opper either in black powder or in shining laminae, ought also to be
neutral. But if the fumarole continues active, hydrochloric acid issues
with the smoke, and often some time after sulphuric acid. Then the
sublimations turn first yellow, then green, and more rarely azure. The
chemical reactions show that these sublimations are chlorides or
sulpho-chlorides, and sometimes sulphides, and they afford reactions,
indicative of soda, magnesia, copper, lead, and traces of other
substances, not excluding ammonia, which I must speak of separately.
This, I have observed, is the general law with the fumaroles of the
tranquil lavas, which occur with long and moderate eruptions--for
instance, the lavas of 1871, and even those of 1872, preceding the 26th
April.

But in the great lavas of the great conflagrations of Vesuvius, chloride
of iron more or less in combination with all the other substances above
mentioned changes the appearance of the sublimations. The fumaroles in
the lava of the 26th April frequently indicated chloride of iron.
Sulphuretted hydrogen, by reaction of sulphurous acid, is decomposed,
and sulphur sublimed, having a particular aspect, collects on the
scoriae. This is never found but in fumaroles of the smaller lavas; it
was therefore absent in those of 1871, but frequently occurred in those
of 1872.

Although the sublimations are generally mixtures, yet sometimes distinct
and crystallized chemical or mineral species are found, such as sulphur,
sal ammoniac, _tenorite_, _cotunuite_, etc. Micaceous peroxide of iron
(feroligiste), so common near eruptive cones, is very scarce on lava;
any found in it has been carried down from the craters, and proofs of
this transport are very abundant and striking in the lavas of this last
eruption. Even the iron found in the bombs is evidently transported;
there is a fumarole on the ridge of the lava in the Fossa di Faraone
which contains micaceous peroxide of iron, and this, at first sight,
appears to oppose what I have affirmed; nevertheless, it gives
additional force to my statement. This fumarole is only a bomb or
rounded mass of enormous size, four or five metres in diameter. Smoke
and hydrochloric acid issued from the aperture in its envelope, and
being partly broken it was seen to contain lapilli and pieces of
antecedent lava, covered with micaceous peroxide of iron. The internal
temperature of this mass was very high; the hydrochloric acid which it
discharged had, in