Mercury is a planet in the zodiac.
In Indian alchemy, elemental mercury plays a crucial role.
The universe is seen in Hindu alchemy as a succession of
bipolar opposites in conflict with one another.
The unification of these competing forces leads to spiritual
advancement and the termination of reincarnation (samsara).
Hindu tantra, a hidden, ritual-based religious discipline,
shares this paradigm of combining or transcending antagonistic energies.
Hatha yoga, which is centered on a sequence of physical
exercises that are also said to alter the subtle body, has a similar idea.
The merger of the sun and the moon is the dominating metaphor
for this clash of opposites.
The sun and the moon are linked to other opposing principles
through a complex network of relationships.
Mercury is conceived as the semen of the god Shiva, and thus
full of healing power, in accordance with this bipolar symbolism.
It's also associated with the moon (possibly due to its
bright silvery hue), healing and restorative properties, and the nectar of
immortality.
The goddess Shakti's uterine blood is associated with the
element sulfur.
The aspirant's body is purified and refined when mercury and
sulfur are mixed and consumed, eventually making it immortal.
Modern descriptions of this practice caution that it should
only be done under the supervision of one's guru (spiritual teacher);
otherwise, these combinations will be harmful, as mercury is a poison in and of
itself.
Obscure Religious Cults, by Shashibhushan B. Dasgupta, was
published in 1962, and The Alchemical Body, by David Gordon White, was
published in 1996.
You may also want to read more about Hinduism here.
Be sure to check out my writings on religion here.