t, so what has been
called the mechanical theory, in a variety of shapes, took its place.

This, in whatever form, takes its lava and other heated products of the
volcano ready made from a universal ocean of liquid material, which it
supposes constitutes the interior or nucleus of our globe, and which is
only skinned over by a thin, solid crust of cooled and consolidated
rock, which was variably estimated at from fourteen to perhaps fifty
miles in thickness. Here was a boundless supply of more than heat, of
hot lava ready made, the existence of which at these moderate depths the
then state of knowledge of hypogeal temperature, which was supposed to
go on increasing with depth at the rate of about 1 deg. Fahrenheit, for
every thirty or forty feet, seemed quite to sustain.

The difficulty remained, how was this fiery ocean brought to the
surface or far above it? To account for this two main notions prevailed,
and, indeed, have not ceased to prevail. Some unknown elastic gases or
vapour forced it up through fissures or rents pre-existent, or produced
by the tension of the elastic and liquid pressure below.

The form in which this view took most consistency, and approaching most
nearly to truth, finds the elastic vapour in steam generated from water
passed down through fissures from the sea or from the land surface. But
to this the difficulty was started, that fissures that could let down
water would pass up steam. The objection, when all the conditions are
adequately considered, has really no weight; and it has been completely
disposed of, since within a few years it has been proved that capillary
infiltration goes on in all porous rocks to enormous depths, and that
the capillary passages in such media, though giving free vent to
water--and the more as the water is warmer--are, when once filled with
liquid, proof against the return through them of gases or vapours. So
that the deeply seated walls of the ducts leading to the crater, if of
such material, may be red hot and yet continue to pass water from every
pore (like the walls of a well in chalk), which is flushed off into
steam that cannot return by the way the water came down, and must reach
the surface again, if at all, by the duct and crater, overcoming in its
way whatever obstructions they may be filled with.

And this remarkable property of capillarity sufficiently shows how the
lava--fused below or even at or above the level of infiltration--may
become interpen