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It has been known since the early days of spectroscopy that there is a group of band spectra associated with the halogen salts of the alkaline earths. Probably the first distinction between the oxide and chloride bands of these elements was made by Lecoq de Boisbaudran when he observed relative intensity fluctuations between two band systems on introducing hydrochloric acid vapour into a flame charged with calcium chloride. The system intensified by the acid vapour he attributed to the chloride. The other halides were also found to yield characteristic spectra. Since their discovery they have been the subject of only one publication of any real note. Olmsted made a careful study of these spectra as they are found under flame excitation, and his catalogue of bands contains the only reliable data concerning these spectra which is available. An exception must be made of the fluoride spectra, which have been closely studied by Dufour, Datta, and others. The origin of the present investigation of these bands was the discovery that it is possible to observe them, very conveniently, in absorption against a continuous background spectrum. In the course of attempts to find band spectra of the alkaline earth metals (corresponding to those of the alkali metals) a pair of bands at about λ 6200 was observed in the absorption spectrum of a column of calcium vapour at temperatures of 900°C. and upwards. On measurement, these bands, which are reproduced in Plate 2, figs. 1, 2 and 5, were found to coincide with two prominent calcium chloride bands. Ordinary commercial calcium had been used for the experiments, and on analysis approximately 0·05 per cent, of chloride was found in it. It seemed evident that the development of the bands was due to this impurity in the metal, and the matter was clinched by adding a trace of bromide to the metal before heating, when the calcium bromide bands in the red also appeared prominently in absorption.

The alkaline earth halide spectra and their origin