Argon - 18Ar: historical information
Argon was discovered by Lord Rayleigh and Sir William Ramsay in 1894. It was isolated by examination of the residue obtained by removing nitrogen, oxygen, carbon dioxide, and water from clean air. In fact, air contains less than 1% argon. The atmosphere of Mars contains less than 2% argon. It was recognized by the characteristic lines in the red end of the spectrum.