b c d_.
If throughout that space we imagine electro-chemical causes to be
continually in operation, even of very feeble power, they might give
rise to heat which, if accumulated at certain points, might melt or
render red-hot entire mountains, or sustain the temperature of stufas
and hot springs for ages.

_Theory of an unoxidated metallic nucleus._--When Sir H. Davy first
discovered the metallic basis of the earths and alkalies, he threw out
the idea that those metals might abound in an unoxidized state in the
subterranean regions to which water must occasionally penetrate.
Whenever this happened, gaseous matter would be set free, the metals
would combine with the oxygen of the water, and sufficient heat might be
evolved to melt the surrounding rocks. This hypothesis, although
afterwards abandoned by its author, was at first very favorably received
both by the chemist and the geologist: for silica, alumina, lime, soda,
and oxide of iron,--substances of which lavas are principally
composed,--would all result from the contact of the inflammable metals
alluded to with water. But whence this abundant store of unsaturated
metals in the interior? It was assumed that, in the beginning of things,
the nucleus of the earth was mainly composed of inflammable metals, and
that oxidation went on with intense energy at first; till at length,
when a superficial crust of oxides had been formed, the chemical action
became more and more languid.

[Illustration: Fig. 93.

Centre of the earth.]

This speculation, like all others respecting the primitive state of the
earth's nucleus, rests unavoidably on arbitrary assumptions. But we may
fairly inquire whether any existing causes may have the power of
deoxidating the earthy and alkaline compounds formed from time to time
by the action of water upon the metallic bases. If so, and if the
original crust or nucleus of the planet contained distributed through it
here and there some partial stores of potassium, sodium, and other
metallic bases, these might be oxidated and again deoxidated, so as to
sustain for ages a permanent chemical action. Yet even then we should be
unable to explain why such a continuous circle of operations, after
having been kept up for thousands of years in one district, should
entirely cease, and why another region, which had enjoyed a respite from
volcanic action for one or many geological periods, should become a
theatre for the development of subterranean heat.