Line Absorption Spectra of Solids at Low Temperatures in the Visible and Ultra-Violet Regions of the Spectrum a Preliminary Study of Gd${\mathrm{Cl}}_3$·6${\mathrm{H}}_2$O from Room Temperature to that of Liquid Hydrogen

This work is the ${\mathrm{beginning}}^1$ of a systematic investigation of the absorption spectra of solids, particularly single crystals of the ions of the rare earths, over a range of temperatures. We expected that the ions of the rare earths in the solid state would exhibit line absorption spectra because their magnetic moments agreed quantitatively with ${\mathrm{theories}}^2$ which had been successful in elucidating the line spectra of gases. The latter definitely established that only the $4_4$ electronic shell is incompletely filled and extrapolation of the energies from x-ray data led to the conclusion that the energy necessary to remove a $4_4$ electron might possibly be measured through quartz.