The gaseous products of volcanoes are important in the investigation of the chemisesl theory of the igneous action. Besides the clouds of vapour of water (so abundant in eruptions, and so often productive of local rains), chlorine, wrote, sulphuretted hydrogen, sulphurous acid, — — — and carbonic acid, are the most common. The evolution of sulphu• retted hydrogen (depositing sulphur) continues under various circum stances after other signs of activity have ceased In particular volcanic regime; and even after the craters have fallen in and become full of water, mineral springs and springs rich in carbonic acid flow with little variation for centuries (many such have been flowing from before the commencement of history to the present day), while azotised waters, riding to the surface along the lines of fissures more ancient than any known volcanic systems, concur with them io demonstrating the almost interminably slow process by which subterranean heat rises to the surface of the earth.
C74emical Hypothesis of Volcanic Action.—The nature of these various products, and the order in which they successively make their appear ance, have been the bases for speculations as to the chemical processes going on in the interior of volcanic regions. Sir H. Davy's discovery of the metallic bases of the earths and alkalies, and of the extraordinary appetcncy for oxygen of several of these bases (potassium, sodium,&e.), suggested to that great chemical philosopher a new and ingenious hypothesis of volcanic action. Water admitted to some of the metallic bodies alluded to is instantly decomposed, and its oxygen absorbed, with an immediate and very remarkable evolution of heat and light, while the metals become earths or alkalies. The substances most abun dant in volcanic products contain these earths, and these alkalies,— namely, potash, soda, lime, silica, alumina, she.,—in various couibina tione, evidently the result of successive crystallisation; from a fluid mass. In this hypothesis it is aasumed that the interior portions of the earth consist in part of tho metallic bases of the earths and alkalies; that water is from time to time admitted to these; that violent com bustion and great heat follow ; that the oxides generated are melted together, constituting lava, while the hydrogen, and some of the water undecomposed, go off to form new combinations with sulphur, ciao. rine, carbonic acid, &c., which are liberated from previous states by the heat and the various chemical agencies set in activity. The power which raises the lava, and throws out the clouds of ashes and scorhe, is the undecomposed and confined steam.
Whoever looks carefully at this hypothesis will find in it much that is admirable, and little that is open to strong objection, if it be regarded merely as a theory of the eruption of volcanoes, not as a theory of the changes in the condition of the interior parts of the globe, of which volcanic action is one of the visible exponents.
It is some recommendation of this view that it seems to unite itself with a general and not improbable speculation regarding the origin of the more ancieut Plutonic rocks, which certainly must be supposed to have passed through a very similar series of changes to those which lava has undergone. Those rocks have the same bases as lava ; it is the natural result of chemical reasoning, that the elements which are now combined in them existed at sonic earlier time in a separate state ; the oxidated and melted granite cruet of the earth is formed by the union of these elements, and, according to the hypothesis of Davy,* the new rocks which volcanoes yield are produced by a somewhat similar process of oxidation and fusion.
But this hypothesis was nevertheless neglected by its author for reasons which do not appear to have been fully stated by himself. It was taken up by Dr. Daubeny, and has been maintained by him with much perseverance and ingenuity of research as a sufficient Chemical Theory of Volcanoes.' We may call it the' Hypothesis of Subterranean Oxidation,' and develop it, according to Dr. Daubcny, as follows ; Below the surface, at a depth of a few miles, the interior of the earth is assumed to contain the earthy and alkaline metalloids, iron and other metals, sulphur and sulphuretted salts. Slow combustion happening amongst them, even under the continents, by slight additions of moisture and air, generates particular gases (nitrogen, carbonic acid, sulphuretted hydrogen, &o.); these rise and combine with springs which issue along lines of natural fissures, or are discovered in artificial wells, often giving to them a temperature higher than that of the country where they occur. Under the sea or large bodies of water, and especially along lines of sea.coaat (where fissures may be supposed more numerous than elsewhero), water may be admitted to the interior more easily and in greater quantity, and may occasion phenomena of the same order, accompanied by other effects more powerful, rapid, and characteristic, until the process ceases for awhile by the choking of the passages which admitted the water.