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discoverer of several gases and photosynthesis
Joseph Priestley was born in Fieldhead, Yorkshire (near Leeds), England, on March 13, 1733, the son of a Calvinist minister. He was educated at Daventry Academy, where he became interested in physical science. He then received training as a minister of the Dissenting church, which comprised various churches that had separated from the Church of England. In 1755, he became minister at a small Presbyterian parish at Needham Market, Suffolk, and there wrote The Scripture Doctrine of Remission. In 1758 he became minister at Nantwich, where he remained until 1761. He then became a tutor at Warrington Academy in Lancashire, where he was noted for his development of practical courses for students planning to enter industry and commerce. In 1761, he published Rudiments of English Grammar, a textbook which differed greatly from older, classical approaches. He was ordained in 1762. His Chart of Biography (1765) won him a doctorate of laws from the University of Edinburgh.
Priestley's interest in the sciences was rekindled when he met Benjamin Franklin, one of the most prominent scientists of the day, while on a trip to London in 1766. As a result of this meeting, Priestley began to dabble in the field of electricity. In 1767, he discovered that graphite can conduct electricity. That same year he published The History and Present State of Electricity, with original Experiments.
In 1767, Priestley became minister at Leeds. Living next door to a brewery, he soon became intrigued by the "air" that floated over the fermenting grain. From his first experiment, he was able to show that this "air" extinguished lighted wood chips. He also noticed that the "air" drifted to the ground around the vat, implying that it was heavier than normal air. After devising a method of producing the "heavy gas" in his home laboratory, Priestley found that it had a very pleasant and tangy taste when dissolved in water. The gas he had isolated is now known as carbon dioxide, and the water he produced is commonly referred to as soda water. For these discoveries he was elected to the French Academy of Sciences in 1772 and received a medal from the Royal Society in 1773.
In 1772, Priestley was employed by William Petty Fitzmaurice, 2nd Earl of Shelburne, as librarian and literary companion, and it was while working in this capacity that he made his next important discovery. He had placed a shoot of a green plant into a container of water, then covered the container and lit a candle in it until it completely burned out. Later, he was able to both burn the candle again and keep mice alive in the air. He had become the first person ever to observe the respiration of plants -- the fact that they take in carbon dioxide and release oxygen.
To further his study of gases, Priestley devised an apparatus that allowed him to collect gases over mercury. Because mercury is a dense liquid at room temperature, it does not absorb gases easily. Priestley floated various materials on top of the mercury and sealed a glass vessel over the top. He then heated the material with a burning lens (a magnifying glass) and collected the released gas in the glass vessel. One of his first experiments with this device (conducted in 1772) resulted in his discovery of nitrous oxide (aka "laughing gas").
In 1774 Priestley placed a piece of mercuric oxide into the test chamber. When he sampled the gas, he found that it had an unusual property -- it made a candle burn brightly. All the other gases that he had tested extinguished the candle's flame. Priestley called the gas dephlogisticated air, based on the phlogiston theory (the idea that combustion is essentially the process of losing a hypothetical substance known as phlogiston). In another experiment, Priestley observed that the green plant material that had grown on the walls of his jars produced a gas when exposed to sunlight. He quickly identified this unknown substance as the same gas that was released from the heated mercuric oxide. Although he did not know it at the time, Priestley had just documented the process of photosynthesis. French chemist Antoine Lavosier would later repeat Priestley's experiments, prove the phlogiston theory wrong, and name the gas that Priestley had discovered oxygen.
In addition to his work with carbon dioxide, nitrous oxide and oxygen, Priestley also isolated and described the properties of ammonia, sulphur dioxide, hydrogen sulphide, and carbon monoxide. In 1770, he discovered that India gum could be used to "rub out" lead pencil marks, thereby making him the inventor of the eraser; he was also responsible for giving the material its common name -- rubber. In 1781, he documented the decomposition of ammonia by electricity.
In 1780 Priestley left his position with Petty because of religious differences and became a minister in Birmingham. By this time he had turned to Unitarian thinking, and was considered a religious radical. His book, History of Corruptions of Christianity (1782), was officially burned in 1785. Due to his open support of both the American and French Revolutions, his Birmingham home and church were burned to the ground by an angry mob in 1791. He moved to London, but the persecution continued. In 1794, Priestley and his family emigrated to the United States, where he was enthusiastically received by the scientific and civic bodies of the day. He settled in Northumberland, Pennsylvania, and retired to a peaceful life as a writer. He died there on February 6, 1804. His posthumously collected Theological and Miscellaneous Works (1817-1832) and Memoirs and Correspondence (1831-1832) cover a wide variety of subjects in science, politics, and religion.
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