Isolation of 'dephlogisticated air' (1774)
Oxygen, obtained by heating mercuric oxide (red calx of mercury) with a burning lens
Science · Chemistry
Joseph Priestley
1733–1804 · English (later lived in the United States)
Pneumatic chemistry; also theology, electricity and political philosophy
Joseph Priestley was a Dissenting minister, teacher and polymath who became the leading pneumatic chemist of his age, isolating several new gases, including oxygen in 1774, which he called "dephlogisticated air". Ironically he remained a defender of the phlogiston theory and rejected Lavoisier's new chemistry built partly on his own discoveries. A Unitarian and supporter of the American and French Revolutions, he saw his Birmingham home destroyed in the 1791 riots and emigrated to Pennsylvania.
How to spot them
- ▸Discovered oxygen ('dephlogisticated air') in 1774 by heating mercuric oxide
- ▸Invented soda water (water impregnated with 'fixed air', carbon dioxide)
- ▸Unitarian minister; member of the Lunar Society; home burned in the Priestley Riots of 1791
- ▸Stayed loyal to phlogiston theory; died in Northumberland, Pennsylvania
Key discoveries & works
Directions for Impregnating Water with Fixed Air (1772)
Method for making carbonated water: the invention of soda water
Experiments and Observations on Different Kinds of Air (1774–1786)
Multi-volume account of his discoveries of gases
The History and Present State of Electricity (1767)
History of electrical research, written with encouragement from Benjamin Franklin
Mouse-and-mint experiments showing plants 'restore' air (1771)
A sprig of mint made air in a sealed vessel breathable again, a step towards understanding photosynthesis
Key ideas
- Pneumatic chemistry: collected gases over mercury, allowing water-soluble gases to be isolated
- Phlogiston theory: interpreted oxygen as air lacking phlogiston, hence especially good at supporting combustion
- Rational Dissent and Unitarianism: argued for religious toleration and against the doctrine of the Trinity
- Suggested that electrical attraction follows an inverse-square law, by analogy with gravity
Quick-fire facts
- •Born at Birstall (Fieldhead), near Leeds, Yorkshire
- •Discovered oxygen while working for Lord Shelburne at Bowood House, Wiltshire; Carl Scheele had found it independently but published later (1777)
- •Also isolated gases including ammonia ('alkaline air'), hydrogen chloride and nitrous oxide
- •Awarded the Royal Society's Copley Medal in 1773
- •Taught at the Dissenting academy at Warrington
- •Member of the Birmingham Lunar Society with Matthew Boulton, James Watt and Erasmus Darwin
- •His house and laboratory were destroyed in the Birmingham 'Priestley Riots' of July 1791
- •Emigrated to the United States in 1794 and died at Northumberland, Pennsylvania
- •The American Chemical Society's highest honour is the Priestley Medal
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Sources
- Science History Institute — Joseph Priestley