appearances on the furface of the lava when cool¬ 
ed, mud have arifen from particles heterogeneous to the 
real lava; and which detach themfelves from it, riling to 
the furface under a variety of forms proportioned to the 
fpaces of time taken up in cooling. Thefe crufts are 
formed of different kinds of fcorias and dirty lava, mixed 
with fand or aflies. At the fame place are alfo found 
great numbers of fmall pieces like thofe of ice heaped 
upon one another after having floated for fome time on a 
river. Beneath thefe the pure lava is met with, and which 
has evidently been in a (fate of perfect fufion. This is 
extremely denfe; and by looking narrowly into its chinks, 
the compofltion of the whole appears to be merely homo¬ 
geneous. “ It is curious (fays he) to obferve, fo near 
one fpecies of lava which is very pure, another which has 
like wife arrived at the fame place in a fluid date, and has 
there undergone fo great a change as fcarcely to retain an 
appearance of its original Hate. It is, however, like iron 
•Urol’s, in grains of unequal fizes, We find it alfo at ya- 
V A. 
rious diffances, fuch as one, two, or more, hundred fa¬ 
thoms. It is fometimes found in large pieces like tables, 
covered over with (harp points, fome longer and others 
(horter. All thefe pieces are quite detached from one an¬ 
other, as if they had been brought thither and fcattered 
from a tumbril. The matter of which the cruft of the 
lava is formed, feems to have iflued from it in the fame 
manner in which froth rifes upon folution of foap in wa¬ 
ter. It appears afterwards to have fvvelled, burlt, and af- 
fumed its prefent form, prefenting to the view various 
fpaces filled with fmall loole fiones. A great number of 
new lavas were likewife obferved, all of them putting forth 
various kinds of effiorel'cences in great quantity. 
3. Lava fcoriacea, l'pongy lava, or Icoria : velicular, 
rough, fiiining internally, of a conchoidal texture. Found 
in ltreams of volcanic lava, generally covering the L. cora- 
padta; black or brown, with often a mixture of heteroge¬ 
neous matters ; the furface appears full of empty bub¬ 
bles, often difpofed in an undulate manner. 
4. Lava porofa, porous or cellular lava: opake, without 
luffre, porous, lightifh. Found in volcanic mountains 
and their neighbourhood, more rarely in thofe which have 
been extinguished, and leems rather to have been thrown 
from the crater than run over at the Tides; colour black 
or brown, fometimes reddifli-brown. It probably contains 
more carbonat of magnefia than the reft, and is more 
fubjeft to deftruftion than compaft lava ; its pores are 
larger near the furface than towards the centre. 
5. Lava pumex, or pumice-llone : opake, without luf- 
tre, parallel, fibrous, porous, light, rough. Found in the 
aflies of mod volcanic mountains, from whence it is walhed 
down into the fea. Colour grey, greyifli-white, brown, or 
reddifli, rarely yellowifli ; the fibres are generally parallel, 
more or lefs dilcernible, and have a filky luffre ; does not 
eftervefce with acids, melts into a white enamel. Pumice 
may be confidered as a loofe vitrified lava, entirely de¬ 
void of iron. The pores of pumice, when obferved in the 
undivided mafs, are always in the direflion of the current 
of the lava of which the pumice makes a part; in pro¬ 
portion as it is nearer or farther from the furface, the pu¬ 
mice is more or lefs porous; and the lowermolt part of 
the bed is generally completely compafl, and pafles into 
the (late of a denfe glafs. In fome l'pecimens the com¬ 
plete tranfition from the one to the other Hate may be 
traced by very delicate gradations. 
There appears to be the lame relation between compaft 
and porous pumice that there is between compafl and po¬ 
rous lava ; the inferior part of a bed of lava being always 
compafl, the fuperior porous. The true pumice, (fays 
M. Faujas de St. Fond,) which we (liould be careful not 
to confound with the lighted lavas, keeps the midway be¬ 
tween the volcanic glades and the enamels ; it generally 
derives its origin from a peculiar kind of vitrification of 
compafl feldfpar, and fome porphyritic rocks. Thole of 
Lipari and Vulcano are the only known volcanoes that 
have produced pumice in coniiderable quantity ; the i(le 
of Lipari, in particular, is the vaft magazine that furniflies 
almolt the whole of the immenfe (lores of this lubftance^ 
confumed for the purpofes of different arts, in almoft all 
parts of Europe. This mafs of pumice is an aggregation 
of numerous beds, or ftrata, of pumices, fucceflively placed 
on each other, diftinguilhable by their colour and by their 
projection from the mountain. Some of thefe pumices 
are fio compafl, that the ('mailed pore cannot be difcerned, 
nor do they exhibit '<the lead trace of a filamentous na¬ 
ture. With a lens they appear to be an irregular accu¬ 
mulation of fmail flakes of ice. Others are lull of pores 
and vacuities of a larger lize, ufually round ; and their 
texture is formed by filaments and (freaks, generally pa¬ 
rallel to each other, and of a Alining filvery whitenefs. 
Of thefe pumices there are three kinds, which the people 
of Lipari dig for fide. One kind is employed in polifii- 
ing different fubffances ; and the other two kinds are uled 
io the conttruflion of arched vaults, and the corners' of 
buildings. 
