GASES GIVEN OFF DURING ERUPTIONS. 191 



renders the position of the volcanoes readily intelligible. Other 

 agencies, such as the accumulation of sediments, have sometimes 

 been supposed to develop additional pressure, and assist in augment- 

 ing the volcanic fires along coasts ; but on the view proposed, 

 \ve seem compelled to adopt the conclusion so long since enunciated 

 by Krug von Nidda, and regard volcanoes in their normal condition 

 as intermittent springs which throw out melted matters. 



Relation of Volcanoes to Springs. There is, in fact, a perfect 

 sequence to be traced from the volcano which pours out molten rock 

 to that in which the water supply has become so great, relatively to 

 the rock-matter, that the materials ejected cease to be molten ; and 

 as the water preponderates they may become more and more invisible, 

 till the condition of a hot spring is reached. Professor Prestwich 

 has drawn attention to the fact that an artesian well at Naples, after 

 passing through 735 feet of volcanic beds, and 787 feet of more 

 or less water-bearing strata, furnished a spring which rose 8 feet 

 above the surface, or 81 feet above the sea-level. He urges that the 

 pressure of the water in the rocks which rise above the sea-level 

 keeps the sea-water out frojn the land. But when the water which 

 is stored in the mountain and neighbouring rocks "becomes expelled 

 and exhausted under the conditions of an eruption, then the pressure 

 is removed on the landward side, and an inflow of salt water from 

 the sea necessarily takes place, and modifies the explosive form of 

 the outburst. 



Decline of Volcanic Activity. After the solid materials cease to 

 be ejected, and before the eruptive throat of a volcano is hermetically 

 sealed, the existence of various gases may be detected, and the de- 

 position of salts observed. Some of the gases appear to be given off 

 all through an eruption, others chiefly at its close. Among the most 

 frequent acids are sulphuric and hydrochloric. The gases comprise 

 nitrogen, hydrogen, and carbonic dioxide. When we seek for the 

 causes of the gaseous eruptions, we shall find some gases to be derived 

 from water, and others from the rocks beneath the surface. It is 

 well known that a considerable amount of atmospheric air is dissolved 

 in water, and that in sea- water there is some amount of carbonic acid 

 gas; it is also known that at a moderate temperature these gases 

 become expelled from water, and at a higher temperature the water 

 itself is decomposed into oxygen and hydrogen. The hydrogen 

 is chiefly found as sulphuretted hydrogen, and in combination with 

 chlorine, in the form of hydrochloric acid; the nitrogen is more 

 common in the form of sal ammoniac than in the free state. The 

 decomposition of the sulphuretted hydrogen has produced frequent 

 deposits of sulphur, which sometimes cap the mountain. The 

 frequent presence of salt, of hydrochloric acid, and the large per- 

 centage of soda in Vesuvian lavas, leave little doubt that sodium 

 chloride in some way obtains access to the heated regions, and in 

 some cases is decomposed. The most abundant gas is carbonic 

 dioxide, and this would appear to be always due to the action 

 of heated matter upon limestones beneath the surface, so that the 



