Yet the Volcano, or rather all volcanic activity as now operative upon our globe, is, as it were, an experiment of Nature's own perpetually going on before us, the results of which, if well chosen—that is, as Bacon says, by keeping to the main and neglecting the accidents—can, when colligated and correctly reasoned upon, in relation to our planet as a whole, give us the key to the enigma of terrestrial Vulcanicity in its most general sense, and at every epoch of our world's geognostic history, and show us its true place and use in the cosmical machine. Let us glance at the history of past speculation on this subject, from which so little real knowledge is to be derived, and then at the salient facts of Vulcanology as now seen upon our earth, and finally see if we can connect these with other great cosmical conditions, so as to arrive at a consistent explanation in harmony with all.
We gain nothing absolutely from the knowledge of the so-called "ancients" as to Volcanoes in Europe at least, where alone historic records likely to refer to them exist. The Volcanoes of Europe are few and widely scattered. The Greeks saw but little of them, and the Romans were all and at all times most singularly unobservant of natural phenomena.
Cæsar never mentions the existence in France of the Volcanoes of Auvergne, so much like those he must have seen in Italy and Sicily; and Roman writers pass in silence that great volcanic region, though inhabited by them, and their language impressed upon the places, as Volvic (volcano-vicus) seems with others to indicate; and though there is some reason to believe that one or other of the Puys was in activity within the first five hundred years of our epoch, the notices which Humboldt and others have collected as from Plato, Pausanius, Pliny, Ovid, etc., teach nothing.
Whatever of mere speculation there may have been, volcanic theory, or what has passed for such, there was none before 1700, when Léméry brought forward a trivial experiment, the acceptance of which, even for a moment, as a sufficient cause for volcanic heat (and it retarded other or truer views for years), we can now only wonder at. Breislak's origin, in the burning of subterranean petroleum or like combustibles, was scarcely less absurd than Léméry's sulphur and iron filings.
Davy, in the plenitude of his fame, and full of the intense chemical activities of the metals of the alkalies which he had just isolated, threw a new but transient verisimilitude upon the so-called chemical theory of Volcanoes, by ascribing the source of heat to the oxidation of those metals assumed to exist in vast, unproved and unindicated masses in the interior of the earth. But Davy had too clear an intellect not to see the baseless nature of his own hypothesis, which in his last work, the "Consolations in Travel," he formally recanted; and it only survived him in the long-continued though unconvincing advocacy of Dr. Daubeny. So far, the origin of the heat had been sought always, in the crude notion of some sort of fuel consumed, whether that were petroleum or potassium and sodium; but as no fuel was to be found, nor any indicated by the products, so far as known, of the volcanic heat, so what has been called the mechanical theory, in a variety of shapes, took its place.
This, in whatever form, takes its lava and other heated products of the volcano ready made from a universal ocean of liquid material, which it supposes constitutes the interior or nucleus of our globe, and which is only skinned over by a thin, solid crust of cooled and consolidated rock, which was variably estimated at from fourteen to perhaps fifty miles in thickness. Here was a boundless supply of more than heat, of hot lava ready made, the existence of which at these moderate depths the then state of knowledge of hypogeal temperature, which was supposed to go on increasing with depth at the rate of about 1° Fahrenheit, for every thirty or forty feet, seemed quite to sustain.
The difficulty remained, how was this fiery ocean brought to the surface or far above it? To account for this two main notions prevailed, and, indeed, have not ceased to prevail. Some unknown elastic gases or vapour forced it up through fissures or rents pre-existent, or produced by the tension of the elastic and liquid pressure below.
The form in which this view took most consistency, and approaching most nearly to truth, finds the elastic vapour in steam generated from water passed down through fissures from the sea or from the land surface. But to this the difficulty was started, that fissures that could let down water would pass up steam. The objection, when all the conditions are adequately considered, has really no weight; and it has been completely disposed of, since within a few years it has been proved that capillary infiltration goes on in all porous rocks to enormous depths, and that the capillary passages in such media, though giving free vent to water—and the more as the water is warmer—are, when once filled with liquid, proof against the return through them of gases or vapours. So that the deeply seated walls of the ducts leading to the crater, if of such material, may be red hot and yet continue to pass water from every pore (like the walls of a well in chalk), which is flushed off into steam that cannot return by the way the water came down, and must reach the surface again, if at all, by the duct and crater, overcoming in its way whatever obstructions they may be filled with.
And this remarkable property of capillarity sufficiently shows how the lava—fused below or even at or above the level of infiltration—may become interpenetrated throughout its mass by steam bubbles, as it usually but not invariably is found to be.