There is one other consideration which has frequently been urged as worthy of especial attention, in dealing with the question of the exciting causes of volcanic outbursts. If volcanoes were, as was at one time almost universally supposed, in direct communication with a great central mass of liquefied materials, or even if any large reservoirs of such liquids existed beneath volcanic districts, as others have imagined, then the different mobility of the solid and liquid portions of the earth's mass would give rise to tidal effects similar to those occurring in the surface waters of the globe. Under such circumstances, volcanic outbursts, like the tides, would be determined by the relative positions of the sun and moon to our globe. It is certain, however, that no very direct relation has yet been established between the lunar periods and those of volcanic outbursts, though recent close observations upon the crater of Vesuvius, by Professor Palmieri, do seem to lend support to the view that such relations may exist.

At the present time, therefore, it must be admitted that vulcanologists have only just commenced those series of exact and continuous observations which are necessary to determine the conditions that regulate the appearance of volcanic phenomena. The study of the laws of volcanic action is yet in its infancy. But the establishment of observatories on Vesuvius and Etna 18 fall of promise for the future, and when we consider the advances which have been made, during the last one hundred years, in our knowledge of the true nature of volcanic action, we need not despair that the extension of the same methods of inquiry will lead to equally important results concerning the conditions which determine and the laws which govern it.

In the meanwhile, it is no small gain to have established the fact that volcanic phenomena, divested of all those wonderful attributes with which superstition and the love of the marvellous have surrounded them, are operations of nature obeying definite laws, which laws we may hope by careful observation and accurate reasoning to determine; and that the varied appearances, presented alike in the grandest and feeblest outbursts, can all be referred to one simple cause—namely, the escape, from the midst of masses of molten materials, of imprisoned steam or water-gas.