John Dalton, English chemist, c 1835.
© Science Museum / Science & Society Picture Library
Daguerreotype. John Dalton (1766-1844) formulated the atomic theory to explain chemical reactions, based on the concept that the atoms of different elements are distinguished by differences in their weights. This theory has influenced scientific thought for more than a century and a half. In 1794 he first described colour blindnes (Daltonism), an affliction from which both he and his brother suffered. He also kept a meteorological journal for most of his life, recording over 200,000 observations.