Priestley, Joseph

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Priestley, Joseph (1733-1804): English Scientist and Political thinker.

Today Joseph Priestley is remembered primarily as one of the men who isolated oxygen and who also identified several other gases. In his own time, Priestley was also known as a teacher, controversial minister, and very prolific author who wrote books and pamphlets on theology, history, politics, education and language in addition to various scientific subjects.

Priestley's considerable achievements in chemistry and other areas might be better remembered today if he had not clung so tenaciously until his death to an outmoded theory of combustion. Priestley was born on March 13, 1733, in Fieldhead, Yorkshire, England, the oldest son of Jonas Priestley, a cloth dresser, and his first wife, Mary Swift. His mother died in 1739, and soon after Priestley was sent to live with his aunt, Sarah Priestley Keighley, until he was nineteen. Thus, he left behind his family's stern Calvinist beliefs and was exposed to the more liberal Presbyterian religion of his aunt. The young Priestley demonstrated a love of learning, and his early education came in local parish schools and from tutors. At nineteen Priestley entered an academy at Daventry run by "dissenters", or Protestants who did not accept the Church of England's close ties to Catholicism.

The teachers at Daventry exposed Priestley to free inquiry into controversial subjects. After completing his studies there in 1755, Priestley attempted preaching at posts in Needham Market, Suffolk, and Nantwich, Cheshire. His unorthodox religious views, and a speech impediment, made these positions unsuccessful. At Nantwich, Priestley opened a school and began his own teaching career; its rapid success led to an invitation to become a tutor at a new dissenting academy at Warrington, where he moved in 1761.

In this position Priestley thrived. He taught numerous subjects and began his writing career, publishing two books on grammar and language theory that advocated usage as the only guide to proper English. Over the next few years, Priestley also wrote several books on educational and political theory. In 1764 he received an LL.D. from the University of Edinburgh and was ordained. The following year he met Franklin, Benjamin and other scientists in London. These men encouraged his work on a history of electricity, which was published in 1767; a history of optics followed five years later. Priestley was elected to the prestigious post of Fellow of the Royal Society in 1766. The following year he left teaching and accepted a position as minister to a large Presbyterian chapel in Leeds. Ironically, here Priestley adopted Unitarian beliefs and began his most controversial religious writing. Also at Leeds Priestley published his histories of electricity and optics and began chemistry experiments with "airs" or gases.

In 1773, William Petty, Earl of Shelburne, persuaded Priestley to join his household as librarian and tutor. During the next decade, Priestley's publications centered in three main areas. His religious works defended and promoted Unitarianism. He became its chief spokesman in England, a role that attracted much criticism from the church establishment opposed to dissent. Priestley's political writing aligned him with the revolutions in America and France, another position intolerable to many in Britain. Between 1774 and 1786, Priestley also completed the volumes known as Experiments and Observations on Different Kinds of Air, which contain results of additional experimental work on gases he performed in his considerable spare time while working for the Earl.

Priestley's work in chemistry made him one of the best-known scientists in Europe, but his political and religious writings made him very controversial. In 1780, Priestley and his wife Mary, whom he had married in 1762, moved to Birmingham to live near his brother-in-law, Joseph Wilkinson. Priestley became co-pastor of a church, a position that only required him to preach on Sunday and thus left time for his research and writing. He joined the Lunar Society, a loosely-organized group of scientists and businessmen that included inventor James Watt and his business partner Matthew Boulton. This comfortable life came to an end in 1791, when a rioting mob—objecting to both his religious and political ideas--burned his church and his house. His books and manuscripts were destroyed. Priestley fled to London, and finally sailed to America where his sons had gone a year earlier. He settled in Northumberland, Pennsylvania, and died there February 6, 1804.

Priestley's earliest scientific works, the histories of electricity and optics, mostly summarized past achievements and current knowledge and added a few of his own experiments. His interest in gases led to more original research. In Leeds his family lived next to a brewery; Priestley observed the bubbles in boiling mash and wondered about their nature. With bladders he extracted this gas, cardon dioxide, which he called "fixed air". Later he learned to produce the gas by pouring sulfuric acid on chalk. Combining the gas with water, Priestley created soda water, which he thought resembled the spring water of spas. He published a pamphlet on the topic, and won the Copley Medal from the Royal Society for this work. This talent for close observation led Priestley to numerous other discoveries in subsequent years. As he refined his methods and improved his equipment, Priestley isolated and described oxygen, ammonia, sulfur dioxide, nitrous oxide, nitrogen dioxide, and silicon tetrafluoride. In his work on oxygen, Priestley independently repeated observations by Karl Wilhelm Scheele, a Swede who actually isolated the gas two years before Priestley, but did not publish his results until after Priestley had published. Priestley called oxygen "dephlogisticated air", since he believed that combustion was caused by release of a substance called "phlogiston". This theory had dominated chemists' thinking about combustion for decades. On a trip to the European continent in 1774, Priestley met Lavoisier, Antoine Laurent, the French scientist and government official who in coming years would revolutionize chemistry with his systematic nomenclature of the elements. Priestley described his new work with "dephlogisticated air", and Lavoisier later repeated his experiments. Lavoisier made the crucial observation that this gas, which he named "oxy-gene", actually made combustion possible and thus he overturned the phlogiston theory. In 1783 Cavendish, Henry and James Watt discovered the compound nature of water. Lavoisier began to construct a theory that would explain such compounds, and which Priestley could not accept. Priestley was a scientist who could make discoveries, but not a theory to contain them. Lavoisier, although not much of an original experimentor, could construct a theory to explain these newly-discovered chemical phenomena. Although he declared himself ready to discard the phlogiston theory if needed, Priestley never did and continued to publish defenses of it even after moving to America. By that time Lavoisier's theory had become widely accepted.

Further Reading:

Bendiner, "Joseph Priestley: Dissenter for All Seasons," Hospital Practice 24 1989.

F.W. Gibbs, Joseph Priestley: Adventurer in Science and Champion of Truth, 1965.

A.J. Wright

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