Such are the general features of action in the vast powers which dwell deep beneath the surface, harmless in most parts of the earth, frightfully perilous in others. Yet even here they often rest for long terms of years in seeming apathy, until men gather above their lurking places in multitudes, heedless or ignorant of the sleeping demons that bide their time below. Their time is sure to come, after years, perhaps after centuries. Suddenly the solid earth begins to tremble and quake; roars as of one of the buried giants of old strike all men with dread; then, with a fierce convulsion, a mountain is rent in twain and vast torrents of steam, burning rock, and blinding dust are hurled far upward into the air, to fall again and bury cities, perhaps, with all their inhabitants in indiscriminate ruin and death.
Theories of Volcanic and Earthquake Action.
Though the first formation of a volcano (Italian, vulcano, from Vulcan, the Roman god of fire) has seldom been witnessed, it would seem that it is marked by earthquake movements followed by the opening of a rent or fissure; but with no such tilting up of the rocks as was once supposed to take place. From this fissure large volumes of steam issue, accompanied by hydrogen, nitrogen, carbon dioxide, hydrochloric acid, and sulphur dioxide. The hydrogen, apparently derived from the dissociation of water at a high temperature, flashes explosively into union with atmospheric oxygen, and, having exerted its explosive force, the steam condenses into cloud, heavy masses of which overhang the volcano, pouring down copious rains. This naturally disturbs the electrical condition of the atmosphere, so that thunder and lightning are frequent accompaniments of an eruption. The hydrochloric acid probably points to the agency of sea-water. Besides the gases just mentioned, sulphuretted hydrogen, ammonia and common salt occur; but mainly as secondary products, formed by the union of the vapors issuing from the volcano, and commonly found also in the vapors rising from cooling lava streams or dormant volcanic districts. It is important to notice that the vapors issue from the volcano spasmodically, explosions succeeding each other with great rapidity and noise.
All substances thrown out by the volcano, whether gaseous, liquid or solid, are conveniently united under the term ejectamenta (Latin, things thrown out), and all of them are in an intensely heated, if not an incandescent state. Most of the gases are incombustible, but the hydrogen and those containing sulphur burn with a true flame, perhaps rendered more visible by the presence of solid particles. Much of the so-called flame, however, in popular descriptions of eruptions is an error of observation due to the red-hot solid particles and the reflection of the glowing orifice on the over-hanging clouds.