Alchemy is the very old study and philosophy of how to change basic substances (such as metals) into other substances. It also studied how substances (and how they are changed into other substances) were related to magic and astrology. People who studied alchemy were called alchemists. Some things alchemists tried to do were change lead or mercury into gold, make the philosopher's stone, and make the "elixir of life," which they thought could cure (make better) any disease and make someone young again.