"River Chariot" ( 河車 heche ) is a term of the science of the Golden Elixir ( 金丹 jindan ). This term already existed in the Han book The Three Ways Unified and Normalized of the Book of Changes ( 周易參同契 zhouyi cantongqi ), which says, "what presides over the five metals is the river chariot in the north". It is also often found in the Yellow Emperor's Book of Formulas of the Divine Elixir of the Nine Cauldrons ( 黃帝九鼎神丹經訣 huangdi jiuding shendan jingjue ), the Classics of Drugs and Acupuncture ( 藥石爾雅 yaoshi erya ), and other books. In early Daoism, "River Chariot" is an ubiquitous expression indeed. There have always been different understandings of this term. Yin Changsheng considered "River Chariot" an alternative name of "lead", while the Handbook of Reversion Elixir Formulas ( 還丹肘後訣 huandan zhouhou jue ) took the joint refinement of lead and mercury to be River Chariot. Since the Sui and Tang dynasties, Daoism has interpreted the term from the point of view of Inner Alchemy ( 內丹 neidan ). According to the elaborations in books such as the Transmission of Dao by Zhongli Chuan and Lu Dongbin ( 鍾呂傳道集 Zhonglü Chuandaoji ), the "River Chariot" has essentially two senses in Inner Alchemy. First, it refers to the "the Vital Breath ( 氣 qi ) of Perfect Oneness ( 真一 zhenyi ) of the water mansion" contained in the kidneys. The two kidneys, one on the left, the other on the right, are just like the sun and the moon in circulation or two wheels running in coordination, hence the term "River Chariot". Then why are the kidneys called "the water mansion"? This is because the kidneys are signified by "water" in terms of their property in the Five Agents ( 五行 wuxing ). Again, why is the River Chariot" related to the north? This is because the agent water corresponds to the north in the aspect of orientation. This also explains why The Three Ways Unified and Normalized of the Book of Changes mentions "north" and "River Chariot" jointly. Second, the "River Chariot" refers to the movement of the "Vital Breath of Perfect Oneness", which circulates and moves without end, just like carrying things with a chariot, hence the term "River Chariot".