is responsible for achieving a balanced life. With the goal of explaining Epicurean philosophy to a Roman audience, the Roman poet and philosopher Lucretius wrote De rerum natura (The Nature of Things) in 50 BC. In the work, Lucretius presents the principles of atomism; the nature of the mind and soul; explanations of sensation and thought; the development of the world and its phenomena; and explains a variety of celestial and terrestrial phenomena. The earliest alchemists in the Western tradition seemed to have come from Greco-Roman Egypt in the first centuries AD. In addition to technical work, many of them invented chemical apparatuses. The bain-marie, or water bath, is named for Mary the Jewess. Her work also gives the first descriptions of the tribikos and kerotakis. Cleopatra the Alchemist described furnaces and has been credited with the invention of the alembic. Later, Zosimos of Panopolis wrote books on alchemy, which he called cheirokmeta, the Greek word for "things made by hand." These works include many references to recipes and procedures, as well as descriptions of instruments. Much of the early development of purification methods were described earlier by Pliny the Elder in his Naturalis Historia. He tried to explain those methods, as well as making acute observations of the state of many minerals. Medieval alchemy The elemental system used in medieval alchemy was developed primarily by the Persian-Arab alchemist Jābir ibn Hayyān and was rooted in the classical elements of Greek tradition. His system consisted of the four Aristotelian elements of air, earth, fire, and water in addition to two philosophical elements: sulphur, characterizing the principle of combustibility, "the stone which burns"; and mercury, characterizing the principle of metallic properties. They were seen by early alchemists as idealized expressions of irreducible components of the universe and are of larger consideration within philosophical alchemy. The three metallic principles (sulphur to flammability or combustion, mercury to volatility and stability, and salt to solidity) became the tria prima of the Swiss alchemist Paracelsus. He reasoned that Aristotle's four-element theory appeared in bodies as three principles. Paracelsus saw these principles as fundamental and justified them by recourse to the description of how wood burns in fire. Mercury included the cohesive principle, so that when it left the wood (in smoke) the wood fell apart. Smoke described the volatility (the mercurial principle), the heat-giving flames described flammability (sulphur), and the remnant ash described solidity (salt).