The greater part of the lava issued from the base of the great fissure in the cone which I have described; and although two other lava streams descended from the top of the mountain, neither proceeded from the crater, but from apertures near it. The great crater, divided in two as already described, opened wide on the morning of the 26th April, destroying the brim of the antecedent crater, and remaking it in another shape with ejected matter, except on the south-west side, where the brim was split. (See [Plate 5].)
From this double crater, copious smoke, bombs and incandescent scoriæ, with ashes and lapilli, issued with violence, and from the depths below came dreadful detonations and bellowings, producing great terror. And yet the lava poured out into the Atria del Cavallo without any noise, and not even a column of smoke marked its origin of issue—namely, from the fissure.
When the eruption was over, the sight of the vertical walls of these deep craters, of almost horizontal strata of scoriæ and lithoidal masses, with a fracture fresh, and as if they had never undergone the action of fire or of acid vapours, without recent scoriæ and without fumaroles, was to me a marvellous spectacle. The fumaroles were almost all on the brims of the craters, with emanations of hydrochloric and sulphurous acid. In a few that were more removed from the brim, sulphuretted hydrogen was perceptible. In the sublimations, chloride of iron was 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 scoriæ I observed a yellowish substance, which looked like sulphur in a viscid state, and which boiled at a temperature of 120°, 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 scoriæ.
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.