Oxygen Discovered by the Batman of the 18th Century

Antoine-Laurent Lavoisier. Line engraving by Louis Jean Desire Delaistre, after a design by Julien Leopold Boilly.Courtesy Blocker History of Medicine Collections, Moody Medical Library, The University of Texas Medical Branch, Galveston, Texas, USA.

Scientists in the late 1700s were fascinated by fire. When wood burns, they noticed that ashes weighed less than wood. This led to the idea that burning things gave off a substance that they called phlogiston. Flammable things were thought to contain phlogiston, and fire released the phlogiston.

But metals behaved differently when they burned, leaving behind an ash called calx. When metals burned, scientists thought that they produced the calx plus phlogiston. But the calxes always weighed more than the metals. If the metal is giving off a substance, it should weigh less, not more.

This brings us to the story of radical Presbyterian minister named Joseph Priestley. He first became interested in science through conversations with Benjamin Franklin in 1766. Their friendship would become very important later in Priestley’s life.

Burning metals wasn’t enough for Priestley. He wanted to fry the calxes, too. Specifically, he was interested in calx of mercury. While mercury is a liquid metal, its calx is a red powder. In 1774, he heated the calx even hotter than was needed to burn mercury, and found something really strange. He regenerated mercury metal! What’s more, he also produced a strange new gas. He gave it the catchy name dephlogisticated air, but today we call it oxygen. Dephlogisticated air made flames burn more brightly. It also made animals hyperactive. Priestley showed that this stuff made up about 20% of the air we breathe.

Meanwhile another scientist named Henry Cavendish was studying gases given off in chemical reactions. Cavendish was a kindly eccentric who was independently wealthy enough that he could spend most of his time on science. Cavendish spent most of his time investigating electricity, but he was also interested in chemistry. Specifically, he looked at what happened when metals react with acids. Reacting zinc with hydrochloric acid, he produced a strange new gas. Cavendish noticed it was lighter than air. He noticed the stuff burned, so he called it inflammable air. He also noticed that burning it produced water. Today we call this gas hydrogen. Because it was so flammable, Cavendish thought that it might be phlogiston itself.

But a French scientist had a different take on things. That person was Antoine-Laurent Lavoisier. In many ways, Lavoisier was the Batman of chemistry. Just as Bruce Wayne was a respectable millionaire socialite by day who fought crime by night, Lavoisier was a respectable millionaire socialite by day, while doing his research after hours. He had some very different thoughts about Priestley’s and Cavendish’s experimental results.

Lavoisier didn’t like the idea that metals released phlogiston but gained weight in the process. He thought that Priestley’s experiment and Cavendish’s experiments could be explained if instead of releasing something when burned, they were combining. Things combined with oxygen when they were burning.

This explained why metals gained weight when they were burned. The extra weight came from the oxygen. It also explained why red calx of mercury gave off oxygen when it was decomposed. What’s more, it explained what was going on with Cavendish’s hydrogen. Hydrogen was combining with oxygen to make water. As for things that lost weight when they burned, like wood, the weight loss could be attributed to the loss of smoke and other gases produced in the burning. Lavoisier’s new explanation of combustion was so revolutionary, it became known as the Chemical Revolution.

Cavendish had reservations about Lavoisier’s ideas, but then, he seemed to have been more worried about electricity than chemistry. He never published a lot of his research, which we now know to be about a 100 years ahead of its time.

Lavoisier was insightful in the lab, but naive at life. He had been a major shareholder in the private corporation that collected the King’s taxes. When the French Revolution came, all sense would have told him to get out of Dodge, but he stayed, believing in its ideals, strangely not grasping how incompatible that was with being a royal tax collector. He lost his head during the Reign of Terror under Robespierre.

Joseph Priestley never accepted Lavoisier’s ideas, strangely enough. Outside of the lab, his life was very eventful. He was a political maverick, with strong democratic and egalitarian ideas. During the French Revolution, many in England feared the revolution would spread across the channel, and radicals like Priestley were eyed with suspicion. After a mob burned down his house, he decided that it was time to pay a visit to his friend Benjamin Franklin, and he hopped on a boat bound for Philadelphia. He eventually settled in Northumberland, PA, where his house is now a museum.

There is an epilogue to this whole story. In more recent times, the work of a once-obscure chemist who lived at the same time as Priestley, Cavendish and Lavoisier has become more widely known. That person was Karl Wilhelm Scheele. Scheele lived in Sweden, and managed to carry out all the work that Priestley and Lavoisier carried out combined, and reached the same conclusions as Lavoisier, before either of them. He discovered oxygen, which he called “fire air,” in 1772, two years before Priestley. But Sweden was far from France and Britain, and communications were slow in those days. By the time his work was published, Priestley’s work was old news. It wasn’t until 1892 that historians discovered that Scheele was really the first person to discover oxygen, and the first person to understand its role in combustion.

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This article was originally published under the title "We're History" in the August 2004 edition of Chemical Engineering Progress magazine. This article was prepared by Mark Michalovic, PhD, education specialist for the Chemical Heritage Foundation.