PHENOMENA OP 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 suppHed by the doctrine of a change 
in interior heat; abundant admission for water is 
afforded by ihQ 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 
