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Priestley turns up twice in your syllabus: in chemistry as a discoverer of oxygen, and in Class 11 biology at the start of the photosynthesis chapter, with a mouse, a candle and a mint plant. This post puts both stories together, quotes what Priestley himself wrote, and shows why the man who isolated oxygen never accepted the modern explanation of it.
Who was Joseph Priestley?
The Science History Institute's biography describes Priestley (1733 to 1804) as "primarily a minister" with strong interests in education and politics. He taught at Warrington Academy, a school for religious dissenters, and wrote a well-received history of electricity in 1767 with encouragement from Benjamin Franklin, which earned him election to the Royal Society.
In the late 1760s he began studying different kinds of "air". He found a way to dissolve fixed air (carbon dioxide) in water, making the first carbonated water, and the Royal Society gave him its top honour in 1773. Through the 1770s he identified at least eight new kinds of air. His first long report, Observations on different kinds of air, filled more than a hundred pages of the Philosophical Transactions in 1772.
What happened on 1 August 1774?
Priestley told the story in the second volume of his Experiments and Observations on Different Kinds of Air, published in 1775. He had obtained a burning lens "of twelve inches diameter, and twenty inches focal distance", and used it to heat substances standing in vessels filled with mercury.
On 1 August 1774, he wrote, "I endeavoured to extract air from mercurius calcinatus per se", and "by means of this lens, air was expelled from it very readily". Water did not absorb the new air. Then came the surprise, in his own spelling: "what surprized me more than I can well express, was, that a candle burned in this air with a remarkably vigorous flame".
Mercurius calcinatus per se is mercury(II) oxide, HgO. In modern terms, heating it decomposes it:
2HgO gives 2Hg + O2
How much oxygen can mercury oxide give?
Here is the kind of calculation an exam might build on his experiment.
Suppose you heat 21.7 g of HgO. Its molar mass is 200.6 + 16.0 = 216.6 g/mol, so you have 21.7 / 216.6 = 0.100 mol.
The equation says 2 mol HgO gives 1 mol O2, so you get 0.0500 mol O2.
Mass of O2 = 0.0500 × 32.0 = 1.60 g.
Volume at 273 K and 1 bar, from V = nRT/p: 0.0500 × 0.08314 × 273.15 / 1 = 1.14 L.
A common mistake is to forget the 2:1 ratio and report 0.100 mol of O2. Write the balanced equation before you touch the numbers.
Why did he call it dephlogisticated air?
In the 1770s most chemists explained burning with phlogiston, a weightless principle of flammability that burning substances were thought to release into the air. As the Science History Institute explains, an air that kept a candle burning longer could, in that theory, soak up the most phlogiston, so Priestley called his new gas "dephlogisticated air".
He demonstrated it to visitors, including Antoine Lavoisier. Lavoisier saw that the gas was the part of the atmosphere that combines with substances when they burn and makes them heavier, renamed it oxygen and built a new chemistry around it. Our post on Lavoisier and conservation of mass covers that side. By the 1790s most chemists had accepted Lavoisier's view.
Priestley never did. His objections were not foolish: the Science History Institute notes he pointed out that the new chemistry could not explain how gunpowder worked, and that it rejected weightless phlogiston while accepting weightless light. But the new system explained far more, and it won. Another chemist, Carl Wilhelm Scheele, had made the same gas independently in Sweden and described it as "fire-air" in his Treatise on Air and Fire of 1777.
What do plants have to do with it?
The NCERT Class 11 chapter on photosynthesis opens its history section with Priestley. A candle in a closed bell jar soon went out, and a mouse in a closed space would soon suffocate. But when he placed a mint plant in the same jar, the mouse stayed alive and the candle kept burning. NCERT summarises his hypothesis as: "Plants restore to the air whatever breathing animals and burning candles remove." Jan Ingenhousz later showed that the plant needs sunlight to do this.
So the gas that made Priestley's candle blaze is the same one your biology book says green plants release in photosynthesis. Our photosynthesis guide for NEET picks up the story from there.
What happened to Priestley later?
His politics made him enemies. In 1791 a mob burned his house, church and laboratory in Birmingham, and in 1794 his family sailed for the United States. He settled in Northumberland, Pennsylvania, where he kept up his criticism of the new chemistry until his death in 1804.
What can a student take from Priestley's way of working?
Write down surprises. His notebook-style account records what he expected and what astonished him. Your practical file should too.
Use the right apparatus. He chose mercury because he judged it the most accurate method. In problems, notice the conditions given; they often decide the answer.
Hold theories loosely. Priestley's data were excellent; his interpretation was not. Be ready to change your explanation when a better one accounts for more of the facts.
If you can follow the history but the idea of oxidation still feels fuzzy, the Fermi Concept Library covers every concept in chemistry, biology, physics, mathematics and computing, one idea at a time.
Tonight, write the balanced equation for heating HgO, then redo the oxygen calculation above starting from 10.0 g instead of 21.7 g.
