Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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Perhaps The Soda Contained In The Lavas Of
The Peak Acts An Important Part In The Formation Of These Deposits
Of Silex.
There may exist in the crater small crevices, the vapours
of which are not of the same nature as those on which travellers,
whose attention has been directed simultaneously to a great number
of objects, have made experiments.
Seated on the northern brink of the crater, I dug a hole of some
inches in depth; and the thermometer placed in this hole rose
rapidly to 42 degrees. Hence we may conclude what must be the heat
in this solfatara at the depth of thirty or forty fathoms. The
sulphur reduced into vapour is condensed into fine crystals, which
however are not equal in size to those M. Dolomieu brought from
Sicily. They are semi-diaphanous octahedrons, very brilliant on the
surface, and of a conchoidal fracture. These masses, which will one
day perhaps be objects of commerce, are constantly bedewed with
sulphurous acid. I had the imprudence to wrap up a few, in order to
preserve them, but I soon discovered that the acid had consumed not
only the paper which contained them, but a part also of my
mineralogical journal. The heat of the vapours, which issue from
the crevices of the caldera, is not sufficiently great to combine
the sulphur while in a state of minute division, with the oxygen of
the atmospheric air; and after the experiment I have just cited on
the temperature of the soil, we may presume that the sulphurous
acid is formed at a certain depth,* in cavities to which the
external air has free access. (* An observer, in general very
accurate, M. Breislack, asserts that the muriatic acid always
predominates in the vapours of Vesuvius. This assertion is contrary
to what M. Gay-Lussac and myself observed, before the great
eruption of 1805, and while the lava was issuing from the crater.
The smell of the sulphurous acid, so easy to distinguish, was
perceptible at a great distance; and when the volcano threw out
scoriae, the smell was mingled with that of petroleum.)
The vapours of heated water, which act on the fragments of lava
scattered about on the caldera, reduce certain parts of it to a
state of paste. On examining, after I had reached America, those
earthy and friable masses, I found crystals of sulphate of alumine.
MM. Davy and Gay-Lussac have already made the ingenious remark,
that two bodies highly inflammable, the metals of soda and potash,
have probably an important part in the action of a volcano; now the
potash necessary to the formation of alum is found not only in
feldspar, mica, pumice-stone, and augite, but also in obsidian.
This last substance is very common at Teneriffe, where it forms the
basis of the tephrinic lava. These analogies between the peak of
Teneriffe and the Solfatara of Puzzuoli, might no doubt be shown to
be more numerous, if the former were more accessible, and had been
frequently visited by naturalists.
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