s lava. To this source
alone he refers the production of the steam by which the melted matter
is propelled upwards, even to the summit of cones three miles in
height.[762]

When treating of springs and overflowing wells, I have stated that
porous rocks are percolated by fresh water to great depths, and that
sea-water probably penetrates in the same manner through the rocks which
form the bed of the ocean. But, besides this universal circulation in
regions not far from the surface, it must be supposed that, wherever
earthquakes prevail, much larger bodies of water will be forced by the
pressure of the ocean into fissures at great depths, or swallowed up in
chasms; in the same manner as on the land, towns, houses, cattle, and
trees are sometimes engulfed. It will be remembered, that these chasms
often close again after houses have fallen into them; and for the same
reason, when water has penetrated to a mass of melted lava, the steam
into which it is converted may often rush out at a different aperture
from that by which the water entered.

The gases, it is said, exhaled from volcanoes, together with steam, are
such as would result from the decomposition of salt water, and the fumes
which escape from the Vesuvian lava have been observed to deposit common
salt.[763] The emission of free muriatic acid gas in great quantities is
also thought by many to favor the theory of the decomposition of the
salt contained in sea-water. It has been objected, however, that M.
Boussingault did not meet with this gas in his examination of the
elastic fluids evolved from the volcanoes of equatorial America; which
only give out aqueous vapor (in very large quantity), carbonic acid gas,
sulphurous acid gas, and sometimes fumes of sulphur.[764] In reply, Dr.
Daubeny has remarked, that muriatic acid may have ceased to be
disengaged, because the volcanic action has become languid in equatorial
America, and sea-water may no longer obtain admission.

M. Gay Lussac, while he avows his opinion that the decomposition of
water contributes largely to volcanic action, called attention,
nevertheless, to the supposed fact, that hydrogen had not been detected
in a separate form among the gaseous products of volcanoes; nor can it,
he says, be present; for, in that case, it would be inflamed in the air
by the red-hot stones thrown out during an eruption. Dr. Davy, in his
account of Graham Island, says, "I watched when the lightning was most
vivid, and the