How did Cavendish’s unexplained residue in air later help identify argon?

In 1785, while investigating the composition of atmospheric air, Henry Cavendish observed that after removing all oxygen and nitrogen from a sample, a tiny residue remained, amounting to approximately 1/120th of the original nitrogen volume.1

Although Cavendish could not identify this substance at the time, his rigorous quantitative methods ensured the accuracy of his observation. Over a century later, in the 1890s, Lord Rayleigh noticed a discrepancy in the density of nitrogen obtained from the atmosphere versus nitrogen derived from chemical compounds. Recalling Cavendish’s earlier work on the oxidation of atmospheric nitrogen, Rayleigh and William Ramsay hypothesized that this unexplained residue was a previously undetected constituent of the atmosphere. Their subsequent research confirmed that this residue was argon, an inert gas, thereby validating the precision of Cavendish’s original experiments.12