VOLCANO

VOLCANO, or VULCANO, a name given to several parts of the earth whence flames, smoke, stones, cinders, &c. are sometimes thrown out with astonishing violence and in prodigious quantity. The name is borrowed from VULCAN the Heathen god of fire, who was fabled by the poets to have his work-shop under Mount Ætna, where he, with the assistance of the Cyclops his servants, forged thunderbolts for Jupiter.

The greater part of the known volcanoes are situated under high mountains, which they perforate by means of their internal heat, and throw out such immense quantities of melted and burning hot matter, seemingly from the bowels of the mountain, that it is wonderful how it continues to support itself, and is not at once precipitated into the huge caverns which must be occasioned by the disgorging such immense quantities of matter. The very summit of volcanoes indeed does from time to time fall into the abyss below; but this is soon repaired again by the quantity of stones, cinders, and ashes, thrown out of the volcano, which form a conical mount, hollow like a funnel in the inside, and having an hole in the middle which leads directly down to the abyss whence the fire proceeds. This conical mount is supported all the way down by the stones and other matter heaped up on each side of the spiracle or chimney through which the fire of the volcano passes. But in process of time the vehemence of the heat, or its long continued action, wears away those supports, and the top again falls in, and soon after is renewed as before. But though the summit of volcanoes thus very often falls in, we seldom or never hear of a burning mountain falling down from the foundation. On the contrary, Ætna, though it has continued to burn from time immemorial, is still a very high mountain. Vesuvius also continues much of the same height as ever, though its summit, and even great part of the mountain itself, has been known to fall

Volcano. fall in. However, it must be remarked, that though volcanoes are generally seated under high mountains, this is not always the case. In the Mediterranean, a burning island arose in the beginning of the present century, upon which there was no eminence of any considerable height. In the island of Tanna, in the South Sea, also, Mr Forster mentions a volcano whose height was much less than that of many hills in the neighbourhood.

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times of
their erup-
tion uncer-
tain.

Though volcanoes generally emit a hot sulphureous and suffocating steam, yet they by no means cast forth flames at all times with violence. This happens only at uncertain intervals of sometimes more than a century together. Nay, sometimes even the emission of the above-mentioned kind of steam ceases, the crater or chimney of the volcano is covered over with earth and vegetables, and it is forgot that ever any eruption took place. This appears to have been the case with Vesuvius before the time of Pliny; for at that time nobody seems to have apprehended the least danger; though it is now pretty certainly determined that many eruptions had happened before this time. The signs of an approaching eruption are enumerated under the article ΑΤΝΑ so fully, that little remains to be added here. It would seem however, from the accounts of Sir William Hamilton and Mr Brydone, that neither the volcanoes of Ατνα, Vesuvius, nor Stromboli, are ever in a state of perfect quietness. Smoke continually issues from them, and explosions and other noises are heard within them. Mr Brydone particularly mentions a small mountain on the side of Ατνα, which disturbed him in the night-time with frequent explosions like the noise of heavy cannon fired at a distance, though the mountain was then as quiet as it is almost ever known. Mr Hamilton mentions it as a common remark at Naples, that Vesuvius grumbles in bad weather; which he explains by supposing that the sea forces itself into some of the caverns under the mountain, and which are at all times of a glowing heat, whence arise those explosions and strange kinds of sounds above-mentioned: but we can scarce suppose this to be peculiar to Vesuvius; nor is there any certainty of a communication between that mountain and the sea. Mr Raspe indeed supposes the sea-water to be necessary for the accension of volcanoes; but of this he brings no proof. It is certain indeed that many of them are below the bottom of the sea; but it seems most probable that the water has not access, or but in small quantity, to the place whence the fire originates. Mr Forster, from more certain observation, concludes, that volcanoes are exasperated by the falling of rain. This he took particular notice of in the volcano in the island of Tanna, and suspects that rain excites the explosions by increasing the fermentation of the mineral substances within the mountain.

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Vesuvius
most noisy
in bad wea-
ther.

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Conjectures
concerning
the accen-
sion of vol-
canoes.

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Prodigious
force of vol-
canoes.

The force with which a volcano explodes seems to be the greatest hitherto known in nature. So great indeed is this force, that we must of necessity suppose the resistance of the air to be exceedingly lessened before the projected materials. Under the article GUN-NERY, it is shown how great this resistance is, and what an effect it has upon globes of solid iron. Upon irregular masses of rock, then, and much more upon streams of liquid lava, the resistance must be such as to stop their motion almost as soon as it began.

Nevertheless, in the great eruption of Vesuvius in 1779, Sir William Hamilton informs us, that a stream of lava of an immense magnitude was projected to the height of at least 10,000 feet above the top of the mountain; and if to this we add the depth of the source whence this lava was derived, the force by which it was impelled must exceed our imagination. Indeed, let us suppose the cause to be what we will, had the air exerted its resisting power upon this lava as it does on a cannon-ball, the force necessary to have carried it to that amazing height must have dashed the stream in pieces at its first emission from the mouth of the volcano. Either the extreme heat of the lava, therefore, or some other cause must contribute to keep off this resistance, without which we cannot suppose that it could have reached to such an height, much less could the lighter materials, ashes, cinders, &c. have arisen to an height sufficient to carry them to the distance of 100 miles from the mountain, which we are assured was the case at that time.

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There are no phenomena in nature whose cause seems more difficult to be explained than volcanoes. The fermentation of pyrites, of sulphur and iron, the explosions of nitre, of aqueous steam violently heated, or of air pent up and greatly rarefied, have all been thought of to explain them, but with very little success. Dr Woodward and others have recourse to a central fire which they suppose to exist in the bowels of the earth, and suppose that volcanoes are so many spiracles or chimneys to prevent it from destroying the whole world.

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It must, however, be very evident, that the immense force which we observe in volcanoes cannot by any means be accounted for from any such principles. It is impossible that beds of pyrites can be so totally inexhaustible as to remain for thousands of years under, or nearly under, the same part of the surface of the earth. And as to nitre, it is never found in a fossil state, nor indeed can we conceive that such quantities of nitre can exist under any one part of the surface as would be sufficient for a single explosion: besides, nitre does not explode instantaneously, unless it is thoroughly mixed with sulphur and charcoal in order to convert it into gun-powder; an operation which cannot take place in the bowels of the earth. As little can we suppose those eruptions owing to the expansion of aqueous steam; for though this will in some cases explode with great violence, yet in others it seems to be in a manner totally deprived of that power. With melted lead, copper, or salts, it explodes with the utmost fury, even when water in a very small quantity is thrown upon the surface of such substances; yet on the surface of melted glass, which is much hotter than any of them, water will not explode, but evaporates quietly, seemingly kept at some distance from the surface of the glass by the violent emission of steam from its under part. Nay, with this violent heat, it does not even appear to boil; but gradually lessens in its dimensions, until at last it quite disappears. Indeed, if we suppose water intimately mixed and dispersed in small quantities through the substance of melted glass or any similar substance, no doubt a very violent explosion will take place; but in this case the matter, instead of being thrown up in a full stream, would be scattered all around in an immense

Volcano. immense number of small drops. Indeed we cannot conceive how any such intimate mixture could take place; for if we suppose any large collections of water to exist under ground at the same time that a lava is by some unknown means melted in such a manner as to come in contact with it, either the water must fall upon the lava, or the lava into the water. If the water falls upon the lava, no explosion will take place, as has been already shown; and if the lava falls into the water, the liquid must no doubt be heated boiling hot, and the steam dissipated with those noisy explosions mentioned under the article ATRA, when a lava of that mountain ran into the sea. On this supposition, however, the lava could never be projected above the mountain; much less can we suppose that solid rocks should be moved from their places, and thrown to great distances, as experience shows them to be. In the eruption of Vesuvius in 1779, Sir William Hamilton measured one which was 108 feet in circumference and 17 feet high, and which was thrown to the distance of more than 1300 feet from the crater of the volcano. If we suppose this stone to have been only twice as heavy as water, it must have weighed considerably more than 700 tons; and the projectile power employed to throw this to such a distance must have exceeded any thing we can conceive, almost, of the force of gun-powder itself.

⁹ Difficulties in ascertaining the true cause.

Indeed, when we consider the immense power exerted by volcanoes, we can scarce find any thing in nature to which it can be ascribed. What only seems to resemble it is the explosions of inflammable air in mines, and of which an account is given under the article DAMPS. But as it is impossible to account for the continual generation of this air, which we know must be destroyed by every explosion, we cannot have recourse to this agent; nor, though electricity seems to be very much concerned in volcanoes, are we acquainted with any laws according to which that fluid acts, by which a succession of violent explosions could be occasioned. The primary consideration, however, in attempting to account for the phenomena of volcanoes, is the means by which the fire is originally kindled, because upon this, some-how or other, all the rest of the phenomena depend. This, from what has been already observed, cannot be occasioned by fermentations of metalline, mineral, or saline substances, which in time would wear out; but must depend on some inexhaustible cause.

¹⁰ Fire of volcanoes originally kindled by the electric fluid.

The only inexhaustible source of fire which we are acquainted with is the electric fluid, by means of which every degree of heat, even to that which vitrifies gold and platina, may be excited. From the many articles in this work, where the nature of the electric fluid has been considered, it must appear very probable that it is no other than the light of the sun himself absorbed by the earth, pervading every part of it, receiving new motions, and becoming subject to new laws, in consequence of this absorption, as if it were different fluid. As the sun perpetually sends down an immense quantity, great part of which is still absorbed, it is impossible that such an absorption can take place without an emission of an equal quantity somewhere or other. The outlets to this fluid are innumerable; but by various causes many of these may occasionally be stopped. In this case it is plain, that the fluid must move with greater velocity through those which remain

open. But it has been shown, under the article VOLCANO, ELECTRICITY, n^o 79. and HEAT, n^o 9. that the violent motion of the electric fluid through any substance is heat; that when this fluid converges to a centre, or diverges from one, heat always takes place in the central point, and that more or less violent according to the quantity of fluid which thus converges or diverges. If then, from any cause, the electric matter diffused through a very large portion of the earth should converge towards one particular spot on the surface, then that spot would undoubtedly be heated, or perhaps set on fire; and if a sufficient quantity of electric matter should constantly be directed in this manner, we must easily see that the heat would be continual. Hence may be understood the origin of fol-fataras, or spots of earth which have a continual heat in them, and of hot springs. But if this violent flux of electric matter should by any means be interrupted and renewed at uncertain intervals, we see, in that case, the heat must occasionally increase and decrease, without any cause visible to those who dwelt near the spots above-mentioned. In volcanoes, however, it is probable that the flux of electric matter always continues to a very considerable degree, but occasionally is increased in such a manner as to melt down the earth, stones, and mineral substances through which it passes into the semivitrified compound called lava. We cannot indeed, as yet particularly know the causes by which this electrical flux is at all times occasioned. In general, we are certain, that whatever has a tendency to solicit the matter to the surface, will increase the violence of an eruption; and whatever can increase it after it has begun, will also contribute to bring it on originally. This will happen when the pressure of the atmosphere directly over the mountain is lessened; when a cloud passes over it whose electricity is opposite to that of the mountain itself, or which, by acting as a conductor, may facilitate the escape of the confined electric matter; or it will readily happen if by any means the surface of the mountain and of the earth round it is rendered a better conductor than usual. But above all, the state of electricity in the neighbouring earth and air must contribute towards the bringing on an eruption, or increasing one which is already begun. Under the article EARTHQUAKE, n^o 29. it has been shown, that when there is a perfect equality between the electricity of the earth and of the atmosphere, there is then great danger of a concussion, because the electric matter can neither get into the atmosphere from the earth, or into the earth from the atmosphere. It must therefore stagnate altogether, and be ready to burst out on all sides, when the pressure upon it becomes sufficiently strong. In such circumstances, if there happens to be a volcano in the neighbourhood, it is probable that it will be set on fire, or perhaps a new volcano may be produced on this occasion.

¹¹ Origin of fol-fataras and hot springs.

¹² Of the lava of volcanoes.

¹³ How a flux of electric matter may be determined to one particular spot.

¹⁴ Why volcanoes are produced or set on fire by earthquakes.

¹⁵ Facts in confirmation of volcanoes being set on fire by electricity.

These causes seem to be sufficient for the production of volcanoes, and setting them on fire from time to time, as usually happens with them; though no doubt there may be others much more unknown to us which take place in the bowels of the earth itself; but as these are entirely concealed from us, we can only guess at their existence. But that all the above-mentioned causes do contribute towards the eruption of burning mountains experience evinces. Sir William Hamilton

mentions it as a common observation, that the inward explosions of Vesuvius increase during bad weather, when we know that the pressure of the atmosphere is lessened, and the descending vapours serve as conductors to the electric fluid. The same gentleman informs us, that, on the 7th of August 1779, as he was observing Vesuvius, a collection of thunder-clouds passed directly over its summit, and mingled with the sulphureous and mineral clouds of smoke, cinders, and ashes, which were then piled up to an amazing height over its top. "At this moment (says he) a fountain of fire was shot up to an incredible height, casting so bright a light, that the smallest objects could be clearly distinguished at any place within six miles or more of Vesuvius." Thus we see that these clouds acting either by their conducting power as aqueous vapours, or by an electricity opposite to that of the volcano, solicited a vast discharge of the fire as soon as they came directly over it. Mr Forster's observation, that the volcano in the island of Tanna took fire after rain, has been already mentioned; and the cause will now be apparent, namely, that by this means the surface of the ground all round was rendered a much better conductor than before, and perhaps supplied with a greater quantity of electricity, the passage of which to the volcano was now free, in consequence of which it took fire and burnt violently till this supply was again exhausted. Lastly, it is well known that some countries which were formerly very subject to earthquakes, have been freed from these destructive phenomena by the breaking out of a volcano. In the time of earthquakes also, temporary volcanoes are often formed, and the neighbouring ones throw out vast quantities of fire. This was particularly observed in the great earthquake of 1692, when Catania was destroyed, at which fatal moment a vast flame was seen to issue from mount Etna. But whether any such appearance was observed during the earthquakes of the present year (1783), which seem not to have been less violent than that of 1692, has not yet been authenticated.

We shall only add one remark more in confirmation of the hypothesis that volcanoes are set on fire by electricity, namely, that in a volcanic eruption the quantity of this fluid emitted from the mountain is quite incredible. For a long time together, the smoke is sometimes so much electrified as to attract the ashes which are thrown out along with it almost into a solid column. The stones thrown out, according to Sir William Hamilton, are sometimes transparent, which we can only attribute to an immoderate electricity. Nay, the same author informs us, that the vast stream of lava which issued in 1779 to the height of almost two miles, was transparent also. Add to this, that the volcanic smoke proves very dreadful by reason of the thunder-storms which issue from it. In the great explosion of 1779, which, however, lasted only twenty-five minutes, a cloud of electrified smoke was produced which threatened the destruction of Naples at six miles distance from the mountain; and though it was happily driven from thence by the wind, the air continued so much electrified, that a Leyden vial armed with a pointed wire soon became considerably charged by being only held out at a window. At Ottaviano, which was involved in this cloud, incessant flashes of lightning added greatly to the horror of the

scene, as well as to the danger of the inhabitants. By another day's eruption the air was filled with meteors resembling falling stars, which give us a yet higher idea of the quantity of electric matter emitted, as these indicate a greater degree of electricity than even the most violent forked lightning. See LIGHTNING, n^o 7.

Thus we have attempted to explain the original cause of volcanoes, the formation of lava, and the reason why the volcanic eruptions are not constant, but only take place at uncertain intervals. It now remains only to assign a reason why volcanoes do not burn quietly as any common fire, but, even during the time of an eruption, by fits and intervals. The explosions indeed are in a manner continued; but, Sir William Hamilton observes, that the burning mountains are subject to what he calls a crisis at noon and midnight. The exacerbations also are very remarkable, so that our author distinguishes them by the name of four fits. All these phenomena, however, may be solved by an attentive consideration of the principles above laid down. The lava is formed by a great quantity of electric matter violently forcing itself through a small spot of earth, in consequence of which that spot is melted, and thus becomes a better conductor of electricity than before, whence the force of the electric blast towards that place is still augmented. Now, as the earth is composed of what we call simply mould, together with stones, and minerals of various kinds, moistened with a certain quantity of water, it is not possible that a mass of this kind can be melted without the production of an incredible quantity of steam of various kinds. It is not only the aqueous vapour that is here concerned, but almost all the different kinds we can imagine. No doubt the steam of water alone shows, in certain circumstances, an incredible degree of strength; but of all vapours hitherto known, the aerial ones are the most powerful in their effects as well as the quickest in their operation. Mr Robins, in his Treatise on Gunning, relates some experiments, from which he concluded, that air, heated to an intense white heat, was expanded into more than four times its usual bulk, and consequently, in such circumstances, has more than four times its usual elasticity. From another experiment he showed, that inflamed gun-powder produces a quantity of permanently elastic steam equivalent to 244 times the bulk of the gun-powder itself. Hence he concludes, that the force of inflamed gun-powder is nearly equal to 1000 times the pressure of the common atmosphere. Now, though the air produced by the inflammation of gun-powder is of a different kind from that which we may suppose to be produced by the melting of a lava, and in much greater quantity in proportion to the bulk of most of the materials, yet it does not appear that the elasticity of the volcanic steam is at all inferior, as far as it goes. From Dr Priestley's experiments it appears, that the expansion of the different kinds of air by heat are in the following proportions:

In order to form some estimate of the force of volcanic steam, however, it is necessary not only that we should know the different elasticity of the airs which may be supposed to be extricated, but the proportions which are yielded by different substances. But here our data are very slender. Dr Priestley, to whom we must in a manner wholly have recourse, has not made experiments on those substances which we can most reasonably suppose to exist in the bowels of the earth. From some, however, air was discharged in the following proportions, with a heat indeterminate indeed, but certainly not greater than that of a volcano.

All these substances we may with good reason suppose to exist in a volcano, or at least substances equally ready to discharge air. Nay, Dr Priestley himself supposes as much dephlogisticated air to be discharged from the melted materials of a lava as is sufficient to continue the burning. But however much he may be in the right with regard to the quantity of dephlogisticated air produced in this manner, he must certainly be mistaken as to the use of it. It is certain that the burning must have commenced before any air could be emitted from the materials, and the cause which produced must also be sufficient to continue it; not to mention that lava could not be kept melted, nor stones red-hot, by blowing air in any quantity upon their surfaces.

With regard to the effects of this steam, we must easily see, that where the quantity of materials is very large, it must be extremely formidable. From the calculations of Dr Priestley above-mentioned, it appears, that fixed air let loose and heated is more powerful than atmospheric air, and consequently more so than even the air emitted by gun-powder, which Mr Robins observes to have much the same elasticity with common air. It is true, that nitre yields pure dephlogisticated air, which is somewhat more elastic than fixed air: but it is instantly phlogisticated by the other ingredients; and Dr Priestley found what remains after the accension of gun-powder to be more phlogisticated than common air, and consequently to have very little more elasticity, as Mr Robins observed. Now, the steam of a volcano must undoubtedly be composed of fixed air, vitriolic acid, some marine acid, dephlogisticated and inflammable air, all blended together at once by the melting of the lava, not to mention a prodigious quantity of aqueous steam rarefied and heated to an extreme degree. As all these are more powerful than an equal quantity of the vapour of inflamed gun-powder, no wonder that such terrible effects ensue from it. From Dr Priestley's calculation, it appears that two ounces of wood-ashes, if a strong white heat could be suddenly applied to them in such a manner as to make them give out all their air at once, would be much more powerful than one ounce of gun-powder; three ounces of pit-coal ashes would be more powerful than an ounce and an half of gun-powder; six ounces of manganese

would be more than an equivalent to one ounce, and 15 pounds of chalk would be the same; though this last we may be assured would in such circumstances yield more than 50 times the quantity of air that Dr Priestley could obtain from it. Lapis calaminaris would be terrible beyond all measure; one ounce of it being nearly equivalent to two of gun-powder. What must be the consequence, then, if thousands of tons of materials equally prolific in such kinds of air are melted at once?

When all this is attentively considered, we will be so far from being surprised at the immense power of volcanoes, that it will rather be matter of surprise that more mischief is not done by them than what actually happens. This however is prevented by the force being constantly directed upwards, and not horizontally along the surface of the earth. The first consequence of the production of a lava must be a violent tendency of it to expand. The reason of this is obvious, viz. because the steam is dispersed through every part of it, and therefore swells and heaves it up, as we see in the case with any glutinous fluid when set over the fire to boil. As this expansion is very violently resisted by the obstacles below ground, the overcoming of these, which in fact is no less than removing and breaking to pieces rocks and huge masses of earth, must be attended with the most tremendous explosions; and hence we may account for the noises below ground before an eruption of a volcano takes place. As the lava is excessively tenacious, it will continue to swell, and keep the steam within itself for a long time, though violent blasts of smoke and elastic steam will now and then get out, as we see that puffs of smoke will get out from any thick matter boiling over a fire. As these blasts of volcanic steam must also be composed partly of the smoke of burnt earth, and partly of elastic air, they are consequently of a black colour; and hence the sign of an approaching eruption is a quantity of black smoke now and then shooting up to an immense height in the air. Along with this black smoke, however, a white one will be emitted without intermission. This white smoke is composed of aqueous vapour, and what comes from the outside of the lava when thoroughly melted, which is always observed to emit a white steam. As the lava advances, a prodigious quantity of earth, stones, &c. are collected on its upper part, so that the blasts of steam cannot get out without breaking through this covering. The earth in the mean time is calcined to ashes, and the stones are made red-hot by the vehement heat of the lava underneath, and by the violent current of electric matter converging towards that point. The volcano therefore begins to throw up red-hot stones, cinders, ashes, and scoria of the lava itself. These falling down into the crater, continually fill it up; and thus confining the steam, give constant occasion for fresh explosions, and the mountain continues to rage with more or less violence, according to the quantity of steam yielded by the lava, and the degree of resistance it meets with. At last, when the lava has reached the summit, or meets with a weak place in the mountain, as the same tendency to expand still continues, it runs over the top, or bursts out thro' the side. Then all the violent explosions and fury of the volcano cease, because the steam gets continual vent by forcing more of the lava out through the opening. But if by any accident the run-

Volcano. ning of the lava happens to be stopped, or the vent happens to be too narrow, all the former phenomena return, sometimes with double violence.

Thus we may in a pretty plausible manner account for the general phenomena of volcanoes; and if extraordinary phenomena sometimes occur, we may reasonably suppose them to be owing to some accidental circumstance. Thus the vast height to which the lava of Vesuvius ascended in 1779, must certainly have been occasioned by some extraordinary pressure, such as indeed we may well be surprised how the sides of the mountain could bear. There are three circumstances which may reasonably be supposed to cause the lava spring up to a considerable height. 1. Its being very suddenly formed; for thus all the elastic steam will be discharged in a manner at once, and the tendency to expand will be inconceivably great. 2. The degree of resistance. And, 3. The capability of the materials to produce elastic steam. All these seem to have concurred to produce the great explosion on the 8th of August 1779. The immense quantity of electric fluid which at this time issued from the mountain, indicated that the lava had been suddenly formed. Immediately before its emission, such a report was heard as shook the houses at Portici, broke the windows, and cracked several walls; and the fitness of the materials at that time in the mountain for producing elastic steam, appeared from the vast quantities of smoke which issued along with the lava.

With regard to the uses of volcanoes, we may observe, that however terrible their eruptions may be, they answer very important purposes in nature, by giving vent to such quantities of electric matter as might otherwise render the earth totally uninhabitable, if not endanger its frame altogether. Sulphur, sal ammoniac, and various other salts, are produced by them; their ashes fertilize the neighbouring country; and the lavas, tufa, basaltes, &c. afford materials for building. By their means also many islands have been thrown up from the bottom of the sea, and afford new habitations for man.