Mercury, Sulfur and Salt are supposed to be the three archetypal forces at work in
alchemy.  Water is also an important component in alchemy,yet it could also allude to "that
which does not wet", but rather dissolves and connects us to aspects of the astral world
not easily understood by the mind.  You will note that in tarot both mercury and sulfur
connect with Fire and intellect and, in fact, mercury often becomes the "water", the fluid
that volatizes and changes and that, like spirit mercurius, or Hermes or Thoth, is
impossible to pin down to one definition.  

Salt is assigned to the Body, to the material world.  The Empress, Key III, represents Salt.
Sulphur is assigned to Key IV the Emperor.

In several decks the Emperor makes the sign of Sulphur with his legs, as we see here in
BOTA's version.

Mercury is assigned to Key 1 the Magician.  

Emperor and Empress; King and Queen; Mars and Venus; Sun and Moon:  these pairs
symbolize the opposites that need reconciliation and union through conjunction and
coagulation.