Stellar, Solar and Laboratory Spectra: The History of Lockyer's Proto-elements

Until now studies on the historical development of atomic spectroscopy have focused on three main aspects-its first applications as a method of chemical analysis, the formulation of spectral laws (the empirical laws of Balmer, Rydberg, Kayser-Runge, etc.), and the rise of the old quantum theory. These developments of spectroscopy were based on the same assumption: the invariance of the atomic spectrum after fixing the chemical element (if any degree of change was expected, this was considered the result of magnetic or electric fields). This paper shows that running alongside these lines of research there was another, no less important area of study based on the negation of this assumption. The focus of these latter studies was the behaviour of atomic spectra as a function of different working conditions. English astrophysicist Sir Norman Lockyer played a central role in this research. While studying the effect of temperature on spectral lines and comparing stellar spectra with laboratory spectra, he discovered a new class of spectral lines (the 'enhanced lines'). In an attempt to explain the origin of these lines, Lockyer, at the beginning of 1897 (a few months before J. J. Thomson's discovery of the electron), hypothesized the 'dissociation' of the chemical elements into simplified forms of matter, named proto-elements. Lockyer's studies are mentioned by Thomson in the paper which introduces the corpuscle (or electron) for the first time. In this paper we have tried to reconstruct the history of Lockyer's proto-elements and highlight the links between them and Thomson's corpuscle. Our analysis has shown that not only the famous studies on discharges in rarefied gases but also Lockyer's stellar spectroscopy played an important role in the work of J. J. Thomson, convincing him that the atom was complex and divisible.