John Dalton
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John Dalton (; 5 or 6 September 1766 – 27 July 1844) was an English chemist, physicist, and meteorologist whose work laid the foundations of modern atomic theory and stoichiometric chemistry. Building on earlier ideas about the indivisibility of matter and his own precise measurements of combining ratios, Dalton proposed that each chemical element consists of identical atoms of characteristic weight, and that compounds are formed when atoms of different elements combine in fixed whole-number proportions. His A New System of Chemical Philosophy (1808) presented a coherent atomic model, supplied relative atomic weights and symbolic notation, and established the quantitative framework that shaped nineteenth-century chemistry and remains the basis of modern chemical thought. Dalton was also a pioneering meteorologist and physicist, keeping daily weather observations for over fifty years, formulating the first empirical law of partial pressures (later known as Dalton’s Law), and studying the behavior of gases through his work on vapor pressure and gas solubility. His investigations into his own color blindness led to the first scientific description of the condition—still called Daltonism in several languages—and helped establish experimental methods for linking perception with physiology. Elected a Fellow of the Royal Society in 1822 and awarded its Royal Medal in 1826, Dalton became the first British scientist to develop a quantitative atomic theory and one of the key figures in the transition of chemistry from a qualitative to a mathematical science. In honour of Dalton's work, a unit of atomic mass, the dalton, symbol Da, is officially accepted for use with the SI.