No lava has flowed over Pompeii since that city was buried, but with Herculaneum the case is different. Although the substance which fills the interior of the buildings in that doomed city must have been introduced in a state of mud like that found in similar situations in Pompeii, yet the superincumbent mass differs wholly in composition and thickness.
Herculaneum was situated several miles nearer to the volcano, and has, therefore, been always more liable to be covered, not only by showers of ashes, but by alluvium and streams of lava. Accordingly, masses of both have accumulated on each other above the ancient site of the city, to a depth of nowhere less than 70, and in many places of 112 feet; while the depth of the bed of ashes under which Pompeii lies buried seldom exceeds 12 or 14 feet above the houses, and it is even said that the higher part of the amphitheatre always projected above the surface.
Yet, strange to say, Herculaneum, though far more profoundly hidden, was discovered before Pompeii, by the accidental circumstance that a well sunk in 1713 came right down upon the theatre where the statues of Hercules and Cleopatra were found. Many others were afterwards dug out and sent to France by the Prince of Elbeuf, who, having married a Neapolitan princess, became proprietor of the field under which the theatre lies buried. Further excavations were, however, forbidden by Government, and only resumed in 1736. But the difficulty of removing the large masses of lava accumulated above the city, and the circumstance of its partly lying under the modern towns of Portici and Resina have confined the exploration of Herculaneum within narrow limits. The large theatre alone is open for inspection, and can be seen only by torchlight, so that its dark galleries, cut through the tuff, are but seldom visited by strangers; while no traveller leaves Naples without having wandered through the ruins of Pompeii, for Italy hardly affords a more interesting sight than that of these streets and forums, these theatres and temples, these houses and villas, which require but the presence of their ancient inhabitants to complete the picture of a Roman town, such as it was eighteen hundred years ago.
CHAPTER VIII.
GAS SPRINGS AND MUD VOLCANOES.
Carbonic-acid Springs—Grotto del Cane—The Valley of Death in Java—Exaggerated Descriptions—Carburetted Hydrogen Springs—The Holy Fires of Baku—Description of the Temple—Mud Volcanoes—The Macaluba in Sicily—Crimean Mud Volcanoes—Volcanic Origin of Mud Volcanoes.
The numerous gas springs which in many countries are evolved from an unknown depth, afford us a convincing proof that the remarkable chemical transformations of which we find so many traces in the past history of our planet are still perpetually taking place in many of the mysterious crevices and hollows of the earth-rind. In Auvergne, the Vivarrais, the Eifel, and along the whole basaltic range from the Rhine to the Riesengebirge in Silesia, carbonic acid gas is exhaled in incredible quantities from the vast laboratories of the subterranean world.
Professor Bischoff found that a single gas spring near Burgbrohl daily produced 5,650 cubic feet of carbonic acid, a quantity amounting in the course of a year to no less than 262,000 pounds in weight; and, according to Bromeis, the great Artesian spring at Nauheim evolves every minute 71 cubic feet of carbonic acid, equal to a weight of 5,000,000 pounds annually. If from these two instances we judge of the produce of the many carbonic acid gas springs of Germany, and if we farther extend our view to the rest of the world, in many parts of which carbonic acid probably escapes in still greater quantities, we can form some idea of the geological importance of these springs, which also exercise no small influence upon the organic world. For the incalculable masses of carbonic acid which are thus constantly pouring from subterranean vents into the atmospheric ocean are again absorbed by millions of plants. They feed the forests and the fields; and thus these chemical changes, which are incessantly but imperceptibly modifying the earth-rind, ultimately tend to the advantage of man.
As a light dipped in carbonic acid gas is immediately extinguished, and every animal inhaling it is liable to instant suffocation, these properties are sometimes made use of for cruel experiments, for which, among others, the insignificant Grotto del Cane, in the kingdom of Naples—a cave or hole in the side of a mountain near the Lake Agnano—has become notorious. Some miserable dogs are thrust into the stratum of fixed air which covers the bottom of the hole, and are alternately almost choked and resuscitated to satisfy the idle curiosity of tourists. Their violent efforts to escape, when about to be plunged into the poisonous vapour, prove the horrible cruelty of the practice.
The carbonic-acid springs in the glen of the Brohl, a small rivulet flowing into the Rhine, near Andernach, are turned to a better purpose, for the manufacture of white lead.
The famous ‘Valley of Death,’ or Poison Valley, in the Island of Java, is nothing more than a funnel-shaped hollow, measuring about 100 feet in diameter at the top, and with a bare space in its centre fifteen feet broad and long, which is frequently covered with a stratum of carbonic acid gas. The sides of the hollow, and even the bottom, with the exception of the above-mentioned naked spot, are everywhere clothed with shrubbery, or even with forest trees.