One of the major storylines of the book is that the main character's grandfather is an alchemist. The themes of alchemy run throughout the story. One of the interesting things I have discovered along the way is that sulfur and mercury have a similar sort of meaning and play and Shiva and Shakti. There are also strong correlations between Sulfur, Mercury, and Salt and the Christian Trinity. This also comes to play in a really important way thematically in the book. Anyway, the more I learn about all of this stuff, the more it all really does seem to meld together. Different religions, similar themes playing out. Here are my notes on Alchemy from articles I've read online:
Sulfur:
The pairing of sulfur and mercury strongly corresponds to the male-female dichotomy already present in Western thought. Sulfur is the active male principle, possessing the ability to create change. It bears the qualities of hot and dry, the same as the element of fire, and is associated with the sun, as the male principle always is in traditional Western thought.
Mercury:
Mercury is the passive female principle. While sulfur causes change, it needs something to actually shape and change in order to accomplish anything. The relationship is also commonly compared to the planting of a seed: the plant springs from the seed, but only if there is earth to nourish it. The earth equates to the passive female principle.
Mercury is also known as quicksilver because it is one of very few metals to be liquid at room temperature. Thus, it can easily be shaped by outside forces. It is silver in color, and silver is associated with womanhood and the moon.
Sulfur and mercury are described as originating from the same original substance, and one might even be described as the gender opposite of the other, for example, suggesting that sulfur is the male aspect of mercury.
Salt:
Salt is an element of substance and physicality. Salt starts out as coarse and impure. Through alchemical processes, the salt is broken down through dissolving, purified and eventually reformed into pure salt, the result of the correct interactions between mercury and sulfur.
Thus, the purpose of alchemy is to strip down the self to nothingness, leaving everything bare to be scrutinized. Through the gaining of self-knowledge about one's nature and one's relation to God the soul is reformed, the impurities expunged, into a united, pure and undivided thing. That is the purpose of alchemy.
Body, Spirit and Soul:
Salt, mercury and sulfur are equated to the concepts of body, spirit and soul. Body is the physical self. Soul is the immortal, spiritual part of the person that defines the person and makes them unique among other people. The concept of spirit is far less familiar to the average reader. Many people use the words soul and spirit interchangeably. Some use the word spirit as a synonym for ghost. Neither is applicable in this context. The soul is personal essence. The spirit is a sort of medium of transference and connection, whether that connection be between body and soul, between soul and God, or between soul and the world.
All metals are a mixture of sulfur and mercury, gold is just perfect balance of them.