May 29, 1766

Henry Cavendish

First part of Cavendish’s Experiments on Factitious Air read to the Royal Society

The substance presented in May 1766 would not be called hydrogen in print for roughly another two decades.

29 May 1766, before the summer recess

The Royal Society in London heard the first part of Henry Cavendish's investigation on 29 May 1766, having received it on 12 May. It was later published as "Three Papers, Containing Experiments on Factitious Air."

This first part concerned "inflammable air" made by reacting iron, zinc or tin with dilute sulfuric acid or hydrochloric acid.

The same air from three metals

Cavendish found that changing the metal, or using either acid, produced air with consistent measurable properties. That supported treating it as a particular species of air rather than ordinary atmospheric air.

He examined its production, combustibility and specific gravity, comparing gases by weighing a bladder before and after it was emptied. He reported a density about 7 to 11 times lighter than common air, depending on the calculation and the experimental source.

Explained as escaping phlogiston

Cavendish interpreted the result within phlogiston theory, proposing that the metal's phlogiston escaped and formed inflammable air.

The May reading was only the first installment. The second and third parts were read on 6 and 13 November, and all three appeared together in Philosophical Transactions volume 56, pages 141–184.

The 1766 paper did not report that burning inflammable air produces water; those experiments belong to Cavendish's 1784 paper "Experiments on Air." Calling him the discoverer of hydrogen is a retrospective convention — earlier experimenters had produced the gas, and he did not identify it as a modern chemical element.

Sources

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What the sources leave uncertain

Calling Cavendish the “discoverer of hydrogen” is a retrospective convention. Earlier experimenters produced the gas, while Cavendish was distinguished by collecting and systematically measuring it as a particular inflammable air.
Cavendish did not identify hydrogen as a modern chemical element; he worked within phlogiston theory.
The familiar claim that Lavoisier named hydrogen in 1783 is disputed by nomenclature scholarship. “Hydrogène” appears in the collaborative Méthode de nomenclature chimique of 1787, apparently first in Guyton de Morveau’s chapter.