The water, decomposed by contact with the metalloids, yields its oxygen to them ; the hydrogen is liberated, but not allowed to escape in great quantity alone, for it readily, under the influence of heat, combines with sulphur into sulphuretted hydrogen, or, with the oxygen of atinoepherio air (if any be present), reconstitutes water. Nitrogen is thins liberated, and may be conceived to pass off partly free, partly combined with hydrogen, so as to constitute ammonia, which again unites with chlorine (derived from the sea-water), and constitutes salammoniac. While oxygen (derived from atmospheric air) is plentiful in the volcanic channels, the hydrogen will not unite with sulphur, which accordingly combiner, with oxygen into sulphurous acid. When the oxygen ie consumed, sulphuretted hydrogen is formed iu abundance, and predominates towards the close of the eruption, and even, by the aid of the residual internal heat operating on sulphur, is evolved for centuries after the volcanic violence is spent..
The evolution of chlorine is easily traced to a double decomposition of sea-salt ; carbonic acid is supposed to rise from calcined limestone rocks ; and specular iron-ore (fer oligiste) is a product of sublimation. Thus, in the opinion of Dr. Daubeny, all the main phenomena conco mitant upon volcanic action seem to admit of explanation if we suppose, first, sea-water, and afterwards atmospheric air, admitted to consider able masses of metals, metalloids, sulphur, &c., the basis of the whole speculation being the abundant decomposition of water at a moderate depth below the surface of the earth. The views of Dr. Daubeny have been controverted by very eminent writers (as Dr. Davy, Prof. Bischof, and others) on particular points ; but we are not aware of any attempt upon other assumptions quite so satisfactory as this of Dr. Datfbeny, to explain generally the chemical products of volcanic eruptions in the order of their occurrence.
Perhaps, therefore, we may concede to this hypothesis the proba bility that in the interior of the earth the metalloids exist in quantity sufficient to cause an abundant decomposition of water, and thus originate a given series of chemical changes such as are witnessed in volcanic eruptions. But before we accept it as a general explanation of volcanic disturbances, other classes of data than those furnished by chemical analysis must be brought into the reasoning.
An important circumstance in the general theory of volcanoes is the connection and reciprocal activity which exists underground between volcanio regions entirely separated on the surface, as between Sicily and Naples ; between the Mediterranean volcanic region, taken gene rally, and the region of extinct (or long suspended) volcanic action in Asia. To this we must add another and larger series of facts regarding the extent and distribution of volcanic action on the surface.
Vo&anic Regions and their Connection.—Volcanic regions, if estimated by the area over which the lava and ashes have been dispersed, con stitute but a small portion (perhaps less than 0th part) of the surface of the globe, but in a survey of these regions we must include not only the active vents and extinct craters, but also " large intermediate spaces where there is abundant evidence that the subterranean fire is at work continuously, for the ground is convulsed from time to time by earth quakes ; gaseous vapours, especially carbonic acid gas, are disengaged plentifully from the soil ; springs often issue at a very high tempera ture, and their waters are usually impregnated with the same mineral matters as are discharged by volcanoes during eruptions. (Lyell, ' Princ.
of Geology,' book ch. ix.) To describe these districts would be entirely foreign to the purpose of this essay, but we may by a simple classification show how much of the grandest features of physical geography is due to volcanic dis turbance.
European Volcanic Districts. —The Icelandic volcanoes, remarkable for abundant lava streams ; the Azores, amongst which new islands have been thrown up ; Sicily, including tEtna and the vanished Gra ham's island, often called Sciacca, which is properly only the name of the opposite point of Sicily ; the Lipari Isles, with Stromboli always burning ; the Neapolitan tract, including Vesuvius ; Ischia and the Ponza Isles; Santorini and some neighbouring islets. The above are all considered as attire volcanic centres, and have been subject to eruptions in historical times. The extinct volcanic systems of Europe are the trachytic domes in the centre of France (Auvergne, [AuvEansis, in GEO°. Div.] the Vivarais, &c.), the Eifel country, the Seven Moun tains, and other trachytic and basaltic parts along the Rhine ; the Westerwald, Vogelsgebirge, Rhongebirge, together with the Kaiserstuhl; and many other scattered basaltic bills in the middle of Germany; Hungary, Transylvania, both remarkably rich in trachytes and the vitreous lavas called pearlstones; the Gleichenberg in Styria. In Italy. the Euganean hills and other smaller points appear in the north .of Italy; while between Rome and Naples large ancient craters occur, and connect Mount Albano with Vesuvius. (Daubeny.) A:mac Volcanic Districts.—Mixed active and extinct volcanic moun tains occur about the Persian Gulf, the Red Sea, and the Dead Sea, in the vicinity of Smyrna, in the Caucasus (especially in Mount Ararat), and in the Elburz Mountains, including Demavend. These are, or appear, detached points of more or loss decayed, though once powerful, action, fed by inland seas. On the southern and eastern shores of Asia the subterranean energies are still unsubdued, and constitute a long chain of lofty islands and promontories from Barren Island, in the Bay of Bengal, through the length of Sumatra and Java, by Bally, Sins'aiwa (already noticed), Flores, Celebes, Sangir, Mindanao, Fugo, Lucca.. vormoaa, Loochoo, Japan, the Kurilian Isles, the magnificent mountains of Kamtehatka, examined by Erman, and the line of the Aleutian Isles, to Alaschka, on the western coast of North America.