80 CHANGES PRODUCED BY VOLCANOES AND EARTHQUAKES. 
seem hardly necessary. It may be regarded almost as a self- 
evident truth, that a hot body abandoned to itself, will part with 
its heat in such a way, that the temperature shall go on diminish- 
ing, according to some regular law, from the exterior surface 
towards the centre, and that every individual particle, wherever 
it may be situated, will grow constantly colder. If this be a cor- 
rect statement of the changes that would take place, the theory 
of Scrope is unquestionably erroneous. 
The subject of volcanoes is one, in regard to which future ages are 
destined to have more accurate knowledge, and more enlightened 
views than we possess at present. The circumstances which at- 
tend an eruption, and the order in which the phenomena succeed 
each other, are calculated to produce the belief, that volcanic ac- 
tion is the result of chemical changes of some kind taking place 
within the crust of the earth ; different masses of the substances 
that act upon each other, being from time to time brought into 
contact, and the combustion in this way renewed and kept up 
from age to age, but the matter thrown out is not, as we have 
already seen, of a nature to warrant our adhering very obsti- 
nately to this opinion. If this view of the subject shall be 
deemed inadmissible, and the hypothesis of a central fire em- 
braced, we shall perhaps find no better refuge from the harass- 
ing inquietudes of doubt and scepticism, than in the doctrines of 
Cordier. Some geologists are inclined to combine the two, and 
to add to the agencies supposed by them, that of water intro- 
duced upon the interior heated mass. 
OF THE CHANGES PRODUCED BY VOLCANOES AND 
EARTHQUAKES. 
4 "2. — 1. The permanent visible effects produced by volcanoes, 
consist in an elevation of the surface of the earth around their 
bases, which is covered with a bed of volcanic ashes and socriae, 
or with a sheet of lava; and a change in the altitude or form of 
the mountain itself, which is their seat. 
The ashes that were thrown out from the mountain Tomboro, 
on the island of Sumbawa, in the East Indies, in April, 1S15, 
were so abundant as to crush the roofs of houses on which they 
fell, at the distance of forty miles, and westward of Sumatra, the 
floating mass was two feet in thickness and several miles in ex- 
tent. The stream of lava that issued from Skaptar Jokul, in 
Iceland, in 1783, was ninety miles in length, at some points from 
twelve to fifteen miles in breadth, and one hundred and in nar- 
row defiles, six hundred feet in depth. Vesuvius was re- 
duced in height about eight hundred feet, by the eruption of 
1822. iEtna has experienced a similar, though not as great a re- 
duction of its elevation, and been afterwards built up again. The 
plain of Malpais, in the western part of Mexico, was'converteil 
into a volcano (Jorullo) 1600 feet in height, on the night of 
