I’ve been reading about the discovery of radium by Pierre and Marie Curie (see the 1938 commemorative stamp above). In particular, I’ve been reading Marie Curie’s impressive doctoral thesis. Prior to calculating the atomic weight of radium, Marie Curie arranged for Eugène-Anatole Demarçay to photograph the spectrum. In her words:
“It was of the first importance to check, by all possible means, the hypothesis, underlying this work, of new radioactive elements. In the case of radium, spectrum analysis was the means of confirming this hypothesis.
M. Demarçay undertook the examination of the new radioactive bodies by the searching methods which he employs in the study of photographic spark spectra.
The assistance of so competent a scientist was of the greatest value to us, and we are deeply grateful to him for having consented to take up this work. The results of the spectrum analysis brought conviction to us when we were still in doubt as to the interpretation of the results of our research.
The first specimens of fairly active barium chloride containing radium, examined by M. Demarçay, exhibited together with the barium lines a new line of considerable intensity and of wave-length λ = 381.47 [nm] in the ultra-violet. With the more active products prepared subsequently, Demarçay saw the line 381.47 [nm] more distinctly; at the same time other new lines appeared, and the intensity of the new lines was comparable with that of the barium lines. A further concentration furnished a product for which the new spectrum predominated, and the three strongest barium lines, alone visible, merely indicated the presence of this metal as an impurity. This product may be looked upon as nearly pure radium chloride. Finally, by further purification, I obtained an exceedingly pure chloride, in the spectrum of which the two chief barium lines were scarcely visible.”
The image below shows the spectrum of radium, as defined by the NIST Atomic Spectra Database Team. Superimposed in colour is this visible spectrum, and (in the grey box) the spectrum measured by Demarçay (click to zoom):