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BOOK X
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219
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All
these earths have the dual nature of both an earth and the congealed
juice contained in them. Nature has given these congealed juices a
fiery force. Pure earths dry and cool, saline earths dry and are also
moderately astringent and cleansing. Alkali earths are more cleansing;
aluminous earths more astringent; atramentiferous earths strongly
astringent and also mordacious; sulphurous and bituminous earths
discutient; and earths containing the juice that has no name are acrid.
Chrysocolla, aerugo caeruleum, realgar, and orpiment make an
earth mordacious. Thus earths possess variable properties depending
upon the congealed juice they contain.
There
are as many different compound substances as there are species of
earths. For example, if any of the congealed juices are added to ocher,
red ocher, or another species of earth they will change the nature of
the compound substance. Since I have discussed the species of earths in
Book II, it will not be necessary to mention them here.
Sometimes
the same earth may contain several congealed juices. When the powdered
rock from Pozzuolo is used in building walls in the sea it is soon
converted into an impregnable stone by the waves, according to Pliny.30
It contains alum, bitumen, and sulphur and is found on the hills of
Pozzuolo in the Baiae district and, according to Vitruvius, in the
fields of free towns near Mt. Vesuvius. According to Pliny a similar
earth is found in the Cyzicena district where it is quarried in great
masses and the earth itself, not a powder, placed in the sea where it
is converted into stone. Similar earth is reported to occur near
Cassandria. Oropus writes that any earth will be changed to stone if
placed in the sea. These earths possess within themselves a quality
which changes them into stones although waters may have this same
quality as I have mentioned elsewhere. It is necessary that this
species of earth be either aluminous, bituminous, or atramentiferous.
I shall now take up the earths that enclose stones and adhere to them. Samian earth sometimes contains samius lapis and
chalk often contains silicious nodules. Very often small pebbles adhere
to lumps of earth. Large masses of earth sometimes contain whole rocks,
marbles, and stones, sometimes fragments of these, so it is to be
expected that small lumps of earth would contain and surround pebbles
of rock, marble, and stone as well as gravel, sand, and even gems. For
this reason, when describing a locality we say that the earth or soil
contains rocks, marble, or stones, or is full of pebbles or gravel, or
it is sandy, or, in some cases, gem-bearing, or full of calcareous
nodules. Since there are so many species of stones, gems, marbles,
rocks, pebbles, gravel, and sand it is apparent that this genus of
compound substances has a very great variety of compositions. For
example, if hematite occurs in a mass of earth it will have a very
80 This is the pozzuolana rock that is used today in the manufacture of hydraulic cement.
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