Joseph Priestley (1733-1804) was a Dissenting theologian and an Enlightenment-era chemist. He is best known for his pioneering study of oxygen, although it occurred within the context of the now-rejected Renaissance theory of “phlogisticated air” which plagued European chemistry until late in his lifetime.
– Youth and Education –
Joseph Priestley was born on March 17, 1733, in Yorkshire, to clothier Jonas Priestley and his wife Mary Swift. He was raised for his first several years by his grandfather, and then by an aunt.
Priestley proved an extraordinarily intelligent youngster, memorizing the Westminster Shorter Catechism at just four years old and thus convincing his adoptive parents to send him to school. His parents had been religious Dissentors and his aunt and uncle hoped his schooling would lead him to a career in the clergy. However, he had a falling-out with his church as a late teenager after growing convinced that the Calvinist Protestant tradition of election for salvation (i.e. God chooses those who may be saved) and instead embraced the long-considered-heretical theory of universal salvation (i.e. God will save all human beings, whether they accept his grace or not). This, coupled with a speech defect, caused him to reject the clergy and instead study mathematics, chemistry, and physics, at the time loosely grouped together as natural philosophy. Priestley later chose to return to theological studies on his own terms anyways, graduating from Daventry academy in 1752.
– Religious and Scientific Career –
In 1755, Priestley moved to small Needham Market and took up a preaching position. It was an unlikely beginning for a man later known as a scientist, and was also a poor fit for a man who rejected tradition and yearned for an urban setting. Three years later he therefore moved to Nantwich, opened a school, and even published a grammar textbook.
That experience propelled him into a faculty position at Warrington Academy in 1761, where he initially taught history and language. Being at Warrington, however, spurred Priestley to return to his original academic interest – natural philosophy. Several years later, he published a landmark study of electricity, including an analysis of conducting and non-conducting substances. Priestley later moved back to the clergy, ministering at Mill Hill Chapel in Leeds, Calne, and finally Birmingham.
During this time, Priestley remained a passionate clergyman, continuing to write, speak, and teach on religious subjects. However, he was very quickly becoming better-known for his scientific work – for which he is most remembered today. At Calne, Priestley turned to a new subject: the study of air. The European chemical understanding of the natural world had struggled from one dubious theory to another since the medieval period, and some still held to the theory of four elements (earth, air, water, and fire) – although Priestley’s experiments disproved those. Instead, Priestley favoured a Renaissance-period theory, of phlogiston. Phlogiston theory argued that fire was ultimately caused by an invisible substance known as phlogiston, which was contained in certain substances and could burst out as fire in the appropriate conditions. In the mid-1770s, Priestley finally isolated what he called “dephlogisticated air” – air that possessed no phlogiston of its own and could therefore absorb phlogiston from other substances very rapidly, causing fire at prodigious rates.
Priestley regarded his discovery as proof of the phlogiston theory, although in fact he had discovered something entirely different – oxygen. Oxygen did not draw phlogiston from flammable substances, but rather caused chemical reactions we know as fire; Priestley, with his 18th-century understanding of chemistry and his passionate commitment to the phlogiston theory, understandably did not realize this. Unfortunately, his discovery did not garner him quite as much fame as he hoped it would. Shortly thereafter, Antoine Lavoisier used the same isolation of oxygen to argue against the phlogiston theory and promote a more modern understanding of chemical elements. Priestley never accepted Lavoisier’s work, and therefore found himself increasingly left behind in the later years of his work.
– Later Years –
Priestley’s later years suffered from a degree of chaos more associated with his religious career than his scientific career. His family was forced to flee Birmingham after his house was destroyed in anti-Dissentor riots in 1791. They moved to Hackney, and finally to Pennsylvania, where Priestley turned down a teaching position and apparently attempted to retire into a quiet country life. He continued to produce occasional scientific publications for the rest of his life, most of them defending his version of the phlogiston theory against mounting criticism from abroad.
In 1804, Priestley died after a lengthy illness. Even though his principal achievement (the discovery of oxygen) had been trumped by Lavoisier, he remained well-regarded for the discovery of numerous other chemical substances as well as his connections with political radicals in the Dissentor ranks. Several colleges and one Main Belt asteroid have been named in his honour.