The bands in the spectra of stars of Class are found to correspond with those of ZrO₂, zirconium oxide.[336] A comparison [Pg 84] of the furnace spectrum of zirconium oxide with that of titanium oxide, which produces the characteristic flutings in Class , indicates that titanium oxide persists to lower temperatures.[337] It is of interest to note that the only oxides, other than water, which have been detected in stellar spectra, are those of elements in the fourth column of the periodic table, namely carbon, titanium and zirconium. Probably this has a chemical interpretation. The presence of silicon dioxide SiO₂ has not been detected, although it might be anticipated. NIOBIUM (41) Rowland identifies some of the lines associated with niobium in the solar spectrum. The series relations are unknown, and the lines are too faint to be detected in stellar spectra. MOLYBDENUM (42) All the ultimate lines of molybdenum are present in the solar spectrum. They are too faint to be detected in the stars. The spectrum has been analyzed into series by Kiess,[338] and by Catalan.[339] RUTHENIUM (44) RHODIUM (45) PALLADIUM (46) The strongest lines in the spectra of the three lighter platinum metals are all present in the solar spectrum,[340] but are too faint to be traced in the spectra of stars. Series relations are as yet unknown. The heavier platinum metals, osmium (76), iridium (77), and platinum (78), are not certainly found in the solar spectrum. SILVER (47) [Pg 85] The ultimate () lines of silver are at 3281 and 3383. They are both faintly present in the solar spectrum. The doublet at 4669, 4476 () is also probably present in the spectrum of the sun. The lines cannot be traced in stellar spectra. TIN (50) A line of neutral tin was reported by Lunt[341] in the spectrum of Scorpii. Series relations are as yet unknown. In view of the absence of identifications of related lines, the attribution cannot be regarded as very certain. The strongest line in the spectrum of ionized tin is stated by the same investigator to lie at 4585.80. This line is either absent from or exceedingly weak in the solar spectrum. BARIUM (56) Neutral barium is not certainly present in stellar spectra. The first two ultimate lines () fall in the red and the ultra-violet respectively, and would therefore escape notice in the stars. No lines of corresponding wave-length appear in Rowland’s table, but they are found in sunspots.[342] The groups are also absent from the solar spectrum. The strength of the barium lines in the sun has been thought by Russell to be abnormal, and the question has been considered by several investigators.[343][344][345] IONIZED BARIUM Ionized barium is represented by the lines which are present, though weak, in the sun, and by the lines, which appear[346] at , and increase in intensity for all cooler stars. The line is at 4555, and its behavior is difficult to trace, as it is much blended. THE RARE EARTHS (57-71)