The Infra-red Optical Properties of Some Sulphides: A Balanced Method of Using the Bolometer
T HE relation between the optical properties of compounds and their chemical properties has been investigated for a number of substances in the visible and ultra-violet regions of the spectrum. Certain organic compounds have also been experimented with in the infra-red spectrum in a systematic manner, but little has been done in this region with classes of inorganic compounds (including minerals) if we except the work of Nichols and his students on the reflection from salts of the alkaline
... f the alkaline earths, 1 of Clark on the reflection from chromates, 2 of Morse on the reflection from carbonates, 3 of Rubens and Hollnagel on the reststrahlen from salts of the alkalies, 4 and of Coblentz on the r61e of water in minerals. 5 Perhaps the field seems unpromising on account of the difficulty of obtaining good pieces of the right dimensions of the solids in question, and polishing their surfaces. Last year Prof. A. Trowbridge, noting the perfect cleavage in one direction of the minerals molybdenite and stibnite, was led to investigate the optical properties of thin layers of these substances. He used an infra-red spectrograph of moderate resolving power, and obtained approximate values for the extinction modulus %x for the two substances. On observing the transmission of very thin cleavage layers of molybdenite there were found broad absorption bands, regularly spaced, beginning at wave-length 5 or 6/x. The same piece exhibited bands close together in the very short region in the red end of the visible spectrum for which it was transparent. It was not known whether these bands were due to interference in the layer, or simply to ordinary absorption. The object of undertaking the present investigation was to repeat these measurements with apparatus of larger resolving power, and determine, if possible, the refractive indices. In addition, it was planned to experiment with other sulphides, and discover any common optical properties 1 PHYS.