Priestley , Joseph

(1733–1804) British chemist and Presbyterian minister

Priestley was the greatest British chemist of the 18th century and also one of the century's greatest men. Born in the English city of Leeds, his father was a cloth dresser and a Congregationalist. Priestley was educated at a nonconformist seminary, later becoming a minister at Needham Market in 1755. After a few years in Nantwich in a similar post he went to teach at Warrington Academy in 1761.

On visits to London he met Benjamin Franklin, who aided him in his History of Electricity (1767). He moved to Leeds in 1767 and, being near a brewery, “began to make experiments in the fixed air that was continually produced in it.” It was around this time that he invented soda water with the ample supply of carbon dioxide (‘fixed air’) from the brewery. In 1772 he became Lord Shelburne's librarian, which involved only nominal duties and allowed him to do some of his most important work. He left in 1780 to become a minister in Birmingham, where he mixed with such members of the Lunar Society as Erasmus Darwin, James Watt, Josiah Wedgwood, and Matthew Boulton. As a dissenting radical he preached against the discrimination suffered by non-Anglicans and, in reply to Edmund Burke's Reflections on the French Revolution, wrote in favor of the principles of the French Revolution. This led a Birmingham mob to break into his home (1791) and burn it, destroying all his books, papers, and instruments. He moved to London for a short while but finding no security there moved to America in 1794 to join his sons who had emigrated there earlier. In Pennsylvania he continued with his scientific and theological work until his death.

Priestley attempted to understand the facts of combustion and respiration. His first insight came from the realization that, since even a small candle uses an enormous amount of pure air, there must be a provision in nature to replace it. After trying various techniques to purify the foul air left after combustion he eventually found that a sprig of mint would revive the air so it could support combustion once more (1771). In the next few years, using a variety of new techniques, he isolated various gases – nitrous oxide, hydrogen chloride, sulfur dioxide – and, in 1774, he produced oxygen. Using a powerful magnifying glass to focus the rays of the Sun, he heated oxides of mercury and lead confined in glass tubes over mercury. He found that they gave off large amounts of a gas in which a candle would burn with an enlarged flame. At first he identified the gas as nitrous oxide but found that, unlike that gas, it was barely soluble in water. He next thought it might simply be ordinary air but on putting mice in it he found that they lived longer than in a similar volume of normal air. Being an ardent believer in the phlogiston theory, he named this gas ‘dephlogisticated air’. Antoine Lavoisier realized the crucial value of this discovery in explaining combustion and named the gas ‘oxygine’.

Priestley continued to discover more compounds. He determined the relative densities of various gases by weighing balloons filled with them and also investigated gaseous diffusion, conductivity of heat in gas, and the effect of electrical discharge on gases at low pressure. He also produced numerous substantial theological works.