Oxygen
Year of Discovery: 1774
What Is It? The first gas separated and identified as a unique element.
Who Discovered It? Joseph Priestley
Why Is This One of the 100 Greatest?
Priestley’s discovery of oxygen sparked a chemical revolution. He was the first person
to isolate a single gaseous element in the mixture of gasses we call “air.” Before Priestley’s
discovery, scientific study had focused on metals. By discovering that air wasn’t a uniform
thing, Priestley created a new interest in the study of gasses and air.
Because oxygen is a central element of combustion, Priestley’s discovery also led to
an understanding of what it means to burn something and to an understanding of the conversion of matter into energy during chemical reactions.
Finally, Priestley established a simple but elegant and effective process for conducting
analysis of new gasses and gaseous elements. What did it look like? Would it burn (first a
candle and then wood splinters)? Would it keep a mouse alive? Was it absorbed by water?
How Was It Discovered?
Reverend Joseph Priestley was more fascinated by air than by his church duties. Air
was one of the four traditional elements (with fire, water, and earth). But Priestley felt
driven to find out what air was made of.
Other scientists wrote of creating new gasses that bubbled up during chemical reactions. Some had described these as “wild gasses” that built up enough pressure to explode
glass jars or to triple the rate at which wood burned. But none had successfully isolated and
studied these new gasses.
Priestley’s imagination soared. He felt compelled to seek out and study these wild, untamed gasses.
In early 1774 Priestley decided the only way to isolate and study these new gasses was
to trap them under water in an upside down (inverted), water-filled glass jar in which there
was no air.
He decided to begin by burning solid mercurius calcinatus and studying the gas that
reaction had been reported to create.
On August 1, 1774, Priestley used a powerful magnifying lens to focus sunlight on a
bottle of powdered mercurius calcinatus. A cork stopper sealed this bottle with a glass tube
leading from it to a washtub full of water, where water-filled glass jars stood inverted on a
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