PHENOMENA OF VOLCANOES. 



189 



rine, will form hydrochloric acid, which is known 

 to be often present in the vaporous exhalations of 

 volcanoes. He supposes that the heat generated 

 by the primary chemical action (oxygenation) and 

 the energetic action of steam, to which part of the 

 water is converted, are sufficient to account for the 

 mechanical phenomena of volcanoes. 



It is unnecessary to state the numerous objec- 

 tions which have been raised to this theory, as it 

 was abandoned by its projector, Davy, as he be- 

 came better acquainted with the nature of these 

 substances, to whose agency he had once attributed 

 volcanic phenomena. It, however, still has its ad- 

 vocates ; and Philips remarks, " it appears to us to 

 be clear, that the union of the two speculations here 

 brought into comparison is not only practicable, 

 but reasonable, and even necessary. A general 

 cause of change of form of the earth's surface and 

 interior parts is supplied by the doctrine of a change 

 in interior heat ; abundant admission for water is 

 afforded by the fractures necessary (upon this view) 

 to adjust the balance of pressures ; and the chemi- 

 cal products can only be properly understood by a 

 suitable hypothesis of chemical action."* 



Volcanic eruptions are usually preceded by de- 

 tonations in the mountain and agitations of the 

 earth, or earthquakes in the vicinity ; after which 

 the mountain vomits forth an abundance of ashes, 

 cinders, and stones, and streams of melted lava 

 flow from apertures made in the side of the cone, the 

 resistance of which becomes unequal to the pres- 

 sure of the melted mass within. The lava very 

 rarely seems to proceed from the lip of the crater. 

 Lava, when observed as near as possible to the 

 point from whence it issues, is, for the most part, 

 a semi-fluid mass of the consistence of honey, but 

 sometimes so liquid as to penetrate the fibre of 



* Treatise of Geology, vol. ii., p. 214. 



