It will be recollected that all volcanoes, at present in a state of activity, are sur rounded by, or situated very near, the sea, hence it appears clear, that the agency of that body is extremely powerful in pro moting the violence of their eruptions, by rushing at uncertain intervals, and from unknown causes, through the caverns of the earth, upon the ever enduring fires there existing; and this supposition is supported by the fact, which has been re: peatedly observed, of the sudden retiring of the sea immediately preceding a violent explosion from a crater, the certain con sequence of a rapid diminution of water on the- shore. Little need be urged to prove the immediate and vehement sepa ration that takes place upon the collision of fire and water, and of the force of steam thus produced ; one instance, how ever, may be safely cited, which will place this supposed collision in a true light, and is extracted from the fourth volume of the " Memoirs of the Academy at Bologna." A bell of enormous dimensions had been ordered to be cast at Modena, and pre parations of the usual description were made under a spacious portico. After the metal had been completely melted, it Was led into the mould, situated at a small depth under the pavement, through a small channel ; the burning fluid had no sooner entered the mould than a dreadful explosion took place, which resembled in every particular the horrid effects of springing a mine ; a deep hole was sunk in the earth, the metal, the mould, and every material of the portico above it, were scattered in the air, and several per sons were killed and severely wounded ; if such were the immediate consequences of a trifling degree of moisture remaining in the sand which composed the mould, it may be naturally interred, that a body of water, meeting with subterraneous fires, is capable of producing eruptions and earthquakes. It seems, however, ex tremely probable from experiments, that this effect principally arises from the in sinuation of water under or below the surface of the sides of those fires, as it has been ascertained that water thrown upon fire evaporates without much violence, and yet, if the vapour thus generated is Confined by super-incumbent earth, or rocks, its struggles for a vent must occa sion the violent disruption of those parts ; the event is different on pouring water on melted tin, which is the only metal that is separated by this means, so as to ren der it a dangerous operation to the expe rimentalist.
Spallanzani concludes many curious and interesting observations derived from experience, by saying, " from this series of experiments I think we are authorised to conclude, that when a quantity of wa ter falls on the burning crater of a volca no, it has not the power of producing ex plosions ; but that the latter on the con trary are very violent when the water pe netrating below, reaches the conflagra tion ; when suddenly reduced to vapour by the heat, it finds no room for its dilata tion ; or when it insinuates itself laterally among the liquified matters ; of which we have a satisfactory proof in the explosion of the lava, violently forced from the con taining vessel, on the introduction of wa ter into a cavity made in it." From what has been already said, a to lerable conception may be formed of the probable causes of volcanic eruptions: it how remains for' us to add a concise nar rative of their visible phenomena, and for this purpose we find ample materials fur nished by Spallanzani, whose ascent of Stromboli deserves every praise for its courage, though we cannot help con demning him for the exercise of very dar ing temerity. The visit we allude to was
made in 1788, when the appearance of the mountain was bifurcated, and the crater situated at some distance from the sum mits, from both of which the operations within it are distinctly visible, and from those the height of the ejections May be ascertained, with tolerable accuracy. Du ring violent internal agitation the matter appears to ascend halt a mile and more, but when the mountain is in actual erup tion, the scattered fragments prove, that the impelling force is very greatly increas ed. After having attentively examined the crater from the summit above alluded to, Spallanzani approached the crater, where he found that the explosions suc ceeded each other so rapidly, that they might almost be said to occur without any intervals of quiet; but they varied in their force; the matter, is some instances, not _ rising more than fifty feet, and falling again into the crater ; and in others it was elevated half a mile ; the sounds, conse quently, are proportionably loud, or the reverse, and resemble a hissing noise ; the fragments of lava were actually fluid during their progress, which was evident from their globular shape, and becoming hard before they fell upon the sides of the mountain, that form is preserved.
The exhalations exhibited a thick cloud several miles in extent which were strong-. ly impregnated with sulphur ; this cloud was impenetrablepby the:beams of the sun, and appeared very black in the midst, but white on the edges, and was, in all proba bility, a mile in depth. The vapour thus floating from the mountain was derived from three distinct sources, though doubt lessly produced by the same cause in the first instance : when an ejection of lava took place, it was always accompanied by a cloud of grey smoke from the crater; to the west of that spot were a number of obscure apertures, each of which sent forth a volume of similar vapour ; and to " the east, a vast cavern emitted a column at least twelve feet in diameter, extremely black and dense.
" Not satisfied with the observations I had already made," observes Spallanzani, "my curiosity impelled me to attempt further discoveries. From the pointed rock on which I stood, I could only see the edges of the inside of the crater. I consider ell, therefore whether it might not be possible to obtain a sight of the lower parts likewise ; and looking round me, I perceived a small cavern hollowed its the rock, very near the gulph of the volcano, into which the rock above prevented the entrance of any burning stones, should they be thrown so far. It was likewise so elevated, that from it the crater was open to my view. 1 therefore hastened to take my station in this cavity, taking ad vantage of one of the very short intervals between the eruptions. To my great satisfaction, my expectations were com pletely fulfilled; 1 could here look down into the very bowels of the volcano, and truth and nature stood as it were unveil ed before me." Thus situated in proba ' ble safety, the intrepid Spallanzani saw the following wonders.