Eva Wong

The true lead was there before sky and earth were born. It is the father who created all things and the mother who nourished them. Therefore it is called the mother and father of all minerals. The Tsan-tung-chi says, “It is the mother and father of al the hexagrams.”
It is the secret residence of cinnabar and mercury.
Cinnabar and mercury originally do not have form or substance. They are hidden within the true lead. They emerge in the chamber of the spirit only when they receive the pure essence of the sun. Therefore the chamber of the spirit is called the secret residence of cinnabar and mercury.

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Interested readers may consult the rich introduction by Eva Wong and the full version of this text in the published edition, Harmonizing Yin and Yang. Shambhala, 1997.