Retrospect and Prospect

In view of their scarcity and failure to form chemical compounds, it is not surprising that noble gases remained unknown until relatively late in the history of chemistry. The first known experimental indication of their existence was a persistent gaseous residue after chemical removal of nitrogen and oxygen from air, as noted by Cavendish in 1784; the residue was small, however, “not more than 1/120th part of the whole,” presumably attributed to experimental error, and in any case subsequently ignored. The first definitive identification came when several observers found a previously unknown line in the spectrum of the solar chromosphere during the 1868 eclipse; this was quickly recognized to belong to a new element, not yet known on earth, which was named helium (ηλιov: sun); of course, no chemical characterization was possible.

The actual “discovery” of the noble gases came principally from the work of Rayleigh and Ramsay in the late nineteenth century. In 1892 Rayleigh reported that nitrogen prepared from ammonia was consistently less dense (by 0.5%) than “nitrogen” prepared from air (by removal of oxygen, carbon dioxide, and water). Both Rayleigh and Ramsay, working in collaboration, followed up this experimental clue; pursuing the possibility that the density difference reflected admixture of a heavier gas in air, they, like Cavendish, found a residue when chemically reactive species were removed from air.