to ensure the secrets of their discoveries never left the closed world of alchemists. However, alchemical texts were often obscured with allegory, or even written in code, for a host of other reasons. Medieval alchemists would naturally have been secretive about their search for the Philosopher’s Stone, for example, thought to turn common metals into gold, and would also have been keen to avoid persecution for what were perceived as occult practices.

Such secrecy lived on alongside alchemy well into the era of Enlightenment science. The 17th century Irish scientist Robert Boyle, best remembered today for Boyle’s Law of gasses, was a founder of the Invisible College, a precursor to the Royal Society to whom he left a list of unsolved scientific problems. Though considered the first modern chemist1667 by Johann Joachim Becher removed Air and Fire, and instead introduced three forms of earth: terra lapidea, the vitreous or glassy earth, terra fluida, the mercuric earth, and terra pinguis which gave substances their inflammable, sulphurous quality. Later renamed phlogiston, this third fundamental earth element was used to explain why things burn and lose mass: burning was simply the process of dephlogisticating. When 18th century experiments showed that metals actually gain mass upon burning, phlogiston theory rapidly began to break apart itself.

Lavoisier showed that burning was in fact the combination of oxygen with the substances being burned. He used this to identify oxygen as an element alongside 54 others, including elements such as chlorine, fluorine and boron which had not yet been isolated from their acids. His