When I were younger I started on Alchemy a few times, but always stopped again by reason of their weird vocabulary, but It’s a question about finding the key.
When studying a new religion, philosophy, or esoteric tradition, always look for the trinities, the three Logoi or their principles, and when you have found them, the rest opens up.
This trinity represents the earthly powers Lucifer, Ahriman, and Christ, but also as the Father, the Mother, and the Son; masculine, feminine, and love; Yang, Yin, and Tao; Brahma, Shiva, and Vishnu; Rajas, Tamas, and Sattva; Pingala, Ida, and Sushumna; Sun, Moon, and Earth; Chaos, Order, and Life; Energy, Substance, and Consciousness; right, left, and center cross at Golgotha; right, left, and center pillar of Kabbalah; and the three soul elements: sentient soul (Orektikon, nephesch), mind and heart soul (Kinetikon, ruach), and consciousness or spirit soul (Dianoetikon, n’schamah).
See, among other, Serpents of the Kundalini Fire
Paracelsus in “The life of Philippus Theophrastus Bombast of Hohenheim…” by Franz Hartmann
The Bible tells us that Man is made out of nothing; that is to say, his spirit, the real man, is from God, who is not a thing, but the eternal reality; but he is made into three somethings or substances, and these three constitute the whole of Man: they are himself, and he is they, and from them he receives all that is good or evil for him. Every state in which man can possibly enter is determined by number, measure, and weight. The “Three Substances” are the three forms or modes of action in which the universal primordial Will is manifesting itself throughout Nature, for all things are a Trinity in a Unity. The “Salt” represents the principle of corporification, the astringent or contractive and solidifying quality, or, in other words, the body; the “Sulphur” represents the expansive power — the centrifugal force, in contradistinction to the centripetal motion of the first quality — it is that which “burns,” i.e., the soul or light in all things; and the “Mercury” is the Life, i.e. that principle or form of will which manifests itself as life, or consciousness and sensation. Each of these forms of will is an individual power; nevertheless they are substantial, for “matter” and “force” are one, and originate from the same cause. The three substances, held together in harmonious proportions, constitute health; their disharmony constitutes disease, and their disruption death.
Salt is of course Yin, the solidifying, the calcifying, Gravity, and Sulphur is Yang, light or warms, negative Gravity, and Mercury is the equilibrium between these two, a liquid metal.
The trinity in Alchemy, based on Jacop Boehme and Paracelsus, are Sulphur, Salt, and Mercury, but in Anthroposophic medicine both Sulphur and Phosphorus are used interchangeable together with Salt and Mercury, with the same general description, except that Phosphorus connects to the Ego and Sulphur connects to the Astral body.
In Spiritual Science and Medicine (VI) by Rudolf Steiner (GA 112)
We should recognise that all that we divide and separate on earth, both in our thoughts and in our actions, in nature is actually united in some way or another. In our thoughts we separate what is subject to gravity, and therefore tends to salt formation, from that which bears the light and is therefore akin to the workings of light; and we separate both these categories from what is contained in the state of equilibrium between the two.But in nature there are no such absolute divisions. All these ways of working are connected one with another, adjusted to one another, so that they form highly intricate constructions, and one of these intricate structural systems is shown in the lustre of the metal gold; for it is through gold that the spiritual realm looks, as it were, right into the external world….
Thus it is most important to notice how in old writings all these primary principles, salt, mercury and phosphorus, were seen to be in every substance in different combinations, and to note the diligence with which it was sought to liberate and extract these three principles from a given substance.
Ancient atavistic knowledge was indeed not without justification in calling phosphorus the Light-bearer. Men saw that phosphorus does carry and contain that imponderable light. What salt repels and holds at bay, phosphorus carries within it. Thus the substances at the opposite pole from salt, are those that appropriate, so to speak, the imponderable entities — principally light, but also others, for instance, warmth —and interiorise them, making them their inner properties….
Here then you have, in external nature, two states which are polar to one another; that which acts in a saline manner and that which acts in a phosphoric manner. And between them, there is a third group: that which acts Mercurially. Just as man is a threefold being, a creature with nerves and senses, with a circulatory system, and with metabolism; and as circulation is the bridge linking nerves and senses to the metabolic functions: so also there is a mediatory function in external nature. It comprises everything that possesses, to a great degree, neither the saline character nor the character of interiorising the imponderables, but — so to speak — holds the equipoise between these two, by manifesting in the form of drops. …. You will find accordingly, that these mercurial substances are mainly linked up with all that is calculated to bring about a the activities for which phosphorous and saline substances are best qualified.
This should be an elementary piece of knowledge familiar to all; everything on our earth that is globular in form, whether within or external to the human frame, is the result of the interplay of two forces, one urging towards life, the other drawing life away….
…But the salt-process belongs to the region that I am now outlining, just as the sulphur process belongs to the region previously described.
Man has an inherent tendency to mineralisation [Salt]; just as the forces fundamental to the development of our internal flora and fauna can get “out of hand [Sulphur],” so also can the mineralising tendency.
In Anthroposophical Spiritual Science and Medical Therapy (II) by Rudolf Steiner (GA 313)
Small doses of phosphorus and sulfur have the effect of throwing out the astral body, which has stablished itself too strongly in the physical and etheric bodies. Sulfur works more on the astral body, phosphorus more on the ego. The ego, however, because it organizes the astral body throughout, actually acts in concert with it.
In Salt, Mercury, Sulphur by Rudolf Steiner (GA 220)
In olden times men did not use the abstract word think to express something that arose in the mind as a picture. When a real knower spoke about ‘thinking’ he spoke of the salt-process just described. Nor did he speak in an abstract way of the ‘will’ but of the astral forces laying hold of the airy element in man, of the sulphur-process from which the will is born. Willing was a process of concrete reality and it was said that the adjustment between the two — for they are opposite processes — was brought about by the mercury-process, by that which is fluid and yet has form, which swings to and fro from the etheric nature to the astral nature, from the fluidic to the aeriform.
Alchemy: The Evolution of the Mysteries by Rudolf Steiner.