We may conclude from what I have stated:
1. That by the assiduous study of the central crater, and the
indications afforded by the "Apparatus of Variations" and the
"Electro-Magnetic Seismograph," we can obtain precursory signals of
eruptions; and that the other premonitory signs pointed out by our
ancestors, such as the drying up of wells, either only happen
occasionally or are mere coincidences, such as those of the coincidence
of a dry or a rainy season, the prevalence of certain winds, etc.[F]
2. That the fumaroles of the lavas are communications between the
external surface of the lava, hardened and more or less cooled, and the
interior lava still pasty, or at least incandescent.
3. That from the lava, while flowing, there is no escape of acid
vapours, neither from the fumaroles at the first period of their
existence, but these, if they last long enough, arrive at an acid
period.
4. That hydrochloric is the first acid that appears, combined afterwards
with sulphurous acid, and, still later, with sulphuretted hydrogen.
5. That vigorous lava streams may have eruptive fumaroles. (See
Translator's Note 2 to p. 94.)
6. That the sublimations follow a certain order in their appearance. In
the neutral period we get sea-salt mixed with some metallic oxides, the
first of which is oxide of copper. But in the great lavas, chloride of
iron appears simultaneously with the acid period. Hydrochloric acid
transforms the oxides into chlorides, which, in their turn, change into
sulphurets or sulphates on the appearance of sulphurous acid.
7. That the acids, by attacking the scoriæ, create new chlorides and
sulphates, which are thus not products merely of sublimation.
8. That micaceous peroxide of iron--so common and abundant near the
eruptive mouths--is very scarce and rare on the lavas, unless conveyed
there from the craters.
9. That chloride of iron--so manifest on the fumaroles of the great
lavas--is only found in small eruptions close to the discharging mouths.
10. That the frequency of chloride of iron in the lavas of great
eruptions masks the order of transformation of the other products.
11. The fumaroles at the summit of Vesuvius present even greater
gradations, for they often emit carbonic acid or pure watery vapour.
12. Lead, which I first discovered in the fumaroles of the lavas of
1855, is a constant product of fumaroles which have a certain duration.
It is often obtained as a distinct and crystallized chloride, and often
is found in combination with other products.
13. Oxide of copper is also a constant and primary (_primitivo_) product
of fumaroles. The chloride and sulphate of copper are formed from the
oxide, directly contrary to general belief.
14. I do not think that the chloride of calcium, which I found on this
occasion in almost all the deliquescent sublimations, is a product
peculiar to this eruption only, in which alone, however, I found it. I
was, therefore, induced to look for it in other sublimates, in which I
might possibly have overlooked it, as, without doubt, my predecessors
have done, owing to the deliquescence of the chloride of iron with which
it was constantly combined. I think that this chloride, in accordance
with the general law, is transformed into a sulphate--a transformation
which readily occurs on Vesuvius.
15. Copious and well-crystallized sal ammoniac is only found on the
fumaroles of those lavas which have covered cultivated or wooded ground.
16. The scarcity of oxygen in the gases of fumaroles may possibly arise
from the formation of the oxides which precede the chlorides.
17. Lavas give a continuous spectrum, although covered with smoke, when
looked at with Hoffmann's spectroscope with direct vision.[G]
18. The smoke gives positive electricity, and the falling ashes negative
electricity.