III.
FUMAROLES OF THE LAVAS.

Smoke generally issues from all lava when it cools down to a certain degree, hence it is more abundant at the edges of the fiery torrent, or is liberated from the scoriæ that form on its surface. But when the lava stops, the smoke issues only from certain vent-holes, through which we can still see the fire, and at the edge of which different amorphous or crystallized matters collect by sublimation. These centres of heat, of more or less duration, are the fumaroles of the lavas. I believe I have on other occasions shown that a fumarole is nothing but a communication between the more or less cooled and hardened surface of the lava and the interior, which is still incandescent. Some fumaroles last but a day, others preserve their activity for weeks, months or years, according to the depth of lava through which they penetrate; and when they cease to be active, that is, when the sublimations are formed, or smoke or other æriform matters issue from them, they still retain a rather elevated temperature. In the lavas of 1858, in a place where they had a transverse width of 150 metres, a vent-hole may still be found where the thermometer registers 60° and the scoriæ are warm. Sometimes, while the lava is in process of cooling, new fumaroles appear, in which the fire is visible. This phenomenon, which appeared marvellous and inexplicable when I first observed it in 1855, is now very easily understood; the cooled and hardened crust of the lava fractures with noise and suddenly, and so a new communication is opened with the incandescent lava below, thus creating a new fumarole.

As the smoke of the fluid lava is perfectly neutral, that is, neither acid nor alkaline, so the fumaroles at the first period of their existence with sublimations of sea-salt, mixed frequently with oxide of copper either in black powder or in shining laminæ, 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 scoriæ. 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 some places, covered the micaceous iron with a yellow coating of chloride of iron. From small apertures, on the lower side of the mass, white and green stalactites of chloride of calcium were visible. In one spot only of lava I found a fumarole, with a small quantity of micaceous peroxide of iron, evidently in a state of formation; but this was the very spot where the lava became eruptive, and whence issued the column of smoke which was so well photographed—the place under the hill of Apicella. (See [Plate 4a].)

I have enumerated the products which are constantly collected in fumaroles, although they are not all found at the same time or place, in order to show that the sublimations follow a certain law in their appearance. Tenorite, for instance, was formerly considered an accidental product of certain eruptions, and I have always found it; but if you visit the fumarole when the acids have had time to transform it, you will no longer see it. I found the crystallized chloride of lead, or "cotunuite," as it is called, for the first time in the lavas of 1855, and thought it a singular circumstance; but from that time I recognised it in all the lavas, though not always so beautiful and abundant; and even when not found as a distinct substance, I observed it in combination with chloride of copper. In the lavas of the 26th April cotunuite and tenorite[E] were not very abundant, because the chloride of iron disturbed the greater number of the sublimations. I found sal ammoniac very abundantly on the fumaroles of the lavas that invaded the cultivated ground. Although chloride of ammonia, contrary to opinion, was not wanting in the sublimations of the fumaroles of the lavas deposited on other lavas, yet it was neither abundant nor crystallized, but combined in small quantities with other substances. It appeared in great abundance in all the fumaroles of lavas which covered cultivated or woody ground. At first it was scarce enough, and mixed with chloride of sodium; but when the rains came the sea-salt was washed away, and sal ammoniac formed beautiful crystals, nearly free from adventitious matters, as was the case with the fumaroles of the last lava. Afterwards, when chloride of iron was produced, ferro-chloride of ammonia was found. Crystals of sal ammoniac were sometimes found of a beautiful amber yellow. This colour was, in the opinion of my colleague, Professor Scacchi, produced by such small traces of chloride of iron that neither Professor Guiscardi nor I, nor indeed any other chemists to whom I submitted specimens for examination, could detect any. What I can affirm with certainty is, that these limpid crystals of a yellow colour were almost always attached to an amorphous substance, soluble in water, composed of various chlorides, in which iron was often detected.

From these remarks, it is evident that in the tranquil lavas the sublimations appear with a certain order of succession, and in the violent lavas, and those which flow most copiously, they are more complicated, and render both chemical analysis and spectroscopic researches more difficult. Notwithstanding, I observed traces of lithium and thallium, which I had previously perceived in some sublimations of 1871. I purpose submitting many sublimations which I have collected to more complete spectroscopic investigation, although I am persuaded that the discovery of traces of certain bodies in the sublimations or in the lavas is a matter of small importance to the science of volcanoes. I must say, however, that calcium was discovered on this occasion in great abundance, not only by the spectroscope, but also by chemical analysis. Sulphate of lime has often been found in larger or smaller proportions, but this was the first time I had observed chloride of calcium both close to the craters, and also in the sublimations of the fumaroles upon the lavas. The white stalactites which I collected beneath the great mass or bomb above described were almost exclusively composed of chloride of calcium, and only a few green drops manifested, with the usual re-agents, the presence of iron.