At the heart of medieval alchemy was the idea that all matter was composed of four elements: earth, air, fire and water. With the right combination of elements, it was theorized, any substance on earth might be formed. This included precious metals as well as elixirs to cure disease and prolong life. Alchemists believed that the “transmutation” of one substance into another was possible; thus we have the cliché of medieval alchemists seeking to “turn lead into gold.”
Medieval alchemy was just as much art as science, and practitioners preserved their secrets with an obfuscating system of symbols and mysterious names for the materials they studied.
– Alchemy in the Middle Ages, by Melissa Snell Historymedren.about.com (http://historymedren.about.com/od/alchemy/p/alchemy.htm)