78 THE CAUSES OF THE PHENOMENA OF VOLCANOES, &C. 
when brought into contact with it. Some geologists appear still 
to believe that volcanic action is produced by the combustion of 
some compound of sulphur and silicon, or of the bases of the alkalis 
and earths. The circumstance that almost all the known active 
volcanoes are situated near the shores of the sea is supposed to 
favour the opinion, that water derived from that great reservoir is 
the agent that determines their activitiy. A part of it is said to 
be decomposed, its oxygen entering into combination with the 
metallic base or silicon, whilst the hydrogen uniting with the sul- 
phur passes off under the form of sulphuretted hydrogen. The 
rest of the water being introduced upon substances already in a 
state of intense ignition, is said to be converted into vapour and 
under that form, and at a very elevated temperature to exert the 
immense explosive force by which fragments of rocks are thrown 
to a vast height into the air. 
Two new theories of volcanic action have been proposed with- 
in a tew years, one by Mr. Poulett Scrope, secretary of the Lon- 
don Geological Society, and the other by M. Cordier, of the Paris 
Academy of Sciences. They both start with the assumption that 
the earth was originally a melted mass, of which the exterior 
crust has parted with its caloric by radiation, and been thus con- 
verted into a rock, whilst the great central portion retains its tem- 
perature and is now in a liquid state. The merits of this hypoth- 
esis we will presently consider. It is a circumstance somewhat 
remarkable, that of these two theories, one attributes the phenom- 
ena of volcanoes to the gradual heating, and the other to the grad- 
ual cooling of that crust of the earth which separates the interior 
liquid mass from the exterior portion, which has already by part- 
ing with its heat been converted into a bed of rock. 
Scrope, supposes that the temperature of the central nucleus is 
gradually propagated to the strata adjacent to it, which consist 
of the materials of lava holding a quantity of water in a state of 
intimate combination, to which their fluidity is owing. By re- 
ceiving an accession of heat from below, the elasticity of the wa- 
ter is so much increased that it separates from the particles of fluid 
rock or lava, and escapes through the solid strata lying above, 
overturning them, rending them to pieces and throwing them in- 
to the air and thus producing the most terrible of the phenomena 
of volcanoes and earthquakes. At the same time a quantity of the 
lava urged onward by the elastic vapour that is struggling to es- 
cape is forced out of the aperture already formed. The separa- 
tion of the steam and flow of the lava continue, until the tempera- 
ture of the focus of activity is so far diminished by the absorption 
of caloric during the conversion of the water into vapour, and the 
liquidity of the lava by parting with the water, on the presence 
of which that liquidity depends, that the causes tending to main- 
tain the activity of the volcanic action, and those by which it is 
repressed, are in equilibrium, when the eruption ceases. In his 
view therefore a volcanic eruption is produced simply by the 
