Introductory remarks

H+3 was discovered by J. J. Thomson (1911), who spent most of his research life on studies of the discharges of electricity through gases. He studied negative (cathode) rays and discovered the electron in 1897; he then studied positive (canal) rays, extending Wien’s work, and identi­ ed the proton. When Aston became his assistant in 1910, an early prototype of a mass spectrometer resulted and H +3 was one of the earliest species to be discovered by the apparatus. A plate showing the trace of H +3 (Thomson 1912) is shown in ­ gure 1. While he correctly identi­ ed in the two papers the signal as being due to H +3 , he seemed to have had `discoverer’s doubt’ as to its identity and he refers to it as X3 in both editions of his monographs on the positive ray (Thomson 1913, 1921). In his autobiography (Thomson 1937), however, he clearly states `one of the ­ rst things discovered by the photographic method was the existence of H +3 ’. The younger generation did not have discoverer’s doubt and readily accepted H +3 . Dempster (1916) demonstrated the predominance of H+3 over H + 2 and H + in hydrogen plasmas and correctly explained the production of H+3 as due to a secondary reaction. Hogness & Lunn (1925) seem to be the ­ rst to have written down the celebrated ion{ neutral reaction,