Argon was the first of the inert gases to be discovered on Earth. Its discovery illustrates curious omissions that sometimes occur in science.
In 1785 Cavendish mixed air with oxygen and performed several electrical discharges over a mixture of potassium hydroxide. By this process, he converted nitrogen of the air into potassium nitrate. Then, he removed the oxygen excess, but still remained a small amount of gas, not larger than 1/120 of the original volume of air. He considered that this difference had been caused by experimental errors.
Almost 100 years after, in 1894, Lord Rayleigh made a series of measurements of the density of nitrogen from several sources. He verified that the density of the nitrogen of the air (1,2572) was always larger than the density of the nitrogen present in compounds (1,2506). This difference was too big to be imputed to experimental errors, leading him to conclude that the atmosphere had to contain an heavier inert substance.
Lord Rayleigh, working with Ramsay, repeated the experience of Cavendish, removing the nitrogen with heated magnesium. The residue revealed an unknown spectrum, and it was shown that this new gas was not combined with the nitrogen. Argon was called (from the Greek word for "inactive").