CAUSES OP GEOLOGICL CHANGES. 79 
ebullition of a compound liquid, consisting of water and lava, and 
somewhat analogous in its constitution therefore, to honey, paste, 
or mud and water, in which it is well known that ebullition may 
be excited. Scrope, supposes it to be evident from the appearan- 
ces presented by the lava of Vesuvius at the instant when it issues 
from the mountain's side and afterwards, that it is a substance 
so composed — that it is not actually in a state of fusion but ren- 
dered liquid by water, which serves as a vehicle for the earthy 
particles which remain after its escape. The liquidity of lava is 
said to be always imperfect, never exceeding that of honey and 
generally such as to require the exertion of a considerable force 
to thrust a stick or blunt rod into it. At the instant of its emis- 
sion it has a brilliant white heat, a considerable quantity of va- 
pour is emitted from it and it consolidates almost instantly. The 
superficial crust thus formed, cracks and splits in all directions, and 
fresh vapours escape from the crevices. Scrope states also that 
there is no evidence of the occurrence of a real combustion in the 
crater of a volcano ; and that what are commonly described as 
flames are in fact jets of red hot sand and scoriae. A further illu- 
sion is frequently produced by the brilliant light often given out 
by reflection, from the cloud, that overhangs the mountain. 
Instead of supposing that there is gradual accession of heat to 
the strata lying just within the consolidated crust of the globe, 
M. Cordier, represents them as parting with their caloric by slow 
communication to the beds of rock lying above them ; and eventu- 
ally by radiation, into the regions of space. The consequence 
of this refrigeration is, a contraction and diminution of the 
capacity of the crust or shell, in which the liquid central nucleus 
is enveloped ; by which a part of the matter of which it is com- 
posed, is forced out through a few small openings (somewhat in 
the way in which we squeeze the juice out of an orange); pro- 
ducing the phenomena of earthquakes and volcanoes. 
When two theories are proposed to us at the same time, nei- 
ther of which has our very hearty approbation, and which are 
altogether at variance with each other, in regard to the principles 
on which they are founded; it is sometimes a matter of conve- 
nience, that there are two of them, as it will be the less neces- 
sary to enter upon a minute examination and discussion of the 
merits of either. It may be enough to array them in opposition 
to each other. If, however, we be called upon to state which of 
the theories just exhibited, is the least liable to objection, and 
capable of being supported by the strongest, arguments, the pre- 
ference seems due to that of Cordier. Mr. Scrope supposes — 
the earth to have been originally a melted mass, of which the 
temperature of the exterior crust was so far depressed, that it was 
consolidated into a rock, and that this very same crust is now re- 
ceiving heat from the interior nucleus, by which it is again heated 
to whiteness. There are, perhaps, no experiments by which the 
changes here supposed, are proved to be impossible, but they 
