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The *Xuan Dong Yu Zhao* states: In the past, Guangchengzi taught the Yellow Emperor to refine external elixirs (Waidan), and thus substances such as lead (Qian) and mercury (Gong) appeared in the formulas. This was done by taking internal alchemy (Neidan) as the standard for emulation. When a person is first born, the Kidney (Shen) forms first. The Kidney belongs to the Water of Ren and Gui in the North, corresponding to Lead (Qian) among the Five Metals (Wu Jin). Next, the Heart (Xin) forms. The Heart belongs to the Fire of Bing and Ding in the South, corresponding to Cinnabar (Sha) among the Eight Stones (Ba Shi). Extracting silver from lead is like the True One Water (Zhenyi Zhi Shui) hidden within Kidney Qi. Extracting mercury from cinnabar is like the True Yang Qi (Zhengyang Zhi Qi) hidden within Heart Qi. Generally, combining the silver within external lead with the mercury within cinnabar naturally forms cinnabar-like matter; with appropriate fire timing (Huo Hou), it is refined into the Great Treasure (Da Bao). Analogizing to humans, one extracts the True One Water from Kidney Qi and the True Yang Qi from Heart fluid. These two substances combine to form the Elixir (Dan), similar to how external objects reveal treasures.

The *Chuandao Ji* states: Possessing the quality of Heavenly One and being the head of the Five Metals is Black Lead (Hei Qian). Lead generates silver; lead is the mother of silver. Sensing the Qi of the Sun and being the master of all stones is Cinnabar (Zhusha). Cinnabar generates mercury; mercury is the son of cinnabar. What is difficult to obtain is the silver within the lead; what is easy to lose is the mercury within the cinnabar. If lead and mercury combine, refinement naturally yields treasure. This is the principle of lead and mercury manifested externally. Speaking internally, when the true Qi of parents combines into one, essence and blood form the conception sac, condensing into the Inner Kidney (Nei Shen). The Inner Kidney is Lead. When Qi arises in the Kidney, water arises from Qi. Combining Kidney Water with the True Yang Qi above Heart fluid, and condensing it in the Huangting (Yellow Court), forms the Elixir. This is called True Lead (Zhen Qian). Combining Kidney Qi with Heart Qi, accumulating Qi to generate fluid, knotting into the Mysterious Pearl (Xuan Zhu), and returning to the Lower Dantian (Xia Dantian) is called Mercury (Gong). When the Elixir is accomplished, True Qi ascends, Kidney Qi enters the crown, and True Water descends. Ascending once and sinking once, they achieve Ji Ji (Hexagram 63: After Completion) before the Twelve Towers (Twelve Towers). Returning to the Lower Dantian after one cycle of Ji Ji, this is called True Mercury (Zhen Gong).

The *Yuanhuang Jun Jue* states: Lead and mercury reside in the Cauldron, burned into priceless pearls. Simply these two characters suffice to understand the books of ten thousand families. Zhengyi Zhenren says: Lead and mercury have been transmitted for tens of thousands of autumns; how many have realized them? How many have cultivated them? If ordinary people could comprehend this principle, immortals in the mortal world would be as numerous as flowing water. Lord Lü says: A single grain of Golden Elixir (Jindan) ensures longevity; one must obtain True Lead to refine Jia and Geng. Take the fire from the red phoenix marrow of the South; seek the black turtle essence from the Northern Sea. Ancient and modern sages differ in their descriptions of the principles of lead and mercury. Their origin lies only in this: Lead represents Kidney Water, while True Lead serves as the Elixir; Mercury represents Heart Fluid, while True Mercury serves as the Medicine. True Lead and True Mercury constitute the state of Ji Ji. Relying on external explanations cannot align with the correct principles.