What did Henry Cavendish discover about the composition of atmospheric air in 1785?

In 1785, Henry Cavendish conducted a precise investigation into the composition of atmospheric air. After determining the proportions of phlogisticated air (nitrogen) and dephlogisticated air (oxygen), he observed a small, persistent residue of gas that amounted to approximately 1/120 of the volume of the nitrogen.1

While this residue was initially considered problematic, it was later realized in the 1890s by William Ramsay and Lord Rayleigh that Cavendish had actually discovered the presence of argon, an inert gas. His findings were a testament to his rigorous experimental methods, use of precision instruments, and careful accounting for potential sources of error.1