ing essentially from those with which we are
acquainted.

All the solid, fluid, and gaseous bodies known to us consist of a very
small number of these elementary substances variously combined: the
total number of elements at present known is less than sixty; and not
half of these enter into the composition of the more abundant inorganic
productions. Some portions of such compounds are daily undergoing
decomposition, and their constituent parts being set free are passing
into new combinations. These processes are by no means confined to
minerals at the earth's surface, and are very often accompanied by the
evolution of heat, which is intense in proportion to the rapidity of the
combinations. At the same time there is a development of electricity.

The spontaneous combustion of beds of bituminous shale, and of refuse
coal thrown out of mines, is generally due to the decomposition of
pyrites; and it is the contact of air and water which brings about the
change. Heat results from the oxidation of the sulphur and iron, though
on what principle heat is generated, when two or more bodies having a
strong affinity for each other unite suddenly, is wholly unexplained.

_Electricity a source of volcanic heat._--It has already been stated,
that chemical changes develop electricity; which, in its turn, becomes a
powerful disturbing cause. As a chemical agent, says Davy, its silent
and slow operation in the economy of nature is much more important than
its grand and impressive operation in lightning and thunder. It may be
considered, not only as directly producing an infinite variety of
changes, but as influencing almost all which take place; it would seem,
indeed, that chemical attraction itself is only a peculiar form of the
exhibition of electrical attraction.[755]

Now that it has been demonstrated that magnetism and electricity are
always associated, and are perhaps only different conditions of the same
power, the phenomena of terrestrial magnetism have become of no ordinary
interest to the geologist. Soon after the first great discoveries of
Oersted in electro-magnetism, Ampere suggested that all the phenomena of
the magnetic needle might be explained by supposing currents of
electricity to circulate constantly in the shell of the globe in
directions parallel to the magnetic equator. This theory has acquired
additional consistency the farther we have advanced in science; and
according to the experiments of Mr. Fox, on the el