In the fourth book, The Alchemist in the novelThe Years of Rice and Salt, the auotherKim Standly Robison writes about Alchemy this is a reference to the belief of Alchemy.

As found from Wikipedia the flowing tells what alchemy is an where and when it was used:

In the history of science, alchemy from Arabic (al-kimia) refers to both an early form of the investigation of nature and an early philosophical and spiritual discipline, both combining elements of chemistry, metallurgy, physics, medicine, astrology, semiotics, mysticism, spiritualism, and art all as parts of one greater force. Alchemy has been practiced in Mesopotamia, Ancient Egypt, Persia, India, Japan, Korea and China, in Classical Greece and Rome, in the Muslim civilization, and then in Europe up to the 19th century.

Also it tells about how alchemist used Alchemy:

Alchemy was known as the spagyric art after Greek words meaning to separate and to join together. The best-known goals of the alchemists were the transmutation of common metals into gold (called chrysopoeia) or silver (less well known is plant alchemy, or "spagyric"); the creation of a "panacea or the elixir of life," a remedy that supposedly would cure all diseases and prolong life indefinitely; and the discovery of a universal solvent.[1] Although these were not the only uses for the science, they were the ones most documented and well known. Starting with the Middle Ages, European alchemists invested much effort on the search for the "philosopher's stone", a legendary substance that was believed to be an essential ingredient for either or both of those goals. The Philosophers Stone was believed to mystically amplify the user's knowledge of alchemy so much that anything was attainable.