Discovery of oxygen

1 August 1774: sunlight on a red powder

In his laboratory at Bowood House in Wiltshire, on 1 August 1774, Joseph Priestley focused sunlight through a burning lens twelve inches across and twenty inches in focal length onto red mercuric oxide — then called mercurius calcinatus per se — held in a glass vessel inverted over mercury.

The material readily gave off air. Priestley collected roughly three or four times the bulk of the starting material, which was already more than the substance had any obvious business yielding.

Water, a candle, and a piece of red-hot wood

He admitted water to the collected gas, expecting it to be absorbed as several of the airs he had worked with were. It was not.

A candle then burned in it with what he called a remarkably vigorous flame, far more intensely than he expected. A red-hot piece of wood put into the gas did not merely glow: it sparkled and was consumed rapidly.

Priestley wrote that he was utterly at a loss how to account for it. He read the result inside phlogiston theory rather than as the isolation of an element, and by the end of the day he had a collected, water-insoluble gas that supported combustion violently — an anomalous experimental result and nothing more settled than that.

The mouse, the name and the priority came later

Priestley soon left with his patron Lord Shelburne for continental Europe and did not resume systematic work on the gas until March 1775; the famous mouse test belongs to 8 March 1775, when he observed survival of about an hour in the new gas against roughly fifteen minutes in ordinary air. Nothing was named oxygen on 1 August: Priestley called it dephlogisticated air, Antoine Lavoisier later proposed the name and folded the gas into a new account of combustion, and Carl Wilhelm Scheele had probably produced it independently in the early 1770s but published only in 1777.

Which is why "the third discovery of oxygen" is a label rather than a fact — first production, first publication, correct identification and naming point to four different moments. Priestley supplied one of them, and spent the rest of his life defending the theory that his own gas would demolish.