§ 18. The Sulphur-Mercury-Salt Theory.
Later in the history of Alchemy, the mercury-sulphur theory was
extended by the addition of a third elementary principle, salt. As in
the case of philosophical sulphur and mercury, by this term was not
meant common salt (sodium chloride) or any of those substances commonly
known as salts. "Salt" was the name given to a supposed basic principle
in the metals, a principle of fixity and solidification, conferring the
property of resistance to fire. In this extended form, the theory is
found in the works of Isaac of Holland and in those attributed to "Basil
Valentine," who (see the work Of Natural and Supernatural Things)
attempts to explain the differences in the properties of the metals as
the result of the differences in the proportion of sulphur, salt, and
mercury they contain. Thus, copper, which is highly coloured, is said to
contain much sulphur, whilst iron
is supposed to contain an excess of salt, &c. The sulphur-mercury-salt
theory was vigorously championed by Paracelsus, and the doctrine gained
very general acceptance amongst the alchemists. Salt, however, seems
generally to have been considered a less important principle than either
mercury or sulphur.
The same germ-idea underlying these doctrines is to be found much
later in Stahl's phlogistic theory (eighteenth century), which attempted
to account for the combustibility of bodies by the assumption that such
bodies all contain "phlogiston"—the hypothetical principle of
combustion (see § 72)—though the concept of "phlogiston"
approaches more nearly to the modern idea of an element than do the
alchemistic elements or principles. It was not until still later in the
history of Chemistry that it became quite evident that the more obvious
properties of chemical substances are not specially conferred on them in
virtue of certain elements entering into their constitution.
