The arguments used to establish an idea, that fires excited and maintained by human means exceed those of volcanic origin in force, lie in a very small com pass indeed ; they are derived from ob serving, that some furnaces "vitrify lavas snore decidedly than volcanoes, and melt schorls which remain perfect in the for mer." Dolomieu places this supposition in a clear point of view, in a memoir pub lished by him of basaltes. "I shall again repeat," observes this celebrated French naturalist, " what cannot be too frequent ly inculcated, that lavas are not vitrifica tions ; their fluidity is similar to that of metals reduced to fusion ; it does not change the order and manner of being, of the constituent parts of the lavas. When they cease to flow, they resume, like metals, the grain, texture, and all the characters of their primitive base ; effects which we cannot produce upon stones in our furnaces, since we know not how to soften them by fire, without changing the manner in which they are aggregated. The fire of volcanoes has not that inten sity which is supposed, and produces its effects rather by the extension and dura tion of its action than by its activity." Arguing upon these various facts, and remarks of bis own leading to the same point, Spallanzani candidly acknowledges, that he had been more than once inclined to believe, that our fires possessed more energy than those of volcanoes ; a num ber of experiments, however, induced him to say, that" these facts prove, first, that it is not always true, that volcanic fires are insufficient for the fusion of shorls ; secondly, by the vitrification of the garnets, they confirm the powerful activity of those fires ; thirdly, that those fires operate in a manner in some measure unknown to us; since, at the same time that they vitrify the garnets, they leave the base in which they are included in a state perfectly recognizable, notwith standing that the former are refractory to the fire of the furnace, while the latter is easily fusible." It has been a generally received assertion, that volcanoes emit flame during eruption, and that flowing lavas are attended by the same accompa niment of fire ; this suppoSition is errone ous, as may be proved by referring to the works of Serao, Father Torre, Rottis, and Sir William Hamilton, all of whom will be found to have omitted the observation of flames. The first expressly says of the lavas of Vesuvius, " that when seen by night, at any distance, they emit a light, not shining, like a bright flame, but of a dead kind, like that of red-hot substances which burn without flame ;" and the last mentions, that he has "observed upon mount Vesuvius, that soon after a lava has borne down and burned a tree, a bright flame issues from its surface ; otherwise I have never seen any flame at tending an eruption :" adding, that the light reflected on the smoke, as it rises from the crater, by the raging of the fire in the gulph beneath, is frequently mis taken for flame. Spailanzani confirms the opinion of these accurate observers, and declares lie never saw flame in, or proceeding from,- any of the craters he examined.
Faujas thought it not improbable that fire united with water may produce some of those combinations of which we know not the origin ; he says on this subject, " I almost incline to be of opinion, that the aqueous fluid, raised to a degree of ebullition and incandessence, of which our feeble furnaces can give us no idea, sometimes concurs with the inactive and concentrated fire which exists in the im mense volcanic caverns, and that from this concurrence results a multitude of combinations hitherto unknown to us, which take effect on the stones and earths that remain perhaps whole ages in these burning gulphs, where the fire, intent to destroy, has for its adversary the water, which incessantly creates and opposes to it all the forms and modifica tions of which the matter is suscepti ble." It will now be necessary to mention some of the effects of gas in the opera tions of these fierce internal fires it is well known that their violent efforts to reach the surface of the liquified masses contained in craters causes it to rise sud denly from the bottom, completely filling their whole circumference, and at length forcing it over the sides in destructive streams, which overwhelm in their pas sage every object, either natural or arti ficial. Spallanzani made ten distinct ex
periments, in order to obtain some idea of the nature and effects of gas as exhibited by volcanoes ; for this purpose be made use of different lavas, enamels, and glas ses, ejected from them, and the conse quence was, a conviction that the bub bles and inflations of various dimensions, observable in these substances, are not produced by the action of any permanent gas, but by that of an ariform fluid, produced by the excessive attenuation of those same products, in consequence of heat." Dr. Priestley made similar expe ments, which differed in some degree from those related by the above celebra ted Italian naturalist. The doctor fused 41 ounces of lava from Iceland in a sand stone retort, and obtained twenty mea sures of air, half of which, at the com mencement of the process, was carbonic acid gas, and the remainder, in purity 1.72, extinguished a candle ; between the interstices of this lava was a sand, which the operator could not separate from it. Five ounces and a half of Vesuvian lava produced thirty measures of air, with a slight appearance of carbonic acid gas, the rest was azotic gas, from the degree 1.64 to 1.38, with respect to what came last. On cooling, the residue broke the retort by its excessive inflation.
Without entering into an examination of the difference of opinion existing be tween these philosophers, we shall give an extract from the works of Spallanzani, that fully illustrates this part of our sub ject; " I shall," he observes, "now pro ceed to enquire what part this aeriform vapour acts in the eruptions of volcanoes. Where it exists in the depths of a volcan ic crater, abundantly mixed with a liquid lava violently urged by subterranean con flagrations, I can easily conceive, that by its energetic force it may raise the lava to the top of the crater, and compel it to flow over the sides and form a current. Art can imitate this grand operation of nature on an infinitely less scale. I plac ed in a glass furnace a cylindrical cruci ble, one foot high, and two inches and a half in breadth, which I filled half full with one of those volcanic products which most inflate and boil in the fire. After some hours, I observed that the liquid matter began slowly to rise, and after wards to rise higher, until it at last overflowed the edges of the crucible, forming small streams down its sides, which, when they reached the plane on which the crucible stood, gave origin to small currents, if that plane was at all inclined. When I put more of the same product into the crucible, the currents became larger. lithe plane was then ta ken from the surface, and the small cur rents, thus produced,examined, they were found full of' minute bubbles, as was like wise the matter which remained in the crucible. This curious experiment I made with several glasses and volcanic enamels, as also with a variety of cellu lar lavas, and always with the same suc cess." Judging from the result of the above trial, it cannot be doubted that a similar elastic vapour, collecting in vast quanti ties under the surface of the earth, must, upon meeting with resistance in its pas sage, produce loud noises resembling thunder, and local tremblings of the sur rounding earth, besides forcing its way upwards through super-incumbent lava: other experiments made by Spallanzani, however, seem to prove that it must be another cause which expels the fiery mat ter with violence out of craters, as the mattrasses he used broke without noise, and without ejecting or scattering the substance; and particularly, as the escape of gases has been frequently ascertained by the hissing sounds attending eruptions; unfortunately, though those vapours offer themselves to examination, it would be impossible to collect any part of them without exposing the life of the expe rimentalist to almost certain destruc tion ; we must therefore admit their ex istence, and conjecture must supply the rest.