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VON TROIL’s LETTERS ON ICELAND. 723
in length, and thirteen in breadth. | The outfide of it has no marks of any roundnefs,
but is quite flat. An exceeding great weight is required to prefsa fick to a flat plate ;
and I cannot conceive how the moft immenfe beds, which muft neceflarily have been
foft when fpread over it, could ever produce this effect. ‘The caufe of this is yet undif-
covered, and will probably remain fo a long time; however fomething may be found
there which feems to fhew, that the bituminous flate has been produced in the fame
manner, as it has not only penetrated the fubftance of the flate, but every thing elfe
which has been laid upon it, for it may yet be obtained by means of diftillation. But by
what means has this been brought thither? How could it be imbibed by the clay, in
cafe this was under water, which however feems to be undeniable, from the prodigious
number of marine animals which are found buried ? and how could the inclofed bodies
have been prefled down horizontally ? All thefe problems I cannot as yet anifwer fatis-
factorily, much lefs explain with any degree of certainty.
14. Very coarfe, heavy, and hard lava, full of bladders, almoft black, intermixed
with white grains refembling quarz, which in fome places have a figure not very unlike
a {quare.
The black matter is not attracted by the magnet; but if a piece of it is held againft
a compafs, the needle vifibly moves. When tried in the crucible, it yields from ten to
twelve pounds of iron in every hundred weight : it does not diflolve in the leaft with
fal fodze, with great difficulty with borax, and hardly vifible by fufible urinous falt.
it feems to contain a great deal of clay-earth in its compofition, which may be extracted
by all folvents of acids.
It is well known that this earth, when it is entirely free from any other mixture, may
by means of heat and drying be brought to that degree of hardnefs, as to give fire with
a fteel, which proceeds from the parts being brought clofer together, and contra¢ted in
afpace only half as large. By being thus contra¢ted, it obtains a folidity and hardnefs ;
and befides, the furface is fo much diminifhed, in proportion to the whole mafs, that the
water cannot penetrate any farther to foften it.
We have almoft daily opportunities in the ftudy of chemiftry of convincing our-
felves, that a fubftance with a fmall furface cannot be changed in any manner by liquid
folvents ; but may however be attacked by them, in proportion to the different degrees
of pulverization; nay, evena fubftance which cannot be reduced by the fineft mecha-
nical divifion, may frequently be feparated, as much as is neceflary, by a chemical one ;
that is to fay, by a preceding folution in another folvent. The attraétion is here in pro-
portion to the extent of the furface; and the larger this is, the {tronger will be the at-
tack: confequently I cannot believe that any clay, petrified by heat or flow drying, can
have undergone any effential change, but only that its parts have fo contraéted them-
felves as to give it the hardnefs of a flint to prevent it from imbibing any vifible quantity
of water. But as foon as it has been diffolved by any acid whatever, and its parts have
by this means been brought out of its former contraction, to the requifite degree of
finenefs and expanfion, it becomes as foft as before, without the acid contributing any
more to it than has been faid, as all kinds of acids fucceed equally well.
I havea very good aflortment of the lava of Solfatera, by which it is very evident that
the fulphureous acid, which had penetrated the black lava, deprived.it gradually, partly
of its combuftible quality, and had alfo whitened it (to effeét which other fub{tances,
particularly filk, are likewife expofed.to fuiphureous exhalations’, and partly had re-
duced it by folution, either to a perfect allum, or at leaft to the common nature of any
loofe clay. | have likewife produced all thefe effeéts with aqua-fortis, or any other acid,
in a lava which had not yet fuffered any change.
42.2 ‘The
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