Johann Gahn, Swedish chemist and mineralogist, c 1780.
© Science Museum / Science & Society Picture Library
Johann Gahn (1745-1818) started his career as a miner before studying mineralogy and developing a way of preparing metallic manganese on a large scale. With Carl Wilhelm Scheele, Gahn discovered the presence of phosphoric acid in the bones. Gahnite, a mineral, is named after him.