The Cantong qi: China's Classic of Alchemy and the Elixir

The Cantong qi (Zhouyi cantong qi, "Seal of the Unity of the Three, in Accordance with the Book of Changes") is the most revered scripture of Chinese alchemy. Traditionally attributed to Wei Boyang of the Han dynasty, it is a cryptic work in verse and prose that uses the symbols of the Book of Changes, the cycles of yin and yang, and the images of lead, mercury and the furnace to describe how the elixir is made. It was read first as a guide to laboratory alchemy (waidan) and, from the Song dynasty onward, as the founding text of inner alchemy (neidan).

Title and meaning of the Seal of the Unity of the Three§

The Chinese title is notoriously difficult. Can can mean "three" or "to tally", tong means "same" or "unity", and qi is a tally or contract, the two halves of a split token that prove a match. A common reading, adopted by Fabrizio Pregadio in his translation, takes the "three" to be the Book of Changes (Zhouyi), the teachings of Huang-Lao Taoism, and the practice of "furnace and fire", that is, alchemy. The text claims that these three are in fact one and the same doctrine seen from different angles, and that understanding any one rightly leads to the others.

The prefix Zhouyi signals that the whole work is built on the language of the I Ching or Book of Changes. Its argument is cosmological before it is chemical: the alchemist reproduces in a vessel the same processes by which the Dao generates and sustains the world.

Wei Boyang and the dating of the Cantong qi§

The traditional author, Wei Boyang, is said to have come from Shangyu in Kuaiji (modern Zhejiang) and to have lived in the second century CE. The earliest biography, in the Biographies of Divine Immortals (Shenxian zhuan) associated with Ge Hong (283–343), tells how he compounded an elixir in the mountains with three disciples. When a dog that tasted it fell dead, he took it himself and also died; one disciple followed his example, the other two left, and then master, disciple and dog revived and departed as immortals. The story illustrates the faith, and the risks, of elixir alchemy.

Modern scholars treat this attribution with caution. Pregadio and others argue that the text as transmitted is a layered compilation: some of its cosmological ideas fit the Han dynasty, but the version that survives probably reached its final form only around the seventh or eighth century, in the Tang. The work includes verse in lines of four and five characters, prose sections, and appended pieces such as the "Song of the Tripod". The early history of the Chinese art is outlined in the guide to Chinese alchemy.

Cosmology of the Book of Changes in the Cantong qi§

The Cantong qi begins from the two trigrams Qian (pure yang, heaven) and Kun (pure yin, earth), which it calls the "gate and door" of all change. In alchemical terms they are the furnace and the tripod, the frame within which transformation happens. Inside them move Kan (water, the moon) and Li (fire, the sun), the active principles of the work. The remaining sixty hexagrams of the Book of Changes are distributed over the thirty days of the month, two for each day, to mark the rise and fall of yin and yang.

Two further schemes became famous. In the first, six trigrams are placed on the phases of the moon, from the first sliver of light in the west on the third day (Zhen), through the full moon (Qian), to darkness at the month's end (Kun). In the second, the "twelve sovereign hexagrams" show yang lines growing from one to six and then yielding to yin, matching the twelve months and the twelve double-hours of the day. Together they gave the alchemist a calendar of the "fire times" (huohou), the precise regulation of heat and timing on which the work depended. The underlying theory of yin, yang and the five agents is explained in the article on yin-yang and the five phases.

Lead, mercury and the reverted elixir§

At the centre of the Cantong qi's alchemy are two substances, lead and mercury, whose union makes the "reverted elixir" (huandan). They correspond to Kan and Li. The trigram Kan has a yang line hidden between two yin lines, so it stands for "true lead", the yang concealed within yin. Li has a yin line between two yang lines, "true mercury", the yin within yang, associated also with cinnabar. Joining them returns each to its source and restores the original wholeness symbolised by Qian and Kun.

The text describes this process in images of marriage, of the dragon and tiger, of the metal and water of the five agents, and of a vessel sealed and heated through cycles of time. Whether these passages were written as instructions for working with real metallic lead and mercury, or as an extended metaphor for cosmic processes, has been debated since medieval times. Pregadio argues that the Cantong qi is primarily a work about the Dao and its manifestation, which uses alchemical language as its central emblem, and that it even criticises some practices of its day.

From laboratory alchemy to inner alchemy§

Tang-dynasty practitioners of external alchemy read the Cantong qi as a laboratory manual, and lead–mercury elixirs became a prominent family of waidan preparations. Some of these compounds were toxic, and the history of elixir poisoning among Tang emperors and officials formed part of the background to a shift in emphasis.

From the tenth and eleventh centuries, practitioners of inner alchemy reinterpreted the same text as a description of the transformation of the body's own essence, breath and spirit. Lead and mercury became internal energies, the furnace and tripod became regions of the body, and the fire times became the regulation of breath and attention in meditation. Zhang Boduan's Wuzhen pian (1075), the other great classic of neidan, is deeply indebted to the Cantong qi, which was honoured as the ancestor of all alchemical scriptures. This development is traced in the article on Taoist internal alchemy, and its goal in the article on Taoist immortality and the elixir.

Commentaries and editions§

Few Chinese texts have attracted as many commentaries. The edition of Peng Xiao (947) divided the work into numbered sections and became one of the standard versions; an anonymous commentary attributed to the immortal Yin Changsheng is also early. In 1197 the Neo-Confucian philosopher Zhu Xi, writing under a pseudonym, produced a critical study of the text, showing that it interested scholars as a work of cosmology and Book of Changes learning, not only as an alchemical manual. Yu Yan's commentary of 1284 and Liu Yiming's in the Qing period are among the most influential later readings, and several versions are preserved in the Taoist canon described in the article on religious Taoism and the Daozang.

In the West, the text became known through a translation by Lu-ch'iang Wu and Tenney L. Davis in 1932 and through Joseph Needham's volumes on Chinese alchemy. Complete modern translations, notably Pregadio's of 2011, now make its structure and commentarial tradition accessible. Comparisons with Western alchemy, which also joined a cosmology of opposites to the marriage of sulphur and mercury, are drawn in What Is Alchemy?.

Frequently asked questions§

Who wrote the Cantong qi?§

Tradition names Wei Boyang, a Han-dynasty alchemist of the second century CE. Modern scholars regard this as a legend and see the transmitted text as a compilation that reached its present form around the Tang dynasty.

Is the Cantong qi about external or internal alchemy?§

It has been read both ways. Tang practitioners used it for laboratory elixirs of lead and mercury, while inner alchemists from the Song dynasty onward read it as a guide to transforming the body's essence, breath and spirit. Some scholars see it chiefly as a cosmological work that uses alchemy as metaphor.

What does "the unity of the three" mean?§

A widely accepted interpretation identifies the three as the Book of Changes, Taoist teaching and the alchemy of furnace and fire. The text presents them as a single doctrine.

Sources and further reading§

• The Seal of the Unity of the Three: A Study and Translation of the Cantong qi — Fabrizio Pregadio, 2011
• Great Clarity: Daoism and Alchemy in Early Medieval China — Fabrizio Pregadio, 2006
• Science and Civilisation in China, vol. 5, part 5: Spagyrical Discovery and Invention: Physiological Alchemy — Joseph Needham & Lu Gwei-Djen, 1983
• The Secret of Everlasting Life — Richard Bertschinger, 1994
• Taoism: Growth of a Religion — Isabelle Robinet, 1997
• Understanding Reality (Wuzhen pian) — Zhang Boduan, 1987