How did Henry Cavendish first detect evidence of a chemically inactive gas in air?

In 1785, Henry Cavendish conducted a rigorous investigation into the composition of atmospheric air using highly precise experimental methods. After carefully removing the known components of air—specifically nitrogen (which he referred to as phlogisticated air) and oxygen (dephlogisticated air)—he observed that a small volume of gas remained.12

This residual gas amounted to approximately 1/120th of the volume of the nitrogen. Although Cavendish could not identify the substance at the time, his meticulous work provided the first evidence of a chemically inactive gas in the atmosphere. A century later, this "problematic residue" was identified by William Ramsay and Lord Rayleigh as argon, the first noble gas to be discovered.12