inactive preparations of radiferous barium are strongly luminous. The writer has seen a preparation of impure radium bromide which gave out a light sufficient to read by in a dark room. The luminosity of radium persists over a wide range of temperature and is as bright at the temperature of liquid air as at ordinary temperatures. A slight luminosity is observed in a solution of radium, and if crystals are being formed in the solution, they can be clearly distinguished in the liquid by their greater luminosity.
117. Spectrum of the phosphorescent light of radium and actinium. Compounds of radium, with a large admixture of barium, are usually strongly self-luminous. This luminosity decreases with increasing purity, and pure radium bromide is only very feebly self-luminous. A spectroscopic examination of the slight phosphorescent light of pure radium bromide has been made by Sir William and Lady Huggins8. On viewing the light with a direct vision spectroscope, there were faint indications of a variation of luminosity at different points along the spectrum. In order to get a photograph of the spectrum within a reasonable time, they made use of a quartz spectroscope of special design which had been previously employed in a spectroscopic examination of faint celestial objects. After three days’ exposure with a slit of ¹⁄₄₅₀ of an inch in width, a negative was obtained which showed a number of bright lines. The magnified spectrum is shown in Fig. 46 A. The lines of this spectrum were found to agree not only in position but also in relative intensity with the band spectrum of nitrogen. The band spectrum of nitrogen and also the spark spectrum9 of radium are shown in the same figure.
Some time afterwards Sir William Crookes and Prof. Dewar showed that this spectrum of nitrogen was not obtained if the radium was contained in a highly exhausted tube. Thus it appears that the spectrum is due to the action of the radium rays either on occluded nitrogen or the nitrogen in the atmosphere surrounding the radium.