« AnteriorContinuar »
proceed all from the same reservoir. In some the water is perfectly clear; in others, thick. The waterspouts up from all, some continually, others at intervals. The altitude of one of these spouts, measured by a quadrant, was ninety-two feet. The force of the vapours which throw up this water, is excessive; it not only prevents the stones which are by way of experiment thrown into the opening, from sinking, but even throws them up to a very great height together with the water.
But, if while acting by itself it is thus powerful, how irresistible must it be, when it comes in contact with metals in fusion in the bowels of the earth. An explosion then immediately takes place, and the parts of the metal are scattered in all directions. The force with wlich this steam can act, is indeed wonderful. About 60 years ago, during the operation of casting some brass cannon, in the presence of a number of spectators, the heat of the metal of the first gun drove so much damp into the mould of the second, which was near it, that as soon as the metal was let into it, it blew up with the greatest violence, tearing up the ground some feet; breaking down the furnace, unroofing the house, and killing many people on the spot.
Thus, as it has been related, the explosion was like thunder; and the force was equal to the noise; for the matter was scattered by the blast, as dust would be before the wind.
So apparent a reason for some of the phænomena of nature, did not fail to strike the minds of reflecting men, and to point out to them the way of satisfactorily explaining difficulties of no inconsiderable stubbornness. It is found by experience, says Burnet, that water, so gentle in itself when undisturbed, flies, when it falls amongst liquid metals, with an incredible impetuosity, and breaks, or bears down every thing that would stop its motion and expansion, This causes the marvellous force of volcanos, when they throw out stones and rocks. This explosion, made by the sudden rarefaction of sea waters, which fall in receptacles of molten ore, and ardent liquids within the cavities of the mountains; and thereupon, follow the noises, roarings and eruptions of those places. Volcanos, says he, are always in mountains, and generally, if not always, near the sea; and when its waters by subterraneous passages, are driven under the mountain, they meet there with metals, and minerals dissolved, and are immediately raiefied, and, by way of explosion, fiy out
at the mouth or funnel of the mountain, bearing before them whatsoever stands in their way.
The explosion and eruption of the various matters of a volcano, proceed indeed in all
probability, from the access of a large quantity of water, which either enters through some crack in the bottom of the sea, or fron sources in the earth *. If the mass of water so admitted, be sufficiently great, it will extinguish the subterraneous fire; if not, it will suddenly be converted into vapour, whose elastic force is known to be several thousand times greater than that of gunpowder. But, as I have already said, the contact of water with metals is that which probably produces the most tremendous effects. If we consider the immense quantity of matter thrown up at different times by volcanos, without lessening their apparent bulk, what frightful hoards of both fire and metals, in readiness for the accession of water, must we not suppose accumulated in the internal parts of the globe. The chymical examination of the volcanic matters thus ejected, proves that iron makes from one-fifth to one-fourth of their whole substance. How enormous the quantity, there
fore, of this metal, or at least of the stones in which it is contained.
The sentiments of chymists concerning the causes of subterraneous explosion, have been various. As it, however, has been looked upon as analogous to the act of fulmination, which is more immediately open to our investigation, we shall for a moment advert to what has been said upon that subject. The mixture of nitre, sulphur and charcoal, composes a matter, the terrible effects of which, is owing to its great combustibility. Its greatest proportion is of nitre, next of charcoal, and last of sulphur. The effects of this mixture, or, as we commonly call it, gunpowder, is thus accounted for : sulpur and charcoal are eminently combustible : the nitre is equally distributed between the molecules of these substances, and as it is in a much larger quantity, cach niolecule of sulphur and charcoal is surrounded by nitre. Each then, has a greater quantity of air than is necessary for itself for inflammation ; for it is known, that when heated, nitre furnishes an abundance of pure air. Thus it happens in this combustion, as it does to a combustible substance plunged into dephlogisticated air, which' burns with infinitely more vivacity than before. And
hence the rapidity of powder, and its explosions when opposed by any obstacle whatever. The explosive force of powder, we in this manner see, is from the blast of air in the nitre, expanded by the fire in the sulphur; and that these are more suddenly kindled and opened by the medium of charcoal. The force or energy of solid materials, are thus out of the question. It is merely the operation of a quiescent air, excited to action, and expanded by fire.
Various substances, as we have formerly observed, when heated to a certain degree, produce a great quantity of elastic fluid, and almost instantaneously; which elastic fluid, gives a concussion to the ambient air, occasioning an explosion. Hence, the impetus, or the force exerted, and the report, are in proportion to the quantity of elastic fluid produced, and also to the quickness of this production. Thus nitre yields by heat, a considerable quantity of dephlogisticated air; charcoal yields by heat, a considerable quantity of inflammable air. The fire employed to inflame the gunpowder, extricates these two airs, and sets fire to them at the instant of their extrication.* The air thus volatilized from fired gunpowder, occupies
about * Cavallo.