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734 VON TROIL’s LETTERS ON ICELAND.
lava, which was already burnt out, or that the lava has flowed over them: fome part
of it however feems to have been more attacked by the fire.
The manner in which the fire and acids operate upon this trapp, is exactly the fame
as with the preceding bafalt. There are likewife fome grains fcattered in it refembling
quarz, which are not folved by fol foda, nor does it caufe any effervefcence ; borax
and fufible urinous falt entirely diflolves them, though flowly.
28. A compact dark brown trapp from Vido, the furtace of which is glafly and
uneven, as if it had been made fluid by fire. It muft alfo be obferved, that cryftalliza-
tions of frefh pyrites are frequently found in thefe glafly rough pieces. The glafly
fubftance eafily becomes fluid with fome ebullition, almoft like fhirl; befides, it ex-
hibits the fame circumftances with acids as the preceding.
You will fee, Sir, by this long letter, that in the eruptions in Iceland argillaceous and
filiceous fub{tances have been principally concerned, as has been the cafe in other parts.
I know very well that Mr. Beaumé maintains, that filex might be made to afford allum
with acid of vitriol after it has been diffolved in liquor filicum with fal fode. But
when the fufions are made in veflels which contain no argillaceous earth, no allum can
be obtained with acid of vitriol, which, however, may be obtained when the vegetable
alkali is kept fluid during a confiderable time in a common crucible, becaufe the alkali
diflolves fome part of the veffel itfelf. Pure filiceous earth is entirely indiffoluble by
itfelf in acid of vitriol, let it be treated in any manner whatfoever.
Let this account of thefe dreadful devaftations be fufficient.
Homo nature minifter et interpres, tantum facit et intelligit, quantum de nature
ordine, re vel mente obfervayerit, nec amplius {cit aut poteft, Baco.
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