A search for possible new members of the inactive series of gases

The gases of the inactive series are five in number, if the emanations from radium, thorium, and actinium be excluded. Beginning with helium, of atomic weight 4, neon follows, with atomic weight 20; then argon 40, krypton 82, and xenon 128. It is conceivable that a gas lighter than helium might be found ; an attempt was made by Mr. J. E. Coates, working in my laboratory in 1906, to isolate such a gas, but without success.* The residues “14” and “ 15,” consisting of helium and neon, have now been exhaustively investigated by Mr. Watson.† There are gaps in the periodic table for two, or possibly three, gases of the inactive series of atomic weight higher than that of xenon. Owing to the kindness of M. Georges Claude, M. Helbronner, and the directors of the “Société Air Liquide," all the heavier portions of gas remaining after the fractional distillation of no less than 120 tons of air were delivered here from their works at Boulogne, near Paris. The investigation was carried out almost entirely by Professor Moore, to whom the account which follows is due.‡. As will be seen, no positive result has been obtained; but it is rendered exceedingly unlikely that any stable gas of the inert series with atomic weight higher than that of xenon exists in the atmosphere. It is possible to go further; it may be stated with certainty that if such heavier gases exist they must he found in the atmosphere. From the known gradation of properties in passing from helium to xenon, it is certain that the missing elements must also be gases; and it is almost equally certain that they would form no compounds. The methods of separation of these gases, though laborious, are simple ; it is not possible to overlook them, or to fail to recognise them spectroscopically, if present. Hence it follows that, if they are not found in the atmosphere, they either do not exist at all or they are so unstable that they decompose or “disintegrate” during the processes of separation.