Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 3 - By Alexander Von Humboldt And Aime Bonpland.
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They Are The
Products Of A Feeble And Prolonged Volcanic Action, As May Be Easily
Ascertained In The Solfataras Of
Puzzuoli and the Peak of Teneriffe.
The alumite of Tolfa, which, since my return to Europe, I have
examined on
The spot, conjointly with Gay-Lussac, has, by its
oryctognostic characters and its chemical composition, a considerable
affinity to compact feldspar, which constitutes the basis of so many
trachytes and transition-porphyries. It is a siliciferous subsulphate
of alumina and potash, a compact feldspar, with the addition of
sulphuric acid completely formed in it. The waters circulating in
these alumiferous rocks of volcanic origin do not, however, deposit
masses of native alum, to yield which the rocks must be roasted. I
know not of any deposits analogous to those I brought from Cumana; for
the capillary and fibrous masses found in veins traversing beds of
lignites (as on the banks of the Egra, between Saatz and Commothau in
Bohemia), or efflorescing in cavities (as at Freienwalde in
Brandenburg, and at Segario in Sardinia), are impure salts, often
destitute of potash, and mixed with the sulphates of ammonia and
magnesia. A slow decomposition of the pyrites, which probably act as
so many little galvanic piles, renders the waters alumiferous, that
circulate across the bituminous lignites and carburetted clays. These
waters, in contact with carbonate of lime, even give rise to the
deposits of subsulphate of alumina (destitute of potash), found near
Halle, and formerly believed erroneously to be pure alumina belonging,
like the porcelain earth (kaolin) of Morl, to porphyry of red
sandstone.
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