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, while gold is associated with the sun and man.
Mercury possesses the qualities of cold and moist, the same qualities ascribed to the element of water. These qualities are opposite those of sulfur.
Sulfur and Mercury Together:
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. Since Christian alchemy is based upon the concept that the human soul was split during the Fall, it makes sense that these two forces are seen as having been once united and needing to be united once again.
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. In Christianity, it is the part that is judged after death and lives on in either heaven or hell long after the body has perished.
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.