volcano, and which has been long celebrated for its medicinal virtues. I collected here specimens of five kinds of lava; but, as in their general qualities they are analogous to those already described, I shall only mention them in a cursory manner.

XIII. The first specimen is a simple or homogeneous lava, in which, notwithstanding the most careful examination, I could not discover either shoerls, feltspars, or any extraneous body. In other respects, like those before mentioned, it is decomposed, adheres to the tongue, is friable, but without crumbling under the finger; its whiteness extends through its whole mass, and wherever it is broken has the taste of sulphate of alumine, (or alum).

XIV. The second specimen, through nearly half of it, exhibits a similar decomposition, and is of a white colour; but the other half, which is of a lead colour, has suffered little, gives sparks strongly with steel, and moves the magnetic needle at two lines distance. This lava has for its base the petrosilex. Both that part of it which is slightly decomposed, and the other which is more so, contain rhomboidal feltspars, of which the largest are about an inch in length. Their alteration is scarcely visible where the lava is least changed; and where it is more, they exfoliate with some facility, but retain a considerable degree of their natural hardness and splendour.

XV. The third specimen is a lava of a dark grey colour, siliceous where fractured, very compact, and which gives sparks with steel. It is of a petrosiliceous base, and contains abundance of feltspars and shoerls. But to shew these, it is necessary to divest it of a thick, whitish, and half-pulverous crust, produced by its decomposition.In this crust the shoerls and feltspars retain some consistence, but have lost, in a great degree, their lustre.

XVI. The fourth specimen contains within it a nucleus of a deep red colour, of the hardness and appearance of the carbonates of lime (calcareous earths), of a fine grain, but which is not dissolved or affected by acids, nor yields sparks with steel. It attracts the magnetic needle at the distance of one line. It contains a number of fissures, through which has penetrated, together with water, a quartzous matter, which has consolidated into a semi-transparent, and somewhat rough, covering. In this lava, which is but little decomposed, are found, dispersed, a number of small masses of sulphur of iron.

XVII. Small shoerls, and large crystallized feltspars, occupy the substance of this last lava, which is somewhat porous, but sufficiently hard to give sparks with steel. It is covered with a whitishyellow crust, which flakes off with a knife, and a reddish tincture has penetrated to its internal part, which is of a blackish ground.

In these lavas of Pisciarelli, the decomposition has, likewise, been much more considerable, than in the feltspars and shoerls which they contain within them.

I do not pretend to be certain that I have enumerated all the species of lava to be found at Solfatara: it is possible there may be others unobserved by me. I am persuaded, however, that I have described the principal; and such as enable me to deduce from their qualities the following conclusions.

1. Almost all the species of lava hitherto described, are more or less decomposed, and this decomposition is usually accompanied with a proportionable degree of whiteness. This observation has been made by several authors; and first by Sir William Hamilton, and M. Ferber, who have endeavoured to account for the fact by a very plausible reason, which is, that the sulphureous-acid vapours, which issue from Solfatara, and must have been produced in an infinitely greater quantity, when the conflagration was at its height, penetrating the lava by degrees, have insensibly softened it, and given it a white colour. And, in fact, similar changes are observed to take place in a piece of black lava, exposed for a sufficient time to the fumes of burning sulphur. But it does not hence follow that this lava will be changed into an argillaceous substance, as the above-mentioned Swedish philosopher would have us believe; since, from a chemical analysis, it appears that an earth of that kind, combined with other principles, pre-existed in it, and has only been rendered manifest by the diminution of aggregation produced by the before-mentioned vapours.

It is likewise not strictly true that the walls, or inclosing sides, of Solfatara are every where white and decomposed, as we might infer from the description of M. Ferber. Those who look toward the south, indeed, are so, but not those which are situated in another direction, and especially those which front the north, which are of a blackish colour, and little, or not at all, decomposed. The Abbé Breislak, Director of Solfatara, who accompanied me when I

made my observations, suggested a very probable reason for this diversity of appearance in the different sides, observing that the sulphureous acid is less powerful to effect the decomposition of lava, and requires longer time, when the lava has considerable humidity; which humidity must be much less on the southern side, where the heat of the sun is greatest. In fact, he exposed a piece of solid lava, to a very humid sulphureous exhalation, at Solfatara, during two months, without producing in it the least decomposition.

2. The observations I have made, convince me that the alterations. here described always take place in the upper part of the lava; and that, in proportion as we penetrate downwards into it, they become gradually less, and, at a certain depth, entirely cease. This, at first view, does not appear to accord with the effect of sulphureous vapours, which, rising from the bottom of Solfatara, and passing through the lava, might be expected to cause a greater change in the lower parts than the higher, from their having there greater heat, and consequently being more active. But we must consider that this may indeed be the nature of their action, where the lava is spongy, or at least very porous, but not where it is compact, and almost impenetrable to such vapours; as is that of Solfatara. And, in fact, we find that the sulphureous fumes which arise there, do not issue from the body of the lava, but always from fissures or apertures in it, or the subjacent tufa. These impediments, therefore, prevent them from acting except on the surface, when, issuing forth, they are driven over it by the wind, and, penetrating the lava, in a long course of time, produce the changes in question. We meet with few decomposed lavas, within which we do not find fragments of sulphur adherent, condensed there by the acids above-mentioned, and which are of the same kind with that produced in such abun dance in Solfatara.

But what productive cause shall we assign for those sulphureous vapours, the slow destroyers of the lava, which continually issue from a number of fissures in Solfatara, in the form of hot white fumes? I can conceive no principle to which they can with greater probability be ascribed than those sulphures of iron, (pyrites), which abound at the bottom of the volcano, and decomposing, in consequence of a mixture with the subterraneous waters, slowly inflame,

and produce those hot sulphureous vapours, which evidently prove that the subterraneous conflagration is not entirely extinguished. The noisy effervescence, likewise which, in more than one place is heard under the plain of Solfatara, seems to give a certain indication of the decomposition of these sulphures

The streams of vapour, which arise from Solfatara, according to Father Della Torre *, in the night, appear like flame. No person can be more competent to ascertain the truth of this fact than the Abbé Breislak, who resides near the place, and who, when I questioned him on the subject, assured me that he had never observed any such appearance. It is, however, not impossible, but that, at the time he observed them, they might have undergone some change.

The vapours which arise from the ground of the Pisciarelli are very few, and almost insensible, though formerly they must have been numerous and strong, as may be inferred from the great decomposition and whiteness of the lavas found there. I have already mentioned the noise with which the springs that bear this name burst from the earth. They resemble a boiling cauldron. The reasons assigned for this phenomenon, by different authors, are various, but, hitherto, all conjectural. On applying the ear to the place where the spring issues, we may hear that the bubbling noise does not proceed from any great depth, but from a small distance from the surface of the earth. Were the ground, here, to be dug into, we might, perhaps, be able to discover this secret, the knowledge of which might prove advantageous to volcanic researches.

3d. We have seen that almost all the lavas of Solfatara contain within them shoerls and feltspars. But it has been proved that the changes occasioned in both the latter, by the action of sulphureous acids, are considerably less than those which take place in the lavas in their matrices; which difference must arise from the nature of these two stones, which is less liable to extrinsic injuries. We find them, in fact, firmly resist the power of the humid elements. To the south of Vesuvius, and at a little distance from Salvatore, I have found several pieces of very ancient lava, porous, and half-consumed by time, which, however, preserved unaltered their black crystallized shoerls.

* Storia del Vesuvio.

It has been observed that the houses of Pompeii, long since overwhelmed by Vesuvius, and now in part dug into and cleared, are found to have been built of lava. I have ascertained this fact on the spot. They are of a reddish colour, very dry to the touch, and some of them will crumble under the finger, evident proofs of the change they have undergone; but no such alteration has taken place in the shoerls they contain; they still retain the hardness and glassy splendour which is appropriate to that stone.

We likewise know that the feltspars are indestructible by the air, as appears in the porphyries of which they are a part.

4th. I have already remarked that the lavas of Solfatara usually have for their basis the petrosilex and the horn-stone. I shall add, that I have also met with the granite in them, though not in a large mass, but in small detached pieces, which induced me to doubt whether they properly belong to this volcano; and as they likewise appeared to me untouched by the fire, I rather inclined to believe them adventitious. This granite consists of two substances, quartz and shoerl.

But another production must not be forgotten, which forms large heaps on one side of the internal crater of this volcano. This is an ash-coloured tufa, of a middling consistence, in strata of various thickness, with the superficies of each stratum covered with a black crust, in which may be discovered manifest vestiges of plants. The Abbé Breislak, who first observed this tufa, after having shewn it nie on the spot, gave me some of these impressions of plants to examine, conjecturing them to be some species of the alga marina, or sea-weed. While I was at Naples, I had not sufficient time to make an accurate examination of them; but this 1 afterwards made at Pavia, from several specimens of the same tufa. Some parts exhibited only the impressions of plants, but in others I found leaves. They are striated, with striæ running lengthwise, and, when touched with the point of a needle, easily break, and appear converted into a carbonaceous substance. At first I doubted whether they were plants of the alga; but, on examining them again carefully with a lens, and comparing the leaves found in the tufa with those of the natural alga, I was fully convinced they were.

This observation appeared, both to me and the Abbé Breislak, to be of considerable importance; since we may conclude from it, that