nt, and of considerable depth, which are
at one point in a state of fusion (as beneath some active volcano); at
another, red-hot; and at a third, comparatively cold--strong
thermo-electric action may be excited.

Some, perhaps, may object, that this is reasoning in a circle; first to
introduce electricity as one of the primary causes of volcanic heat, and
then to derive the same heat from thermo-electric currents. But there
must, in truth, be much reciprocal action between the agents now under
consideration; and it is very difficult to decide which should be
regarded as the prime mover, or to see where the train of changes, once
begun, would terminate. Whether subterranean electric currents if once
excited might sometimes possess the decomposing power of the voltaic
pile, is a question not perhaps easily answered in the present state of
science; but such a power, if developed, would at once supply us with a
never-failing source of chemical action from which volcanic heat might
be derived.

_Recapitulation._--Before entering, in the next chapter, still farther
into the inquiry, how far the phenomena of volcanoes and earthquakes
accord with the hypothesis of a continued generation of heat by chemical
action, it may be desirable to recapitulate, in a few words, the
conclusions already obtained.

1st. The primary causes of the volcano and the earthquake are, to a
great extent, the same, and must be connected with the passage of heat
from the interior to the surface.

2dly. This heat has been referred, by many, to a supposed state of
igneous fusion of the central parts of the planet when it was first
created, of which a part still remains in the interior, but is always
diminishing in intensity.

3dly. The spheroidal figure of the earth, adduced in support of this
theory, does not of necessity imply a universal and simultaneous
fluidity, in the beginning; for supposing the original figure of our
planet had been strictly spherical--which, however, is a gratuitous
assumption, resting on no established analogy--still the statical figure
must have been assumed, if sufficient time be allowed, by the gradual
operation of the centrifugal force, acting on the materials brought
successively within its action by aqueous and igneous causes.

4thly. It appears, from experiment, that the heat in mines increases
progressively with their depth; and if the ratio of increase be
continued uniformly from the surface to the interior, the