"It is amongst extinct volcanoes, the surfaces of which have been removed by the action of air and water, and in which the interior parts of strata of lavas are exposed, that the most instructive examples of the operation of slow cooling upon heated masses are to be found. It is difficult to conceive that water could have been the solvent of the different granitic and porphyritic formations; for, in that case, some combinations of water with the pure earths ought to be found in them. Quartz ought to exist in a state of _hydrate_, and Wavellite, not Corundum, ought to be the state of alumina in granite. "To suppose the primary rocks, in general, to have been produced by the slow cooling of a mass formed by the combustion of the metallic bases of the earths, appears to me the most reasonable hypothesis; yet aqueous agency must not be entirely excluded from our geological views. In many cases of crystallization, even in volcanic countries, this cause operates; thus in Ischia, siliceous _tufas_ are formed from hot springs; and in the lake Albula, or the lake of Solfaterra, near Tivoli, crystals of calcareous spar and of sulphur separate from water impregnated with carbonic acid and hepatic gas; and large strata of calcareous rocks, formed evidently in late times by water impregnated with carbonic acid, exist in various parts of Europe. The Travertine marble (_Marmor Tiburtinum_) is a production of this kind; and it is of this species of stone that the Colosseum at Rome, and the cathedral of St. Peter, are built. It is likewise employed in the ancient temple of Pæstum, and it rivals in durability, if not in beauty, the primary marble of Paris and Carrara." CHAPTER XII. Sir Humphry Davy suggests a chemical method for unrolling the ancient Papyri.--He is encouraged by the Government to proceed to Naples for that purpose.--He embarks at Dover.--His experiments on the Rhine, the Danube, the Raab, the Save, the Ironzo, the Po, and the Tiber, in order to explain the formation of mists on rivers and lakes.--His arrival and reception at Naples.--He visits the excavations at Herculaneum.--He concludes that it was overwhelmed by sand and ashes, but had never been exposed to burning matter.--He commences his attempt of unrolling the Papyri.--His failure.--He complains of the persons at the head of the department in the Museum.--He analyses the waters of the Baths of Lucca.--His return to England.--Death of Sir Joseph Banks.--He is elected President of the Royal Society.--Some remarks on that event.--He visits Penzance.--Is honoured by a public dinner.--Electro-magnetic discoveries of Oersted extended by Davy.--He examines Electrical Phenomena in vacuo.--The results of his experiments questioned.--He enquires into the state of the water, and aëriform matter in the cavities of crystals.--The interesting results of his enquiry confirm the views of the Plutonists.