Joseph Priestley and “Dephlogisticated Air”
In 1772 William Petty, the second earl of Shelburne (1737–1805), invited Joseph Priestley (1733–1804), with his wife Mary and their three children, to live on his estate near Calne, Wiltshire, where Priestley would work as Petty's librarian and tutor to his children. The Priestleys moved to the Shelburne estate the following year. That is where their third son was born, and it is where Priestley made his most famous discovery—of oxygen. He did not call it oxygen, however. It was Priestley's friend, the French chemist Antoine-Laurent Lavoisier (1743–94) who gave the gas its name, oxygene, derived from the Greek words oxu- meaning “acid” and -genes meaning “born.” Lavoisier believed (incorrectly) that all acids contain oxygen.
In 1772 Priestley had shown that a gas given off by plants is necessary to animal life. In 1774 he found a way to prepare the gas—or air as he always called it—by heating mercuric oxide (HgO) or minium, also called red lead (Pb3O4). When he placed mice inside a sealed container filled with this air, he found they remained conscious for twice as long as they did when the container held only ordinary air. He tried breathing the air himself and found it superior to ordinary air. By heating these oxides, Priestley had driven off the breathable air (oxygen), but that is not what he believed had happened. Priestley believed in the phlogiston theory, according to which roasting the metal (mercury or lead) in air had driven off the phlogiston it contained, leaving behind a substance known as the metallic calx. The heating process was known as calcination.
With certain metals, heating the calx allowed phlogiston to enter the calx, thereby phlogisticating it and restoring the original metal. Air containing too much phlogiston was unbreathable, so removing the phlogiston by phlogisticating the mercury or lead calx improved the air quality. Priestley called the product of this process “dephlogisticated air.” Lavoisier did not believe in phlogiston—it was one matter on which the friends disagreed—and he was able to recognize the gas as a chemical element and a natural constituent of ordinary air.
Dephlogisticated air, or oxygen, was only one of the gases Priestley discovered. He also isolated nitrous air (nitric oxide, NO), alkaline air (ammonia, NH3), acid air (hydrochloric acid, HCl), and dephlogisticated (also called diminished) nitrous air (nitrous oxide, N2O). He later discovered vitriolic acid air (sulfur dioxide, SO2) and also isolated carbon monoxide (CO), but failed to recognize it as a distinct air.
Priestley also invented soda water, publishing a description of the method he used for the benefit of the crew sailing on James Cook's (1728–1779) second voyage to the South Seas—he mistakenly believed it would cure scurvy. Priestley made no money from soda water, but a German silversmith and watchmaker called Johann Jacob Schweppe (1740–1821) patented the process in 1783.
Joseph Priestley was born on March 13, 1733, in the small town of Birstall, West Yorkshire, about 6 miles (10 km) from Leeds, the oldest of the six children of Jonas Priestley, a finisher of cloth, and his wife, Mary Swift. His mother died when Joseph was six, and when his father remarried in 1741 the boy went to live with his wealthy uncle and aunt, John and Sarah Keighley. He attended local schools where he learned Latin, Greek, and Hebrew. He also studied French, German, Italian, Chaldean, Syrian, and Arabic. The family were Calvinists and therefore religious Dissenters, and his education continued at a Dissenting academy (a school for the children of Dissenters) in Daventry, Warwickshire. He matriculated in 1752, and in 1758 he became a clergyman, with parishes in Needham Market, Suffolk, and later at Nantwich, Cheshire. In 1761 he was appointed to teach modern languages and rhetoric at the Dissenting Warrington Academy in Cheshire. Until the laws were repealed in 1829, Roman Catholics and Dissenters, also known as Nonconformists although the terms are not strictly synonymous, were discriminated against in England. They were not permitted to hold public office, stand for election to Parliament, serve in the army or navy, or attend the universities of Oxford or Cambridge.
On June 23, 1762, Joseph married Mary Wilkinson. While at Warrington, Priestley conducted scientific experiments, mainly on electricity, and lectured on anatomy. It was at this time that he met Benjamin Franklin (1706–90), who encouraged his growing interest in science. In 1767 Joseph, Mary, and their daughter, Sarah, moved to Leeds where Joseph became minister of Mill Hill Chapel. They remained there until 1773, and two sons, Joseph and William, were born during their time there. Their youngest son, Henry, was born at Calne in 1777. While he was at Mill Hill, Priestley sent five scientific papers to the Royal Society, describing his experiments with electricity and optics. The Royal Society awarded him their Copley Medal in 1773.
The family moved to Birmingham in 1780, following a disagreement with Lord Shelburne. Priestley became a minister again, and he also joined the Lunar Society, a group of scientists, engineers, inventors, and manufacturers who met once a month when the Moon was full, to minimize the risk of being attacked on the unlit streets. For many years Priestley had devoted considerable effort to theological and political campaigning, which made him a controversial figure. Their support for the American and French Revolutions had made the Dissenters increasingly unpopular, and in 1791 riots broke out in Birmingham. Joseph and Mary Priestley fled from their home, which was attacked and burned to the ground, destroying all their possessions and Joseph's laboratory. Cartoons were published attacking him, and an effigy of him was burned. He was forced to resign from the Royal Society, was attacked in speeches in Parliament, denounced by preachers, and his sons were unable to find work. The sons decided to emigrate to America, and, although Joseph had been made an honorary citizen of France, he and Mary decided to accompany them, escaping shortly before the government began arresting those who spoke out against them. The Priestleys sailed for America on April 17, 1794, arriving to a warm welcome in New York. They then moved to Philadelphia, where Joseph was offered, but declined, the professorship of chemistry at the University of Pennsylvania. Instead, the family moved to the town of Northumberland, where their son Joseph and others were establishing a colony for English Dissenters. Henry Priestley died in 1795, and Mary died the following year. Joseph's health deteriorated, and by 1801 he was unable to work. He died at Northumberland on February 6, 1804. He had been elected to the membership of every leading scientific society in the world.
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- Nehemiah Grew, Plant Reproduction, and Comparative Anatomy
- Nikolai Vavilov and the Origin of Cultivated Plants
- Gote Turesson and Plant Ecotypes
- Asa Gray and the Discontinuous Distribution of Plants
- Charles Darwin and Evolution by Means of Natural Selection
- Adolphe-Theodore Brongniart, Father of Paleobotany