IV.
BOMBS, LAPILLI AND ASHES.
The bombs ejected from the craters are like those carried down by the lavas, but of smaller size, and they seldomer contain a nucleus similar to those found in the latter. With the bombs properly so called, many pieces of incandescent lava were thrown up, and in their fall went beyond the base of the cone. A quantity of small scoriæ varying in size accompanied these projectiles, and those fragments, which we call lapilli, fell at a greater distance. With the lapilli, and sometimes without them, the smoke carried a very minute dust or sand, which is generally called ashes. These ashes, when washed with water, lose soluble constituents which they have collected in the smoke—such as chloride of sodium and other chlorides and often free acids. The insoluble part originates in the detritus of lava, and with the microscope we can detect abundant fragments of those crystals which most frequently occur in the lava of the same eruption.
The lavas of 1871, which were eminently leucitic, and almost entirely deprived of pyroxene, resembled the ashes, which appeared to be fragments of crystals of leucite, more or less enveloped in the paste of the lava, so that having triturated the scoriæ of the lava, and looked at the powder through the microscope, it was apparently quite the same as the ashes.
But at the beginning of the eruption of the 26th April, a white sand fell in the Atria del Cavallo, close to the Crocella[5], which on the dark scoriæ of 1871 looked like snow. Its fall had a limit so well defined that one passed without any gradation from white to black. Having collected some of this sand that very morning, I put it up in white paper, for at that moment it was impossible for me to examine it. Taking it out some days after, I found it had become reddish, and having put it under the microscope, I observed that it was exclusively formed of little pebbles more or less round, of a transparent vitreous matter, partly covered with a red substance. Fragments of green crystals occurred in this sand, upon which no red was perceptible. I consulted our eminent crystallographer, Arcangelo Scacchi, whether these little pebbles were leucite, as I suspected, and whether the green particles were pyroxene: he confirmed my suspicion, and remarked that the red colour was superficial only. We then washed a little of the sand in hot water, and saw the pebbles become whitish; but having heated some on platinum, we observed that they first turned black and then became perfectly white, proving that the red was a deposit of organic matter. To see these leucites, rounded like small pebbles transported by a torrent, deprived of the soluble chlorides which generally accompany Vesuvian ashes, is a matter worthy of attention. Whilst heating this sand upon platinum, decrepitation was audible, which indicated the cracking of some of the little pebbles. It is evident, therefore, that crystals of leucite raised to a certain temperature may break, and thus we can understand how almost all Vesuvian ashes contain fragments of the said crystals enveloped in the paste of the lava. It is evident that the soluble part of the ashes is obtained from the smoke through which it passes. On this occasion the smoke from the craters did not apparently contain much acids, for no bad smell was perceptible, and the water in which I washed the ashes scarcely reddened litmus paper. Even chloride of iron, which was so abundant in the lavas, was scarcely perceptible in the smoke, which almost exclusively deposited sea-salt on the surrounding rocks; I say sea-salt advisedly, and not chloride of sodium, to show that I include all that sea-salt contains. The slight disturbance it manifested with chloride of barium, and the small precipitate with oxalate of ammonia, reveal sulphate of lime, without excluding the possibility of the chloride.
But how can these ashes do so much injury to the vegetation of the ground they cover, especially at the first fall of rain? I think that the damage is due partly to the sea-salt, and partly to the acids contained either in the ashes or in the rain-water itself. Upon watering the tender tops of some plants with a saturated solution of the salt from Vesuvius itself, I noticed that they withered away after a few hours. But very often the rain alone which traverses the smoke of Vesuvius, or is produced by condensation from it, gives manifest acid reactions, and destroys the grass and the tops of the trees. The peasants believe that the rain is warm or of boiling water, from observing that the tender parts of the plants are, by its deposit, all burnt up. Vegetation is now recovering, but without flowers, and consequently without fruit.