DALTON, JOHN. - Chemist, b. at Eaglestfeld (Cumberland) on 6 Sept. 1766, d. at Manchester on 27 July 1844. From a poor family, he earned his living by giving lessons as an elementary school teacher; he studied mathematics and physics on his own; he never held any public or notable positions: his life was marked by great modesty and a lively passion for study; one of his biographers says that he «was a model of virtue without ostentation and of religion without fanaticism».
He is to be regarded as the founder of the modern atomic theory based on experimental evidence: to him we owe the law of multiple proportions (1802–1808; V. chemistry) and, through his influence, Proust’s law of constant proportions (1799–1806). The genesis of his ideas is to be found in the studies of his friend Henry (1744–1836) on the influence of pressure on the solubility of gases in liquids. The work in which he set forth his doctrine is the *New System of Chemical Philosophy* (3 vols., Manchester 1808, 1810, and 1827); its dissemination, besides the truth and fruitfulness of the theory itself, was due to the eloquent manner in which it was popularized by his friend T. Thomson (1773–1852).