Henry Cavendish
Royal Society hears Cavendish's "Experiments on Air"
View full image ↗ How was this image made?
The creative brief sent to the image model. These are instructions, not a record of what actually happened.
Original prompt & settings (JSON) ↗The experiment now famous for undermining phlogiston was explained, by the man who ran it, using phlogiston.
15 January 1784: gas vanishes, glass goes wet
Henry Cavendish's paper "Experiments on Air" began to be read before the Royal Society of London on 15 January 1784; that is the read date printed on the paper itself.
He described igniting inflammable air together with common or dephlogisticated air inside closed vessels. Gas disappeared and dew formed on the inside of the glass. The reading reportedly ran across two meetings, so 15 January marks where it started rather than where every page was heard.
Testing the dew
Cavendish did not simply note the moisture. He examined it: no colour, no taste, no smell, and no sediment left behind after evaporation. On that basis he judged it to be water.
Cavendish also measured the gases quantitatively. He reported that 423 measures of inflammable air were nearly sufficient to phlogisticate 1,000 measures of common air, and that roughly one-fifth of the common air was converted to dew; to collect more of it he used a cylinder eight feet long.
A modern result in phlogiston terms
In modern terms the experiment synthesized water from hydrogen and oxygen. Cavendish used neither name and no modern formula — he presented his conclusion through a modified phlogiston theory.
He was not the first to see moisture after such an explosion either. John Warltire and Joseph Priestley had reported dew from exploding inflammable and common air, and it was their observation that sent Cavendish looking closer; James Watt independently proposed an account of water's composition, producing a later priority controversy.
Sources
Researched 23 Aug 2026 5 sources 2 audit passes, 1 issue open
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
What the sources leave uncertain
- The date is secure, but “reveals the composition of water” is a retrospective modern description of the experiment.
- Priority and interpretation are distinct questions: Cavendish produced and measured the water, while Watt and Lavoisier offered competing or differently framed accounts of what the result meant.