Johann Joachim Becher, German physician and chemist, c 1670s.
© Science Museum / Science & Society Picture Library
Engraving by Krugner. In 1667, Becher (1635-1682) published a clasification of substances, particularly minerals, in which he expounded a three 'element' theory of matter. Becher's theories were later taken by Georg Stahl (1660-1734) and developed into the phlogiston theory - a theory widely adopted by 18th century chemists, who believed that phlogiston was a substance that existed in all combustible bodies, and was released during combustion.