most abundant, in combination with other chlorides, for
example, of sodium, magnesium and calcium. This last chloride was
frequent even among the sublimations of the fumaroles of the lavas, and
it was the first time it was ever remarked, but I do not think it was
the first time that it was ever produced: being in combination with
chloride of iron, and very deliquescent, it did not attract attention
from anyone. In a hollow fragment of scoriae I observed a yellowish
substance, which looked like sulphur in a viscid state, and which boiled
at a temperature of 120 deg., and evolved hydrochloric acid. Having
collected this substance and poured it into a glass phial, it quickly
coagulated into an amorphous mass of the same colour; but before I
reached the Observatory, I found that it had become liquid by
deliquescence. It consisted of a mixture of the aforesaid chlorides,
according to an analysis made by Professor Silvestro Zinno and myself.
In some fumaroles, where I perceived the smell of sulphuretted hydrogen,
I found sublimed sulphur under the scoriae.

At the source of the lava stream that flowed towards the Camaldoli, on
the seaward flank of Vesuvius, I observed large fumaroles of steam only,
pure aqueous vapour.

There was no trace of carbonic acid in these fumaroles, but that fact
does not imply that there was none at a later period, for, since the
first investigations of Deville, it is known that carbonic acid is found
under certain conditions on the very summit of Vesuvius.

VI.

THE ELECTRICITY OF THE SMOKE AND ASHES.

Our ancestors could judge that a great amount of electricity was
occasionally evolved in the smoke, from their observation of the
lightning flashes that darted through the Vesuvian pine tree; but they
had no proper instruments for ascertaining whether this evolution of
electricity was constant or accidental, or what laws regulated its
manifestations. My _apparatus, with movable conductor_, by which
comparative observations of electric meteorology can be made, and the
errors arising from dispersion corrected, supplied me with an easy
method of studying the electricity evolved during eruptions.

I must begin by describing the bifilar electrometer, in order to explain
the apparatus which I have named as above, "_Apparechio a conduttore
mobile_."

_A A_ (Plate VIa, Fig. 1) is a glass cylinder, the lower edge of which
is ground, well varnished with gum lac, and let into a wooden base, B,