There are four types of Alchemy
1) Physical Alchemy
2) Psychological Alchemy
3) Biological Alchemy
4) Trascendental Alchemy
Alchemy is related to Astrology, as the Seven Revolving Heavens represents the metals seven
Gold - Sun
Silver - Moon
Jupiter - Tin
Venus - Copper
Saturn - Lead
Mars - Iron
Mercury - Mercury
Gold is the metal of the Sun. The Sun represents the Absolute Self, the Divine Solar Consciousness in World Mysticism.
Hence the purpose of Transcendental Alchemy is to transmute our earthly nature ( base metals ) into gold ( divine nature ). Meditation Triune is prescribed for this transmutation
Alchemy is effected by the Fourfold Yoga, via Wisdom, which involves the Discriminative Intellect, which discriminates between the Eternal and the Temporal. The Eternal is the Self, the Witness Consciousness in us. It is like a Mirror, which mirrors everything. It is filled with perceptions of the outer world. Hence it does not experience its own nature. Negating the Ureal and affirming the Real is genuine Wisdom.
Union via Action is renouncing all actions for the Ego and doing all actions in rhythm with Natural Law, for the Absolute Self.
Union via Psychic Control is controlling the Mind and transmuting it into a diviner mind.
Union via Love is the most important. Yogis says that they who do not love grow old quickly. You have heard of the adage " Judge not or ye be judged ". We should be Heart centred and should be non-judgemental !