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18
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DE NATURA FOSSILIUM
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it
is moistened and from which mud can be made when it is saturated with
water. True earth may be found in veins and veinlets within the earth
as well as on the earth's surface in fields and meadows. Earth is by
definition a universal thing. Although the harder earths do not readily
form mud when saturated with water nevertheless this is true of even
the hardest if they are left in water for a sufficient length of time.
There are many species of earth, some of which have been given names.
A congealed juice, called iypos πυκτό$ by
the Greeks, is a dry, rather hard mineral body which is either not
softened in water but dissolves or, if it softens when sprinkled with
water, it differs from an earth in unctuousness or in composition.
Although sometimes it has the hardness of stone it can be distinguished
with ease since it retains the form and appearance of congealed juices
which are softer than stone. Congealed juices are divided into harsh
and unctuous. There are three species of the harsh because they form
from three different things, namely, a liquid mixed with either an
earth, a metal, or a mixed substance. In the first we find halite and nitrum; in the second, chrysocolla, aerugo, iron rust, and caeruleum; in the third, atramentum sutorium, alum, and an unnamed acid juice. Atramentum sutorium and alum form from pyrite, the unnamed acid juice from cadmia. Unctuous congealed juices include sulphur, bitumen, realgar, and orpiment. Although atramentum sutorium and
alum may be somewhat unctuous they will not burn. These latter minerals
also differ in origin from the unctuous juices. The latter are driven
from the earth by the force of heat, the former are produced by the
action of moisture on pyrite.
Stone
is a dry, hard mineral body that may soften a little after standing in
water for a long time and is reduced to a powder in fire or is not
softened in water and melts in only the very hottest fire. In the first
genus we find stones that have been hardened by heat, in the latter
genus those that have been congealed by cold. These two genera are of
similar material. Writers on natural subjects who have considered the
quantities, qualities and values of stones have divided them into four
genera. The first genus, without a special name other than the general
name stone, includes magnetite, hematite, and agate. Minerals of the
second genus are called gems and embrace unusually hard minerals which
are either transparent or sparkle with many beautiful colors. Minerals
of the third genus are called marbles and these are only pleasing to
the eye when polished. Minerals of the fourth genus are called rocks
and are obtained from quarries. Rocks are of indispensable use in
buildings and in sculpture. They do not show marked colors or take
sharp edges. Few stones are lustrous and even fewer are transparent. At
times marble cannot be distinguished from non-transparent gems except
by size or quantity. Rocks can always be distinguished from true stones
by size. Gems are found, characteristically, in veins and stringers
that run through rocks and marbles. There are many species in each of
these four genera which I shall describe later.
A metal is a natural mineral body which is either liquid or solid and will
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