When Joseph Priestley discovered
oxygen in 1774, he answered age-old questions of why and how
things burn. An Englishman by birth, Priestley was deeply involved
in politics and religion, as well as science. He emigrated to
America when his vocal support for the American and French revolutions
made remaining in his homeland untenable.
The American Chemical Society, the world's largest scientific
society, designated Priestley's Pennsylvania home a National
Historic Chemical Landmark in 1994. ACS joined the Royal Society
of Chemistry in designating Bowood House, the site of Priestley's
English laboratory, an International Historic Chemical Landmark
in 2000.
Some 2,500 years ago, the ancient Greeks identified air —
along with earth, fire and water — as one of the four elemental
components of creation. That notion may seem charmingly primitive
now. But it made excellent sense at the time, and there was
so little reason to dispute it that the idea persisted until
the late 18th century. It might have endured even longer had
it not been for a free-thinking English chemist and maverick
theologian named Joseph Priestley.
Priestley (1733-1804) was hugely productive in research and
widely notorious in philosophy. He invented carbonated water
and the rubber eraser, identified a dozen key chemical compounds,
and wrote one of the first comprehensive treatises on electricity.
His unorthodox religious writings, and his support for the American
and French revolutions, so enraged his countrymen that he was
forced to flee England in 1794. He settled in Pennsylvania,
where he continued his research until his death.
But the world recalls Priestley best as the man who discovered
oxygen, the active ingredient in our planet's atmosphere. In
the process, he helped dethrone an idea that dominated science
for 23 uninterrupted centuries: Few concepts "have laid firmer
hold upon the mind," he wrote, than that air "is a simple elementary
substance, indestructible and unalterable."
 In
a series of experiments culminating in 1774 — conducted
with the kind of equipment on display in his Pennsylvania home
— Priestley found that "air is not an elementary substance,
but a composition," or mixture, of gases. Among them was the
colorless and highly reactive gas he called "dephlogisticated
air," to which the great French chemist Antoine Lavoisier would
soon give the name "oxygen."
It is hard to overstate the importance of Priestley's revelation.
Scientists now recognize 92 naturally occurring elements-including
nitrogen and oxygen, the main components of air. They comprise
78 and 21 percent of the atmosphere, respectively.
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