Priestley Joseph (1733–1804), British experimental chemist, theologian, and philosopher. In 1774 he prepared oxygen by heating mercuric oxide. Although he continued to favor the phlogiston hypothesis, his work did much to discredit that idea. He discovered many gases, including ammonia, sulfur dioxide, carbon monoxide, and hydrochloric acid. While studying the layer of carbon dioxide over a brewing vat, he conceived the idea of dissolving it under pressure. The resulting ‘soda water’ was famous throughout Europe.
His Essay on Government (1768) influenced Jefferson’s ideas in the American Declaration of Independence. The essay also contributed to the utilitarianism of Bentham, supplying the phrase ‘the greatest happiness of the greatest number.’ Priestley modified the associationism of Locke, Hume, and Hartley, holding that a sharp distinction must be drawn between the results of association in forming natural propensities and its effects on the development of moral ideas. On the basis of this distinction, he argued, against Hume, that differences in individual moral sentiments are results of education, through the association of ideas, a view anticipated by Helvétius. Priestley served as minister to anti-Establishment congregations. His unpopular stress on individual freedom resulted in his move to Pennsylvania, where he spent his last years. R.E.B.