The periodic law (see Element) permits a grouping of the elements according to their valency as follows: - Group 0: helium, neon, argon, krypton and xenon appear to be devoid of valency.
Travers have obtained evidence of the existence in the atmosphere of three new gases, besides helium, to which have been assigned the names of neon, krypton and xenon.
In addition to these, he mentions other auroral lines as very probably krypton lines, but in their case the wave-lengths which he quotes from Paulsen (22) are given to only three significant figures, so that the identification is more uncertain.
The majority of the krypton lines which Baly identifies with auroral lines require for their production a Leyden jar and spark gap.
C. C. Baly (21), making use of the observations of the Russian expedition in Spitsbergen in 1899, accepts as the wave-lengths of the three principal auroral lines 557 o, 4276 and 3912; and he identifies all three and ten other auroral lines ranging between 5570 and 3707 with krypton lines measured by himself.