VOLCANOES, mountains which emit ignited matter and smoke through aper tures, communicating with cavities in the depths of the earth, where eternal fires are situated, that burn with more or less force, as they are influenced by causes which it is impossible should ever be explained by actual observation, but that may be conjectured with tole rable success from experiments. Such have been made by many naturalists (who deserve every praise for their assiduity and research), though not with an accu racy that distinguishes those of the cele brated Abbe Lazarro Spallanzani, profes sor of natural history in the university of Pavia : this gentleman, though far ad vanced in life, acted with a vigour and hardihood seldom found even in youth, and braved danger and death, in a thou sand horrid forms, in pursuit of his fa vourite object, the elucidation of the phenomena of volcanic fires. To whom, then, can we with more propriety have recourse, in our attempt to explain their causes and effects ? The Abbe observed, in the course of his various examinations of craters, that volcanoes emit vast quan tities of gas ; and he found that stony sub stances were invariably, when complete ly heated by the subterraneous fire, mt rified, inflated, and rendered cellular, by their elasticity ; which effect is observ able in numbers of lavas, glasses, and en amels, • ejected during eruptions : and he discovered, in addition, that their vio lence continually raised the liquified matter from the interior of the craters to their very borders, over which it flowed at each impulse.
He was at the same time equally atten tive to the nature and force of the fire which acts in the bowels of volcanic mountains ; and, in the course of his re searches, discovered that Vesuvius, /Et na, the Eolian Islands, and Ischia, are immense'mountains, composed of rocks that hact been liquified, and even vitri fied, by the violence of the subterraneous conflagration. " What fire," he ex claims, " can we produce equivalent to these effects !" Humble, however, as all experiments appear with our means, this venerable philosopher justly thought imperfect knowledge of volca noes preferable to contented ignorance, and, undismayed by the magnitude of the object, proceeded to ascertain, as far as possible, what man is permitted to know on this terrific subject. "1 have," he observes, " discovered, that Die .fire of the glass-furnace will completely refuse the vitrifications, enamels, pumices, sco ria, and lavas, of these and other volcanic countries. The same will, in like man ner, vitrify rocks congenerous,, to those from which these mountains. have origin ated, by the means of subterranean con flagrations. A less intense fire, on the contrary, produces no such effect on any of these substances. Determined to ex ercise the most rigorous research, and to ascertain, with the greatest possible pre cision, the exact degree of heat requisite to produce the above effects, he had re course to the pyrometer of Wedgwood, which he compliments and praises, by saying nothing could be, better adapted for his purpose.
The terrific appearance, of a volcano in eruption is so appalling, so grand, and altogether so wonderful, that it is by no, means astonishing the world should sup pose the vast volumes of smoke, ignited matter, and stones, hurled into the air with inconceivable violence and rapidity, exclusive of the torrents of liquified sub stances which roll down its sides in so lemn and destructive majesty, were caus ed by more powerful fires than those man has been permitted to kindle ; in saying the world, we wish to be understood as meaning those whu, have seen or read of eruptions without examining the subject further. Of natural philosophers, there were many who coincided with this ge neral opinion ; and others have maintain ed the direct contrary supposition, as serting, that volcanic fires are extremely feeble in their operations : following the example of Spallanzsni, we shall give the substance of the arguments of each, in order that the reader may draw his own conclusion. It is evident that we must have recourse to the same rule for ascer taining the intensity of volcanic fires, which we make use of in measuring the effects of our fires when in activity on bodies immersed in them ; and we have already mentioned, that Wedgwood's pyrometer answers for the purpose, as nearly as the nature of the pursuit will permit : but long before the invention of this instrument, attempts were made to attain the object in question, particularly by the academicians of Naples, who, at the time of the great eruption in the year 1737, made an experiment on the lava near the Torre del Greco, in a valley where it had accumulated ; and though it had ceased its motion several days, yet retained a heat equal to that of red-hot iron. They, formed a piece of lead, weighing two ounces, in a conical shape, which they placed on the red-hot surface ofthe lava; the metal became soft in two minutes and a half, and in one minute more it was completely melted ; another piece of lead, .in every respect exactly similar, was then deposited on a plate of red-hot iron, rendered so by coals beneath it, when they found that it required six minutes and a half to soften, and seven and a half to liquify it. Water placed on the lava, boiled furiously in three minutes, and on burning coals, one minute later. Judging from these facts, the academicians concluded that lava, though exposed to the external air for some days, and consequently far less in tensely heated than when first issuing from the crater, was much mere fiery in its nature than red-hot iron or burning coals 4' but this conclusion is obviously in correct ; because the plate of iron, be ing surrounded by air, could not acquire all the heat which was applied to it ; nei- ther was it fair.to rest an opinion of this description upon a result produced by means so unequal, as a vast depth of ig nited matter opposed to a thin plate of iron.