t A 
Very improbable, to have been in that (late detached from 
the reft of the lava. For the tranfparency, therefore, (ac¬ 
cording to fome authors,) we mult have recourfe to elec¬ 
tricity; which in fome of our experiments has the pro¬ 
perty of rendering opaque bodies tranfparent. Indeed it 
is fcarcely pollible but the iava and every other matter 
thrown out of a volcano mult be in the higheft degree 
electrical, if the fire itfelf take its rife from electricity. 
The lava, after having once broke out, does not con- 
ftantly continue running from the fame vent, but often 
lias intermiffions, after which it will burft out fometnnes 
at the fame place, and fometirnes at another. No real 
flame ever appears-to come from the lava. In the day¬ 
time its progrefs is marked by a thick white fmoke, from 
■which the light of the red-hot matter being reflected in 
the night-time, makes it appear, like flame. But, if, dur¬ 
ing its progrefs, it meets with trees or other combuftible 
fubftances, which in frequently does, a bright flame im¬ 
mediately ifi’ues from its furface, as hath aifo been remarked 
by fir William. Hamilton. This liquid fubftance, after 
having run pure for about 100 yards, (more or lefs, no 
doubt, accordio.; to different circumltances,) begins to 
collect cinders and ftones, and a feum is formed on.the fur- 
face: Our author informs us, that the lava which he ob- 
ferved, with its feum, had the appearance of the river 
Thames, as he had feen it after a hard froft and a great 
fall of fnow, when beginning to thaw, carrying down vaft 
mafies of fnow and ice. In fome places it totally clifap- 
peared, and ran in a fubterraneous paffage formed by the 
feum for feverai paces; after which it came out pure, hav¬ 
ing left the feum behind, though a new one was quickly 
formed. This lava at the farthelt extremity from its fource 
did not appear liquid, but like a heap of red-hot coals, 
forming a wall in fome places to or 12 feet high, which 
rolling from the top foon formed another wall, and fo 
on. This was the appearance alfo put on by the lava 
which iffued in the great eruption of 1783 in Iceland; 
with this difference, that the wall was at one time 210 feet 
high, and the general thicknefs of it was more than too : 
While a lava is in this ftate, fir William is of opinion, 
.that it is very practicable to divert it into another chan¬ 
nel, in a manner fomewbat fimilar to what is prabtifed 
with rivers. This he was afterwards told had been done 
with fuccefs during the great eruption of Etna in 1669: 
that the lava was directing its courle towards the walls of 
Catanea, and advancing very flowly, when they prepared 
a channel for it round the walls of the town, and turned 
at into the fea. A fucceflion of men,covered with fheep- 
Ikins wetted, were employed to cut through the tough 
flanks of lava, till they made a pafiage for that in the cen¬ 
tre, which was in perfect fufion, to difgorge itfelf into 
the channel prepared for it. But this, it is evident, can 
only take place in fmall ftrearos of this burning matter; 
with that above-mentioned it would have been impoflible. 
It has been alfo obferved of the lavas of Etna, that they 
do not conftantly fall down to the lowed places, but will 
fometirnes afeend in fuch a mariner as to make the valleys 
rife into hills. On this fir William Hamilton has the fol¬ 
lowing note : “ Having heard the fame remark with re¬ 
gard to the lavas of Vefuvius, I determined, during an 
eruption of that volcano, to watch the progrefs of a cur¬ 
rent of lava; and I was foon enabled to comprehend this 
feeming phenomenon, though it is, I fear, very difficult 
to explain. Certain it is, that the lavas, while in their 
molt fluid ftate, follow always the laws of other fluids ; 
but, when at a great diftance from their fource, and con- 
fequently encumbered with fcoriae and cinders, the air 
likevvife having rendered their outward coat tough, they 
will fometirnes (as I have feen) be forced tip a Ihiall af- 
cent, the freffi matter pulhing forward that which went 
before it, and the exterior parts of the lava afting always 
as conductors (or pipes, if I may be allowed the expref- 
fion) for the interior parts, that have retained their flui¬ 
dity from not being expofed to the air.” 
From the year 1767 to 1779? this gentleman made many 
V A. 279 
curious obfervations on the lavas of Vefuvius. He found, 
that they conftantly formed channels in the mountain as 
regular as if they had been made by art ; and that, whilffc 
in a ftate of perfect fufion, they continued their courfe in 
thofe channels, which were fometim.es full to the brim, 
and at others more or lefs lo according to the quantity of 
matter thrown out. Thefe channels, after fmall erup¬ 
tions, were generally from two to five or fix feet wide, 
and leven or eight in depth. They were often hid from 
the fight by a quantity of fcoriae that had formed a cruft 
over them, and the lava, having been conveyed in a co¬ 
vered way for fome yards, came out again frefit into an open 
channel. Our author informs us, that he had walked in 
fome of thefe fubterraneous galleries, which were exceed¬ 
ingly curious, the fides, top, and bottom, being very 
finooth and even : others were incrufted with what he 
calls very extraordinary fcoriae, beautifully ramified white 
fairs in the form of dropping ftalabfites, &c. 
On viewing a ftream of lava while in its fluid ftate in 
the month of May 177,9, he perceived the operation of it 
in the channels above delcribe’d in great perfection. After 
quitting them, it fpread itfelf in the valley, and ran gently 
like a river that had been frozen, and had mafies of ice 
floating upon it. The wind happening then to fhift, our 
traveller was fo incommoded by the fmoke, that the guide 
propoled to crofs it, which was inftantly put in execution 
without any other inconvenience than the violent heat 
with which the legs and feet were affebted. The cruft 
was fo tough, that their weight made no imprefiion upon 
it; and the motion fo flow, that they were in no danger 
of falling. This circumftance, according to fir William, 
points out a method of efcape fhould any perfort happen 
to be enclofed betwixt two lavas, but ought never to be 
tried except in cafes of real necefiity; and indeed, if the- 
current of melted matter was very broad, mutt undoubt¬ 
edly be attended with extreme danger, both from the heat 
of the upper cruft,and the chance of its breaking and fall¬ 
ing down with the paflenger into the burning liquid be¬ 
low. That which fir William Hamilton crofted was about 
fifty or lixty feet broad. Having paffed this burning 
ftream, our travellers walked up along the fide of it to its 
very lburce. Here they faw it boiling and bubbling vio¬ 
lently up out of the ground, with a hilling and crackling 
noife like that which attends the playing off an artificial 
fire-work. A hillock of about fifteen feet high was formed 
by the continual fplafhing up and cooling of the vitrified 
matter. Under this was an arched hollow, red-hot 
within, like a heated oven ; the lava which ran from it 
being received into a regular channel railed upon a fort 
of wall of fcoriae and cinders, almoft perpendicularly, of 
about the height of eight or ten feet, and refembling an 
ancient aqueduct. On quitting this fountain of lava, they 
went quite up to the crater, where as ufual they found a 
little mountain throwing up ltones and red-hot fcoriae 
with loud expiofions; but the fmoke and lmell of fulph'u'r 
were fo intolerable, that they were obliged to quit the 
place with precipitation. 
By the great eruption in Auguft 1779, the curious chan¬ 
nels above-mentioned were entirely deftroyed, the cone 
of the mountain was covered with a ftratum of lava full 
of deep cracks, from whence continually iffued a fiilphu- 
reous fmoke that tinged the fcorise and cinders with 3 
deep yellow, or fometirnes white, tint. The lava of this 
eruption appeared to be more perfectly vitrified than that 
of any former one he had obferved. The pores of the 
freffi lava were generally full of a perfeft vitrification ; and 
the fcoriae themlelves, viewed through a magnifying-glafs, 
appeared like a confufed heap of filaments of foul vitrifica¬ 
tion. When a piece of the folid lava had been cracked in 
its fall, without feparating entirely, fibres of perfect' glafs 
were always obferved reaching from fide to fide within 
the cracks. The natural fpun-glafs which fell in fome 
places along with the afhes of this eruption, and which 
has likewife been obferved ih other places, he is of opi¬ 
nion muff have proceeded from an operation-of the kind 
1 juff; 
