The methods now in use for obtaining liquid air, referred to in a subsequent chapter, enable large quantities of that material to be obtained readily; and it was in investigating spectroscopically the residues left after volatilising a quantity of liquid air that Ramsay and Travers, in 1898, detected the existence of two new monatomic gaseous constituents of the air which they named respectively _krypton_ (χρυπτός, hidden) and _neon_ (νέος, new), the former heavier and the latter lighter than argon. By fractional distillation of the argon, simultaneously procured, a gas was obtained which in the spectroscope showed the characteristic lines of helium—previously recognised in atmospheric argon by Kayser and Friedländer—together with a complicated spectrum consisting of a number of lines in the red, orange, and yellow due to the new element neon. On cooling this mixture to -252° by means of liquid hydrogen, the neon solidified, while the helium remained gaseous and could thus be separated. Krypton was obtained from the residues left on the evaporation of a large quantity of liquid air. Mixed with the krypton was a third gaseous constituent of air, to which the name _xenon_ (ξενος, the stranger) was given. The boiling-point of krypton at atmospheric pressure was found to be -152°, and its melting-point -169°; the boiling-point of xenon was -109° and its melting-point -140°. Their critical temperatures were respectively -62°.5 and +14°.7. Hence xenon could be liquefied by pressure a very little below the mean temperature of the air. Neon boils at -243° and freezes at -253°. They form colourless liquids freezing to ice-like solids. All of them, with the exception of argon, which is present to the extent of about 1 part in 107 parts of air, are contained in extremely small amounts in the atmosphere, approximately in the following proportions: Helium 1 part in 245,300 parts by volume. Neon 1 ” ” 80,800 ” ” Krypton 1 ” ” 20 millions ” ” Xenon 1 ” ” 170 ” ” ” Many tons of liquefied air have since been systematically fractionated, but no other gas than those above named has been obtained.