by Faye, and,
although it has met with opposition, appears to be that which accords
best with our present knowledge of the chemical and physical
conditions of matter, such as we must suppose it to exist in the
condensing gaseous mass which, according to the nebular hypothesis,
should form the centre of our solar system. Taking this, as we have
already done, for granted, it matters little whether we imagine the
different planets to have been successively detached as rings during
the rotation of the primal mass, as is generally conceived, or whether
we admit with Chacornac a process of aggregation or concretion,
operating within the primal nebular mass, resulting in the production
of sun and planets. In either case we come to the conclusion that our
earth must at one time have been in an intensely heated gaseous
condition, such as the sun now presents, self-luminous, and with a
process of condensation going on at first at the surface only, until
by cooling it must have reached the point where the gaseous centre
was exchanged for one of combined and liquefied matter.

"Here commences the chemistry of the earth, to the discussion of which
the foregoing considerations have been only preliminary. So long as
the gaseous condition of the earth lasted, we may suppose the whole
mass to have been homogeneous; but when the temperature became so
reduced that the existence of chemical compounds at the centre became
possible, those which were most stable at the elevated temperature
then prevailing would be first formed. Thus, for example, while
compounds of oxygen with mercury or even with hydrogen could not
exist, oxides of silicon, aluminium, calcium, magnesium, and iron
might be formed and condense in a liquid form at the centre of the
globe. By progressive cooling, still other elements would be removed
from the gaseous mass, which would form the atmosphere of the
non-gaseous nucleus. We may suppose an arrangement of the condensed
matters at the centre according to their respective specific
gravities, and thus the fact that the density of the earth as a whole
is about twice the mean density of the matters which form its solid
surface may be explained. Metallic or metalloidal compounds of
elements, grouped differently from any compounds known to us, and far
more dense, may exist in the centre of the earth.

"The process of combination and cooling having gone on until those
elements which are not volatile in the heat of our ordinar