Within a few years the study of obscure radiation has been greatly advanced by systematic inquiry into the laws of dispersion of the infra-red rays by Langley, Rubens, Rubens and Snow, and others. Along with this advancement has come the more extended study of absorption in this region. The absorption of atmospheric gases has been studied by Langley and Angstrom. Angstraom has made a study of the absorption of certain vapors in relation to the absorption of the same substances in the liquid state, and the absorption of a number of liquids and solids has been investigated by Rubens.
In the present investigation, the object of which was to extend this line of research, the substances studied were: plate glass, hard rubber, quartz, lamp-back, cobalt glass, alcohol, chlorophyll, water, oxyhemoglobin, potassiun alum, ammonium alum, and ammonium-iron alum.