How Did Joseph Priestley Discover Oxygen? Key Experiments
Joseph Priestley isolated oxygen in 1774 by focusing sunlight on mercuric oxide inside a sealed glass container. He called the resulting gas “dephlogisticated air” because it seemed to lack the theoretical substance phlogiston that was then believed to cause combustion. Priestley’s experiment showed that a candle burned brighter in this gas and that a mouse could survive longer inside it than in ordinary air. He later shared his findings with Antoine Lavoisier, who recognized the gas as a distinct element and named it oxygen.
The Phlogiston Problem
Before Priestley’s work, chemists explained burning and rusting through the phlogiston theory. According to this idea, any material that could burn contained phlogiston, a colorless, odorless substance that escaped into the air during combustion. A candle flame went out inside a closed jar, supporters argued, because the air inside had become saturated with phlogiston and could absorb no more.
Priestley accepted this framework. He did not set out to disprove phlogiston theory but to understand how different kinds of air behaved. The Royal Society of Chemistry notes that Priestley was one of the first scientists to systematically collect and test gases, which he called “airs” Royal Society of Chemistry. He built simple pneumatic troughs to capture gases over water or mercury, allowing him to study their properties.
The Key Experiment: Heating Mercuric Oxide
Priestley performed his most famous experiment on August 1, 1774. He placed a sample of red mercuric oxide (HgO) into a glass tube and used a large burning lens to focus sunlight onto the compound. The heat caused the oxide to decompose, releasing a colorless gas.
Priestley collected the gas over mercury, which was important because mercury does not dissolve gases as readily as water does. He then performed several tests. He inserted a glowing splint into the gas, and the splint burst into flame, burning much more brightly than it did in ordinary air. He placed a small mouse in a container filled with the gas, and the mouse survived about four times longer than a mouse in a similar volume of ordinary air. He breathed the gas himself and reported that it felt “light and easy” in his lungs.
These observations convinced Priestley that he had found a superior kind of air. Because it supported combustion and respiration so well, he reasoned that it contained little or no phlogiston. He called it “dephlogisticated air.”
The burning lens was a twelve-inch diameter glass lens that could focus sunlight into a spot about one inch across. The temperature at the focal point was high enough to melt gold or silver. Using sunlight as a heat source let Priestley heat substances in sealed glass containers, which prevented any combustion gases from mixing with the sample. The Science History Institute points out that Priestley’s skill with pneumatic chemistry and his willingness to test gases on himself set his work apart from earlier gas researchers Science History Institute.
The Role of Antoine Lavoisier
Priestley visited Paris in October 1774 and told Lavoisier about his discovery. Lavoisier repeated the experiment and quickly realized that Priestley’s “dephlogisticated air” was actually a new element. Lavoisier had been developing an alternative to phlogiston theory based on the role of a substance he called oxygen (from Greek words meaning “acid former”).
Lavoisier showed that combustion and rusting both involve combination with oxygen, not loss of phlogiston. He also demonstrated that Priestley’s gas made up about one-fifth of ordinary air. This reinterpretation was critical because it replaced a vague theoretical substance with a measurable, weighable element. Priestley, however, never accepted Lavoisier’s oxygen theory. He continued to defend phlogiston until his death in 1804.
Why the Discovery Matters
Priestley’s experiment with mercuric oxide had several lasting consequences. It provided the first clear method for isolating oxygen in pure form. It gave Lavoisier the experimental evidence he needed to overthrow phlogiston theory. It also helped establish pneumatic chemistry as a serious branch of science.
The discovery changed how scientists thought about respiration. Before Priestley, most researchers believed that breathing simply cooled the blood or helped circulate it. Priestley’s work showed that air actually participates in the body’s chemistry, although the full details of cellular respiration would not be worked out for another century.
Modern medicine relies on oxygen therapy for conditions ranging from pneumonia to carbon monoxide poisoning. The American Academy of Ophthalmology notes that supplemental oxygen is also used to treat certain eye conditions, such as retinal artery occlusion, by increasing oxygen delivery to the retina American Academy of Ophthalmology. This application traces back to Priestley’s observation that his “dephlogisticated air” could sustain life more effectively than ordinary air.
Key Takeaways
- Joseph Priestley discovered oxygen in 1774 by heating mercuric oxide with a burning lens and collecting the released gas.
- He called the gas “dephlogisticated air” because he interpreted his results within the phlogiston theory that most chemists accepted at the time.
- Priestley’s tests showed that the gas supported combustion and respiration more vigorously than ordinary air.
- Antoine Lavoisier repeated the experiment, recognized the gas as a new element, and used it to develop the modern theory of combustion.
- Priestley never accepted Lavoisier’s oxygen theory, but his experimental work provided the foundation for modern chemistry and oxygen therapy.
