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the fire by fufible urinous fait ; but it
fucceecls a little better with borax,
though with fome difficulty ; with
fal fodæ it diifolves very little, though
in the firft moments fome ebullition is
perceived, and the whole mafs is afterwards
reduced to powder.
From hence it may be concluded,
that the Icelandic agate has been produced
by an exceffive fire out o f the
lava defcribed in 14. I have found
no cryftals ot this glafs in the colleaion.
I f any perfons have fuch in
their pofleffion, they fhould be examined
to fee i f they are ex a a iy of the fame
nature and fubftance with the above
defcribed agate, and i f their form has
• not been produced accidentally by
burfting afunder.
17. More or lefs light, fpongy, and
burnt-out pumice-ftoue, particularly
black and reddifli brown.^ Quarz cryf-
t als are fometimes found in them ; but
often the rays and grains refembling
quarz mentioned in N° 14.
18. Stones thrown out o f the volcano,
grey or burnt brown, w h i c h feem
to confift of a hardened clay mixed
with
with fliiceous earth. They are fprin-
kled with rays and grains refembling
quarz, and fome few flakes o f mica.
They fufb with great difficulty in
the fire ; with fal fodæ they ihew fome
effervefcence at firft, but however it
foon ceafes : the parts refembling
quarz do not produce any motion
at all. From this we may conclude,
that the lava mentioned in N° 14, prin^
cipally originates from this fubftance.
The other loofe ftones which I have
received from you, Sir, to all appearance
have no abfolute conneflion with
the eruptions o f fire, though fome
have been fufpcifted o f it. I will
enumerate them here feparately.
19. Red and green jafper, which,
in fome places where it is broken, is
quite fmooth and iliining ; this cir-
cumftance diftinguiflies it from the
common jafper, which is dull and
clay-like where it is broken. It has
befides all the qualities o f true jafper;
ftrikes fire with the fteel, does not
melt in the moft violent fire, but
is diffolved by fal fodæ with an
effervefcence, &c. &c. As to the
fmooth