There's clearly Salt, Sulphur and Mercury. But then there's what appears to be an Aquarius glyph at the bottom that tends to confuse things a bit. I believe I've found an answer to what Waite is saying here in symbol. It starts with some background on Waite's philosophy. He refers frequently to the necessity in the Great Work of first volatilizing, then fixing Mercury which in its natural state is fluidic and lethargic. Here's an example from an
Occult Review article of October 1908, "The Hermetic and Rosicrucian Mystery":
"The mercury of the sages is that which must be fixed and Volatilized—naturally it is fluidic and wandering . . ."
And another
Occult Review article from November the same year, "Pictorial Symbols of Alchemy":
"This, it will be seen, is the crowned or philosophical Mercury, bearing in either hand the caduceus, which is his characteristic emblem, and having wings upon his shoulders, signifying the volatilized state. But there are also wings beneath his feet, meaning that he has overcome this state, and has been fixed by the art of the sages, which is part of the Great Work . . ."
Here's a picture that accompanies the text.
Mercury Fixed and Volatilized
Mercury in its volatilized state Waite refers to as Quicksilver; it has been enlivened but is still immature. In its fixed state he refers to it as Philosophic Mercury. In his Fellowship of the Rosy Cross Adeptus Major Initiation, there's an interesting part that reveals Waite's thinking about Salt, Sulphur and Mercury. Salt [Will] corresponds to the right pillar on the Tree of Life, Sulphur [Desire] to the left side, and Mercury [Mind] to the middle pillar. As the initiate moves up the Tree these are purified and sublimed, mind also becomes fixed in Tiphareth. Here's a quote:
"I have something to say to you concerning Philosophical Mercury, the symbol of which you bear upon your left side, as an Auxiliary Frater Adeptus. The Keepers of the Secret Tradition tell us in their parables that it is coagulated by its own Sulphur, which is the conjunction of their Sun and Moon, or the Marriage of Adam and Eve. Now this is to be understood mystically, for they say also that it is an union of heaven and earth. The explanation of such hidden language is to be found in the Tree of Life, as this is understood by the Order of the Rosy Cross. The natural principles of our humanity, symbolised in their correspondences with Fire, Water, Air and Earth, are collected in Malkuth, which is the World of Action, and are centred therein upon things manifested. The thoughts, desires and will of man are contained within earth and his senses. In other words, the native Mercury [thoughts], Sulphur [desires] and Salt [will] have not been made subject to the operations of Divine Wisdom. They begin to be purified and prepared in the World of Formation, containing three Sephiroth, allocated respectively to Mind [Yesod], Desire [Hod] and Will [Netzach], and corresponding to three spiritualised planets, the Moon [Yesod], Mercury [Hod] and Venus [Netzach]. These Sephiroth constitute the Second Reflected Triad in the Tree of Life. The World of Creation, or Third Order of the Rosy Cross, is the First Reflected Triad, and it answers to the same principles of our nature, when they have been changed by the work of Wisdom. That which corresponded in Yesod to the reflected light of the Moon has become Philosophical Mercury in the Grade of Tiphereth, or the Mind permeated and transmuted by the Sun of Righteousness. You will understand therefore that in the Higher Grades of the Third Order, analagous [sic] transmutations of Desire and Will are symbolised, so that in the language of the Secret Tradition they become Philosophical Sulphur and Salt. The transmutation of Desire fixes Mind . . ."
Don't be confused by the correspondence of the planet Mercury to Hod; it plays a role but is distinct from alchemical Mercury corresponding to the middle pillar. I've tried to simplify this complex subject as much as possible while still getting the message across. Some of it may not make sense, but the main point is the volatilizing and fixing of Philosophic Mercury.
Here's a
diagram I made from the FRC Tree that makes it easier to see how it all fits. Salt, Sulphur and Mercury retain their positions on the pillars and are transformed as they move up.
On the card the Sphinx is sitting on the circle directly above the Mercury glyph illustrating the fixed state. In the PKT Waite says:
"The Sphinx is the equilibrium therein."
and
"Sometimes the Sphinx is represented couchant on a pedestal above, which defrauds the symbolism by stultifying the essential idea of stability amidst movement."
The water glyph below illustrates the mind in its natural, fluidic state.
This concept of mind in a fixed state is also mentioned by Waite in his comment on the Magician:
"It is also the unity of individual being on all planes, and in a very high sense it is thought, in the fixation thereof."
I believe it can also be seen in the Hanged Man and Temperance, though Waite doesn't specifically say it.