All the principal volcanoes are situated close to the sea, and therefore the hypothesis that a communication exists between them is practically certain; their proximity to the sea, and the deposition of salt from the fumes of lava, as Sir Humphry Davy noticed, are two strong facts. But for all that, it does not prove satisfactorily that salt is solely the result of volcanic agency, and indirectly from the sea, because there is not the slightest trace of the remains of marine organisms, unless they are totally destroyed and obliterated when it is in a state of fusion; if so, it is more conclusive that salt such as we find it is solely due to volcanic force. Salt may have been in times past, as the observations of Sir Humphry Davy seem to corroborate, and as confirmed by more recent chemists, deposited by volcanic agency in the same way that salt is deposited by fumes of hydrochloric acid, which are emitted with the lava during eruptions of such volcanoes as Vesuvius and Etna, by reason of some communication with the sea. As hydrochloric acid is found in the vapours which are disengaged from red-hot lava, and as magnesia, which is not volatile, is left in the lava itself, constituting one of its most important elements, it would certainly lead one to surmise that there is a communication which, though not always in existence, may be periodically caused by the action of the volcano. Both MM. St. Claire Deville and Fouqué have succeeded in demonstrating the perfect accordance of the chemical composition of the products of volcanic eruptions, both gaseous and solid, with the doctrine that salt water has been largely present in volcanic foci. If so, why are there no salts of magnesia in volcanic fumeroles? These salts are readily decomposable by hot steam, and when water and heat are present they produce hydrochloric acid and magnesia. M. Fouqué affirmed that he witnessed an eruption of Mount Etna in 1865; the gaseous emanations agreed in kind with those which we might have looked for if large volumes of sea-water had gained access to reservoirs of subterranean lava, and if they had been decomposed and expelled with the lava.[48] We have obtained three facts, viz., that communications probably exist between volcanic foci and sea-water; that fumes of hydrochloric acid which accompany the lava deposit common salt; and that the salts of magnesia are decomposed by heat; and what more probable than that all living organisms which pass with the sea-water are utterly obliterated? By the preceding observations, the reader will see that salt is not subject to geological laws, by reason of its being confined to no particular strata, and by the absence of organic remains; and that it is not derived from sea-water, because there are no marine organisms to be found in it.