But let us make a study of some of the peculiarities of Vulcano, another of the Lipari islands, which lies north of Sicily. Vulcano affords a curious example of a volcano that has been harnessed by man, or made to do work for him. All volcanoes bring from inside of the earth different kinds of chemical substances, in the form of vapors, gases, or molten materials. Now, these materials acting on one another, produce chemical substances some of which, such as sal ammoniac, sulphur, and boracic acid, possess commercial value. This is especially true in the case of Vulcano, and since the eruptions are not generally violent, a chemical works has actually been erected by a Scotch firm on the side of the mountain, where the materials are collected from the crevices. This effort to harness a volcano was for a time so successful that the same people contemplated the building of great leaden chambers over the principal fissure at the bottom of the crater, so that the large volumes of ejected vapors might be condensed and collected. But Vulcano, like all other volcanoes, could not be relied on continually to keep the peace. One day it suddenly burst forth more fiercely than usual, so that the workmen were compelled to abandon the factory and fly down the mountain for their lives, but not, however, before some of them were severely injured by the explosions. Vulcano is an instance of a volcano in an almost exhausted or dormant condition. It has had, however, many eruptions during the past few centuries, some of which have been very violent, for example, that of 1783, and that of 1786. There still remains to be considered the volcanic region of the Santorin group of the Grecian Archipelago. The island of Santorin or Thera, is the southernmost of the Cyclades. It is an exceedingly curious island, being a submerged volcano, with most of the top of the crater remaining above the waters, so that the entire island has the shape of an irregular circle or crescent broken at several points. Its formation is, probably, due to the gradual sinking of a volcanic mountain until its crater has been almost completely submerged, only the higher parts of the edges of the crater being left above the surface of the waters. Suppose, for example, a mountain like Vesuvius at the time the crater Somma existed, was sunk below the level of the Mediterranean until only the highest parts of the crater remained above the waters. If, now, one or more volcanic eruptions occurred, producing craters or volcanic islands inside the submerged rim, you would have a condition of affairs seen in the island of Santorin. CHAPTER VII ORIZABA, POPOCATEPETL, IXTACCIHUATL, AND OTHER VOLCANOES OF MEXICO