John Dalton, English chemist, 1814.
© Science Museum / Science & Society Picture Library
Later engraving by Worthington after the oil painting by Allen. Dalton (1766-1844) formulated an atomic theory to explain chemical reactions, based on the concept that the atoms of different elements are distinguished by differences in their weights; a theory which has influenced scientific thought for more than a century and a half. In 1794 he first described colour blindnes ('Daltonism'), and he also kept a meteorological journal for most of his life, recording over 200,000 observations.