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Summary Article: Priestley, Joseph (1733-1804) from The Hutchinson Dictionary of Scientific Biography

Place: United Kingdom, England

Subject: biography, chemistry

English chemist and theologian. He entered chemistry when it was making the transition from alchemy to a theoretical science. An outstanding practical scientist, he combined experimental flair with quantitative accuracy - skills that led him to discover several new gases, including oxygen. He was less dynamic as a theorist; his conservatism made him a lifelong supporter of Georg Stahl's phlogiston theory of combustion despite mounting evidence - much of it provided by Priestley himself - refuting the principle. Outside his scientific work his life was far from harmonious. He was an outspoken man of radical views, which brought him notoriety and eventually drove him to leave his native country.

Priestley was born in Fieldhead, near Leeds, on 13 March 1733, the son of a cloth-dresser. His mother died when he was only seven years old and he was brought up by an aunt, who introduced him to Calvinism. In 1752 he attended the Dissenting Academy at Daventry, and three years later he entered the ministry as Presbyterian minister at Needham Market, Suffolk. He moved to Nantwich, Cheshire, in 1758, and in 1761 became tutor in languages at Warrington Academy. A year later he married May Wilkinson, sister of the ironmaster John Wilkinson. On a trip to London in 1766 he met the US scientist Benjamin Franklin, who aroused in Priestley an interest in science; thereafter he combined scientific research with his clerical and social duties.

In 1767 Priestley returned to Leeds as minister of a chapel at Mill Hill. He did his most productive work 1773-80, when he was librarian and literary companion to Lord Shelburne, whom he accompanied on a journey to France in 1774. While in Paris he met Antoine Lavoisier and told him of his experiments with ‘dephlogisticated air’ (soon to be named oxygen by Lavoisier).

By 1780 Priestley's outspoken criticisms as a Dissenter had become an embarrassment to Lord Shelburne, who retired his companion on a small pension. Priestley moved to Birmingham to become minister of a chapel called the New Meeting. He also joined the Lunar Society, in company with the inventors James Watt and Matthew Boulton, Josiah Wedgwood, Erasmus Darwin (grandfather of Charles Darwin) and a number of less notable inventors and scientists. In Birmingham, Priestley continued to voice loudly his opposition to the established church and his support of the French Revolutionaries. In 1791, on the second anniversary of the storming of the Bastille, the people of Birmingham rioted and vented some of their wrath on Priestley and other Dissenters, whose homes were ransacked. Priestley escaped to London and settled for a while in Hackney, but his unpopularity mounted, exacerbated by an offer of citizenship from France (by the very people who executed Lavoisier in 1794). In that same year Priestley emigrated to the USA, to Northumberland in Pennsylvania. He rejected the offer of a professorship at the University of Pennsylvania, preferring to live a life of comparative solitude in Northumberland, where he died on 6 February 1804.

Influenced by Franklin, Priestley's early work of 1767 onwards was in physics, particularly electricity and optics. He established that electrostatic charge is concentrated on the outer surface of a charged body and that there is no internal force. From this observation he proposed an inverse square law for charges, by analogy with gravitation. Priestley's house in Leeds was near a brewery, and it was his interest in the process of fermentation that turned him to chemistry, particularly gases. He experimented with the gas produced during fermentation - the layer of ‘fixed air’ (carbon dioxide) over a brewing vat - and showed it to be the same as that reported by Joseph Black in 1756. He dissolved the gas under pressure in water, beginning a European craze for soda water.

At Lord Shelburne's estate at Calne, Wiltshire, Priestley continued experimenting with gases. He used a large magnifying glass to focus the Sun's rays to produce high temperatures. He invented the pneumatic trough for collecting gases over water, and overcame the problem of handling water-soluble gases by collecting them over mercury.

An early discovery, in 1772, was ‘nitrous air’ (nitric oxide, or nitrogen monoxide, NO). Priestley found that a sample of the gas left in contact with iron filings and sulphur decreased in volume and that the new gas produced supported combustion. He had reduced nitric oxide to nitrous oxide (dinitrogen monoxide, N2O), Humphry Davy's ‘laughing gas’. In the same year he became the first person to isolate gaseous ammonia by collecting it over mercury (previously ammonia was known only in aqueous solution).

It had long been known that burning sulphur gives off a choking gas. In 1774 Priestley made the same gas by heating oil of vitriol (concentrated sulphuric acid) with mercury. He also produced it by heating the acid with copper turnings, a method still used today to make sulphur dioxide (SO2).

Priestley's most famous discovery was that of oxygen. In 1772 he had shown that a gas necessary to animal life is liberated by plants. Two years later he prepared the same gas by heating red calyx of mercury (mercury(II) oxide, HgO) or minium (red lead, Pb3O4). His investigation of the properties of the new gas showed it to be superior to common air. A mouse trapped in a container of it stayed conscious twice as long as in ordinary air, and breathing it had no adverse effects (apart from leaving a peculiar light feeling in the chest). When he mixed the new gas with nitrous air (NO) there was a diminution in volume and yet another, red gas (nitrogen dioxide, NO2) was formed. From all of these observations Priestley concluded that he had prepared dephlogisticated air - that is, air from which the fiery principle of phlogiston had been removed. The Swedish chemist Karl Scheele independently prepared oxygen in 1772, but his tardiness in publication resulted in Priestley being credited with the discovery.

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