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elastic vapours. The enormous expansive force of the gases is attested by 
the violence of the eruptions, and by the height to which lava is raised. 
Explosions require two conditions— the force to be produced instantaneously 
—and the mass moved to be small. Perhaps steam accumulates in the upper 
part of a cavity, in quantity and tension, until the surface of the molten lava 
is depressed below the lower end of the volcanic vent, and then the steam 
suddenly forces up the lava and any solid masses included in the lava. 
Possibly explosive action may be connected with some modification in the 
process of generating elastic vapour. 
In treating on geological phenomena we must refer them to natural causes 
acting under conditions, the former existence of which may be deemed 
admissible. There are two hypotheses of volcanoes — one assumes that the 
earth has been in a state of fluidity, and this does not preclude the supposition 
of a prior gaseous state — the other assumes an original, solid, unoxidized 
nucleus. This the author thinks is less simple than the first hypothesis. 
The latter, or chemical theory, was proposed by Davy, and opposed by 
Gay Lnssac. It assumed the admission of water and air to the volcanic mass. 
Gay Lussac objected that if fissures were open the lava would till them and 
prevent the passage of water. Hopkins concurs in this view, and thinks 
that the suggested process involves a mechanical impossibility. The late 
eminent Professor of Chemistry at Oxford, Dr. Daubeney, maintained this 
theory during his whole life, and in his work on volcanoes based it chiefly 
on the products evolved, both mineral and gaseous. 
Bischoff supposed that the earth's crust does not exceed twenty or thirty 
miles, and that water enters through a passage to the fluid mass, and forces, 
by its conversion into steam, the lava to rise in another channel. Hopkins 
makes the same objection to this theory. 
The remaining theory assumes the original fluidity of the globe, and 
includes the elevation of parts of the crust. 
This leads the author to treat of the form, solidification, and thickness of 
the crust of the globe. The form of the earth would become in either case 
an oblate spheroid, and the density would increase from the surface towards 
the centre. But as the law of increase is not known, the ellipticity cannot 
be calculated. Nevertheless, the "motions of precession and nutation of 
the pole of the earth and a corresponding small inequality in the motion of 
the moon" are consistent with the spheroidal form, if the earth be assumed 
to be solid or to have a crust solid to the depth of one fourth or one fifth of 
the radius. If the earth had been originally a solid sphere and then rotated , 
the centrifugal force would have produced ellipticity, but not so great as 
would have resulted on the supposition of original fluidity. And that 
