ACS Chemistry for Life

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.”

Visit National Historic Chemical Landmarks to read more about Joseph Priestley’s discovery of oxygen.

Excerpted with permission, National Historic Chemical Landmarks Program
www.acs.org/landmarks