proportion
as is necessary for the continuity of the same. And this is termed by
Aristotle the ' Drying and Congealing Virtue of the earth.' And this
is the cause why stones cannot in any way be melted by the desiccative
heat in the same manner as the metals are melted. For in metals the
moisture has not been completely squeezed out, for which reason the
matter of metals remains capable of fusion. For which reasons we must
maintain that Heat, that digests and repels moisture, and Cold, that
constringes moisture after it is acted upon by the dryness of the
earth, are the peculiar instruments of the Efficient or Mineral Virtue
of stones. And this is the doctrine laid down by Aristotle in his
treatise ' On Minerals,' viz., that stones are produced in two ways,
either by congelation or by conglutination ; as already stated."
Aristotle's
disciple Theophrastus has elaborated the same theory into the following
compact and intelligible form :— " Of things growing within the earth,
some are of Water, others of Earth. Of Water, are the metals, such as
silver and gold and the rest: of Earth, are stone, and all the more
precious kinds of stones, and also whatever other peculiar varieties
there be of earths properly so called; peculiar, that is to say, on
account of their colour, their polish, their density, or any other
quality. The subject of metals has been considered elsewhere; at
present let us discuss the latter substances, stones and earths,
"
All these therefore, we ought, speaking generally, to consider as made
up of a certain pure and homogeneous matter, produced either by a flux
or a filtration through some medium, or else secreted in some different
manner, as has already been stated. For it is possible that some are
formed in the latter, some in the former way; others again by a
different process: from the which causes in fact they derive their
smoothness, their density, their brilliancy,
