SALINE INCRUSTATIONS ANALYSED.

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salt. From the 5th December to the 20th February the lava flowed in larger or smaller quantities, so that at night a stream of ignited matter was always visible, more or less interrupted by cooled lava. It changed its direction according to the obstacles it encountered; never extending apparently so much as a mile from its source. During the whole of this time, the craters, two in number, were in activity; the large one throwing up showers of ignited ashes and stones to a height of from 200 or 300 feet; the smaller one projecting steam with great violence. Whenever the crater could be approached, it was found to be coated with saline incrustations; and the walk to the edge of the small crater on the 6th January lay through a mass of loose saline matter principally common salt coloured by muriate of iron, in which the feet sunk to some depth. It was easy even at a great distance to distinguish between the steam disengaged by the one crater, and the earthy matter thrown up by the other; for the steam appeared white in the day, and formed perfectly white clouds, which reflected the morning and evening light, of the purest tints of red and orange; while the earthy matter appeared always as a black smoke, forming black clouds; and in the night it was highly luminous at the moment of the explosion.

On the 20th February, the small crater which had been disengaging steam and elastic matter, began to throw out showers of stones. On the night of the 23d, after an explosion which shook the windows of Sir Humphrey's bed-room at Naples, a column of ignited matter was seen ascending from

Vesuvius to a height at least equal to that of the mountain from its base, which illuminated the whole horizon with volcanic light, both direct, and reflected from the clouds above the column of ignited matter. On observing the lava, it appeared at its origin much broader and more vivid; and it was evident that a fresh stream had broken out to the right of the former one. When he visited the mountain on the morning of the 24th, he could not ascend to the top which was covered with clouds, nor could he examine the orifice from which the lava issued; but near the termination of the stream it was from 50 to 100 feet broad. The saline matters condensed on some of the masses of scoria, were of the same nature in their constituent parts, as those of the lava of the 26th January, but with a larger proportion of sulphate of soda, and a smaller proportion of muriate of iron. The dense white smoke emitted in immense columns by the lava during the whole of its course, was most probably produced by the same substances.

In May, 1814, flame and steam issued from the bottom of the funnel-formed crater; along with fumes of sulphurous and muriatic acids. There was no indication of carbonaceous matter from the colour of the smoke; nor was any deposited upon the yellow and white saline matter surrounding the crater, which was principally sulphate and muriate of soda, with muriate of iron, and occasionally muriate of ammonia.

In March 1815, the crater presented very different appearances; there was no lava aperture. The volcano remained quiet for minutes together, and

SOUNDS PRELUSIVE OF ERUPTION.

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then burst out into explosions with considerable violence, with the ejection of fluid lava and ignited stones to the height of many hundred feet in the air.

These eruptions were preceded by subterraneous thunder, apparently coming from a great distance, and lasting sometimes for a minute. During the four times that Sir Humphrey was upon the crater in the month of March, he had at last learned to estimate the violence of the eruption from the nature of the sound; a considerable explosion being betokened by loud and long continued subterraneous thunder. Before the eruption the crater appeared perfectly tranquil; and its bottom, apparently without an aperture, was covered with ashes. Soon indistinct rumbling sounds were heard as if at a great distance; these gradually approached, till discharges, like artillery, were felt under his feet. The ashes then began to rise and to be thrown out with smoke from the bottom of the crater; and lastly the lava and ignited matter were ejected with a most violent explosion. "I need not say that when I was standing on the edge of the crater witnessing this phenomenon, the wind was blowing strongly from me; without this circumstance, it would have been dangerous to have stood on the edge of the crater; and, whenever from the loudness of the thunder the eruption promised to be violent, I always ran as fast as possible from the seat of danger.'

Immediately after the eruption, the ashes and

Sir H. Davy. Phil. Trans. 1828, Part I., p. 248.

stones which rolled down the crater seemed to fill up the aperture, so that it appeared as if the ignited and elastic matter were discharged laterally, when the interior of the crater assumed the same appearance as before.

Having interrogated nature by these admirable experiments and observations, he next proceeds to offer the following judicious remarks on the theory of the phenomena. It appears almost demonstrable, says he, that none of the chemical causes anciently assigned for volcanic fires can be true. Among these, the combustion of mineral coal is one of the most current; but it seems a most inadequate explanation. However large the stratum of pit coal might be, its combustion under the surface could never produce violent and extensive heat; for the production of carbonic acid gas, when there was no free circulation of air, must tend continually to impede the process; and if such a cause existed, carbonaceous matter would certainly be found in the lava, disengaged along with the saline and aqueous products from the bocca or crater. In England we have many instances of strata of mineral coal which have been long burning; but the results have been merely baked clay and clay schists, without any thing at all resembling lava.

Were Lemery's old idea correct, that the action of sulphur on iron may be a cause of volcanic fires, sulphate of iron ought to be the great product of the volcano; which is known not to be the case, and the heat produced by the action of sulphur on the common metals, is quite inadequate to account for the appearances.

ORIGIN OF VOLCANIC PHENOMENA.

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When it is considered that volcanic fires occur and intermit with all the phenomena that indicate intense chemical action, it seems not unreasonable to refer them to chemical causes. But for phenomena upon such a scale, an immense mass of matter must be in operation, and the products of the volcano, ought to give an idea of the nature of the substances primarily active. Now, what are these products? Mixtures of the earths in an oxidated and fused state, under intense ignition; water and saline substances, such as might be furnished by the sea and air, altered in such a manner, as might be expected from the formation of fixed oxidized matter. But, it may be said if the oxidation of the metals of the earths be the causes of the phenomena, some of those substances ought occasionally to be found in the lava, or the combustion ought to be increased at the moment the materials passed into the atmosphere. To this objection, it may be replied, that the changes which occasion volcanic fires evidently take place in immense subterranean cavities; and that the access of air to the acting substances occurs long before they reach the day.

The ground under the solfaterra is unquestionably hollow; nor is there any reason to doubt of a subterraneous communication between this crater and that of Vesuvius; for whenever Vesuvius is in an active state, the solfaterra is comparatively tranquil. Sir H. examined the bocca of the solfaterra on the 21st February, 1820, two days before the activity of Vesuvius was at its height. The columns of steam which usually rise in large quantities when Vesuvius is tranquil, were now scarcely