LAVA. served. The pores of the fresh lava were generally full of a perfect vitrification, and the scorie themselves, viewed through a magnifying glass, appeared like a confused heap of filaments of foul vitrification. When a piece of the solid lava had been cracked in its fall, without separating entirely, fibres of perfect glass were always observed reaching from side to side within the cracks. The natural spun glass which fell in some places along with the ashes of this eruption, and which has likewise been observed in other places, he is of opinion must have proceeded from an operation of the kind just mentioned; the lava cracking and separating in the air at the time of its emission from the crater, and by that means spinning out the pure vitrified matter from its pores or cells; the wind at the same time carrying off the filaments of glass as fast as they were produced.
Our author observed a kind of pumice stone sticking to some very large fragments of the new lava. On close inspection, however, he found that this substance had been forced out of the minute pores of the solid lava itself; and was a collection of fine vitreous fibres or filaments confounded together at the time of their being pressed out by the contraction of the large fragments of lava in cooling, and which had been bent downwards by their own weight. "This curious substance (says he) has the lightness of a pumice, and resembles it in every respect, except that it is of a darker colour."
When the pores of this lava were large, and filled with pure vitrified matter, the latter was sometimes found blown into bubbles on the surface; probably by the air which had been forced out at the time the lava contracted itself in cooling; and from these thin bubbles it appeared, that this kind of volcanic glass has much the same transparency with our common glass bottles, and like them is of a dirty yellow colour; but when large pieces of it were broken off with a hammer, they appeared perfectly black and opaque.
In the lava of this eruption it was observed, that many detached pieces were in the shape of a barley-corn or plum stone, small at each end, and thick in the middle. Some of these did not weigh above an ounce; but others could not be less than 60 pounds. Our author took them to be drops from the liquid fountain of fire, which might naturally acquire such a form in their fall. There were also many other curious vitrifications, different from any he had seen before, mixed with this huge shower of scorie and masses of lava.
In treating of Mount Etna, M. Houel makes mention of a piece of lava which, after having been once ejected by the volcano, was swallowed up, and thrown out a second time. The intense heat to which it was then subjected, had such an effect upon it, that it appeared all full of chinks to a considerable depth, and which ran at right angles to one another. He had also an opportunity of observing to great advantage some of the hollow channels formed by the lavas of Etna, similar to those described by Sir William Hamilton, but on a much larger scale. Here the great eruption of water in 1755 had overturned, in a vertical direction, a huge tube of this kind for the length of half a mile. The tube itself appeared to be composed of enormous
masses, somewhat resembling planks; each two feet thick and twelve or fifteen in breadth, continued in a straight line through the whole of that space. At the same time by the action of the lava a kind of walls had been formed, from ten to sixteen feet in height, and curved at the top. Some of these walls appear rolled together like paper; and M. Houel is of opinion, that these various appearances on the surface of the lava when cooled, must have arisen from particles heterogeneous to the real lava; and which detach themselves from it, rising to the surface under a variety of forms proportioned to the spaces of time taken up in cooling. These crusts are formed of different kinds of scorie and dirty lava, mixed with sand or ashes. At the same place are also found great numbers of small pieces like those of ice heaped upon one another after having floated for some time on a river. Beneath these the pure lava is met with, and which has evidently been in a state of perfect fusion. This is extremely dense; and by looking narrowly into its chinks, the composition of the whole appears to be merely homogeneous. "It is curious (says he) to observe, so near one species of lava which is very pure, another which has likewise arrived at the same place in a fluid state, and has there undergone so great a change as scarce to retain an appearance of its original state. It is, however, like iron dross, in grains of unequal sizes. We find it also at various distances, such as one, two, or more hundred fathoms. It is sometimes found in large pieces like tables, covered over with sharp points, some longer, and others shorter. All these pieces are quite detached from one another, as if they had been brought thither and scattered from a tumbril. The matter of which the crust of the lava is formed, seems to have issued from it in the same manner in which froth rises upon solution of soap in water. It appears afterwards to have swelled, burst, and assumed its present form, presenting to the view various spaces filled with small loose stones. A great number of new lavas were likewise observed, all of them putting forth various kinds of efflorescences in great quantity.
The hardness, density, and solidity of lavas, no doubt proceed from the degree of heat to which they have been exposed, and which seems to be greater or less according to their quantity. Hence the Icelandic volcanoes, which pour forth the greatest quantities of lava, produce it also in the greatest degree of liquefaction, and Dr Van Troil observes, that what he saw must have been liquefied to an extreme degree.
The composition of the lavas of different volcanoes, and even of different parts of those of the same volcano, is extremely different. Sir William Hamilton is of opinion that this difference in composition contributes not a little to the facility or difficulty with which they afterwards receive earth capable of vegetation. "Some (says he) have been in a more perfect state of vitrification than others, and are consequently less liable to the impressions of time. I have often observed on Mount Vesuvius, when I have been close to a mouth from whence the lava was disgorging itself, that the quality of it varied greatly from time to time. I have seen it as fluid and coherent as glass when in fusion; and I have seen it farinaceous, the particles separating as they forced their way out, just like meal coming from under the grindstone. A stream of lava of this