It is to the violent escape of these gases from the molten rock-masses, as the pressure upon them is relieved, that nearly all the active phenomena of volcanoes must be referred; and it was the recognition of this bet by Spallanzani, while he was watching the phenomena displayed in the crater of Stromboli, which laid the foundations of the science of Vulcanology. [Sidenote: SOURCE OF THE ABSORBED GASES.] But here another question presents itself to the investigator of the phenomena of volcanoes: it is this. At what period did the molten rock-masses issuing from vents absorb those gaseous materials which are given off so violently from their midst during eruptions? Two different answers to this question have been suggested. It may be that the original materials of which our globe was composed consisted of metallic substances in a state of fusion which had absorbed many gases, and that, in the fluid masses below the solid crust, vast quantities of vapour and gas are stored up, which are being gradually added to the atmosphere during volcanic outbursts. The fact that meteorites, which, as we have seen, in all probability closely resemble the materials forming the earth's interior, sometimes yield many times their volume of hydrogen and other gases, may be thought to lend some support to this idea. If it be the correct one, we must regard our globe as gradually parting with its pent-up stores of energy, in those absorbed gases and vapours held in bondage by the solid and fluid materials of its interior.