Krypton and Xenon: Liquid-Air Residues Cleaned and Fractionated at the Temperature of Liquid Air (Ramsay and Travers, 1901)

When liquid air boils away, the heavy noble gases stay in the last of it. Ramsay and Travers took that residue, stripped out the oxygen and nitrogen, and separated what was left by condensing it in liquid air and letting it boil off in fractions: argon first, then krypton, and xenon last of all. Result: krypton and xenon gases (a litre of liquid air gives at most 0.1-0.2 cc of xenon). Source: M. W. Travers, The Experimental Study of Gases (1901), pp. 218-222. Hydrogen is flammable; liquid air is very cold. Picture drawn by Youblob.