As to the kinds of earth, that which is filtered through water
passes into stone in the following manner:—The water which mixes
with the earth and is broken up in the process changes into air,
and taking this form mounts into its own place. But as there is
no surrounding vacuum it thrusts away the neighbouring air, and
this being rendered heavy, and, when it is displaced, having been
poured around the mass of earth, forcibly compresses it and
drives it into the vacant space whence the new air had come up;
and the earth when compressed by the air into an indissoluble
union with water becomes rock. The fairer sort is that which is
made up of equal and similar parts and is transparent; that which
has the opposite qualities is inferior. But when all the watery
part is suddenly drawn out by fire, a more brittle substance is
formed, to which we give the name of pottery. Sometimes also
moisture may remain, and the earth which has been fused by fire
becomes, when cool, a certain stone of a black colour. A like
separation of the water which had been copiously mingled with
them may occur in two substances composed of finer particles of
earth and of a briny nature; out of either of them a
half-solid-body is then formed, soluble in water—the one, soda,
which is used for purging away oil and earth, the other, salt,
which harmonizes so well in combinations pleasing to the palate,
and is, as the law testifies, a substance dear to the gods. The
compounds of earth and water are not soluble by water, but by
fire only, and for this reason:—Neither fire nor air melt masses
of earth; for their particles, being smaller than the interstices
in its structure, have plenty of room to move without forcing
their way, and so they leave the earth unmelted and undissolved;
but particles of water, which are larger, force a passage, and
dissolve and melt the earth. Wherefore earth when not
consolidated by force is dissolved by water only; when
consolidated, by nothing but fire; for this is the only body
which can find an entrance. The cohesion of water again, when
very strong, is dissolved by fire only—when weaker, then either
by air or fire—the former entering the interstices, and the
latter penetrating even the triangles. But nothing can dissolve
air, when strongly condensed, which does not reach the elements
or triangles; or if not strongly condensed, then only fire can
dissolve it. As to bodies composed of earth and water, while the
water occupies the vacant interstices of the earth in them which
are compressed by force, the particles of water which approach
them from without, finding no entrance, flow around the entire
mass and leave it undissolved; but the particles of fire,