At Boughton under Blee (about five miles from Canterbury) the pilgrims are overtaken by a Canon and his servant, who have ridden hard to catch up. The Yeoman greets the company and, on the Host's questioning him, boasts of the power of his master. The Canon, fearing the revelations his Yeoman may make, rides away. Since he is gone, the Yeoman says, he will tell all his master's business. In Part I of his tale, the Yeoman tells of his master's failed attempts to find the Philosopher's Stone, and he laments his own fascination with the craft. In Part II, the Yeoman tells of the trickery of a canon (not his master, he says) who convinces a London priest that he has the power to change base metals into silver and gold. The greedy priest eagerly pays for the recipe; the tricky canon disappears. Lo, the secrets of alchemy will never be found; the writers use terms too dark for our understanding, explaining the unknown by the unknown. God does not want the philosophers to reveal the secret; therefore, let it go; he who works contrary to God will never succeed.
Beginning readers may want to consult an interlinear translation.
There are no existing tales on which Chaucer might have based his Canon's Yeoman's Tale. Even the figure of the dishonest alchemist was rare before Chaucer's time. (For an example, see the selection from Sercambi's Novelle printed with English summary in Sources and Analogues of the Canterbury Tales, ed. W. F, Bryan and Germaine Dempster (New York, 1958; pp. 685-98).
For a survey of the subject see: Edgar H. Duncan, The Literature of Alchemy and Chaucer's Canon's Yeoman's Tale: Framework, Theme, and Characters, Speculum (1968), 633-56. (This article is in JSTOR; click here for an explanation).
The Renaissance rather than the Middle Ages was the age in which alchemy flourished; however, alchemy was becoming increasingly popular in the fourteenth and fifteenth centuries, and it was discussed in popular treatments of science, such as that found in Gower's Confessio amantis, which includes a survey of scientific knowledge, and in the popular handbook for princes, the Secreta secretorum, which purported to be a letter from Aristotle to Alexander containing instructions necessary to a ruler.
Gower on alchemy
For a general discussion of alchemy, with references, see John Reidy's introductory notes in The Riverside Chaucer. For an important fifteenth century work on the subject see Thomas Norton's Ordinal of Alchemy, ed. John Reidy, EETS 272, 1975. For a work on modern alchemy see Steven Skinner,In Pursuit of Gold: Alchemy in Theory and Practice, London, 1976.
There is a great web resource for alchemical texts, inmagery, bibliography, and much more -- The alchemy web site and virtual library. It includes translations of some of the alchemical texts that Chaucer mentions, including an eighteenth-century translation of The Rosary of the Philosophers.
Older scholars speculated that Chaucer knew about alchemy from bitter experience, that he had been hoodwinked by William Shuchirch, a canon at Windsor (see esp. Manly, Some New Light on Chaucer, pp. 245-52. This is only speculation; it does seem likely that Chaucer had some personal experience of chemical processes -- one of the false canon's tricks depends upon the knowledge that mercury evaporates at high temperatures.
For a bibliography of critical and scholarly works on the Canon's Yeoman's Tale click here.