It is well known to chemists, that the metallization of oxides, the most
difficult to reduce, may be effected by hydrogen brought into contact
with them at a red heat; and it is more than probable that the
production of potassium itself, in the common gun-barrel process, is due
to the power of nascent hydrogen derived from the water which the
hydrated oxide contains. According to the recent experiments, also, of
Faraday, it would appear that every case of metallic reduction by
voltaic agency, from saline solutions, in which water is present, is due
to the secondary action of hydrogen upon the oxide; both of these being
determined to the negative pole and then reacting upon one another.

It is admitted that intense heat would be produced by the occasional
contact of water with the metallic bases; and it is certain that, during
the process of saturation, vast volumes of hydrogen must be evolved. The
hydrogen, thus generated, might permeate the crust of the earth in
different directions, and become stored up for ages in fissures and
caverns, sometimes in a liquid form, under the necessary pressure.
Whenever, at any subsequent period, in consequence of the changes
effected by earthquakes in the shell of the earth, this gas happened to
come in contact with metallic oxides at a high temperature, the
reduction of these oxides might be the result.

No theory seems at first more startling than that which represents water
as affording an inexhaustible supply of fuel to the volcanic fires; yet
is it by no means visionary. It is a fact that must not be overlooked,
that while a great number of volcanoes are entirely submarine, the
remainder occur for the most part in islands or maritime tracts. There
are a few exceptions; but some of these, observes Dr. Daubeny, are near
inland salt lakes, as in Central Tartary; while others form part of a
train of volcanoes, the extremities of which are near the sea.

Sir H. Davy suggested that, when the sea is distant, as in the case of
some of the South American volcanoes, they may still be supplied with
water from subterranean lakes; since, according to Humboldt, large
quantities of fish are often thrown out during eruptions.[761] Mr. Dana
also, in his valuable and original observations on the volcanoes of the
Sandwich Islands, reminds us of the prodigious volume of atmospheric
water which must be absorbed into the interior of such large and lofty
domes, composed as they are entirely of porou