OF CONTENTS Introduction Physical factors affecting spectra Chemical factors affecting spectra Adsorption of substance on particles of dispersing medium Chemical reaction of substance with medium Polymorphism of substance Mixed-crystal formation Distortion of the crystalline structure Formation of complexes between substance and medium THE fundamental principles of the analysis of gases, liquids and solutions by infra-red spectroscopy are well established, and the techniques that have to be used are generally well known.Very early on, infra-red spectroscopists were faced with the problem of finding a method of recording solid-phase spectra. The method based on mulling the sample with Nujol, or with fluorinated or chlorinated hydrocarbons, has been in use for several years and is still standard practice for many purposes, such as quantitative analysis and structural diagnosis.There are, however, some practical difficulties in the use of solid-phase spectra for quantitative analysis: accurate control of sample thickness, reflectivity and scatter is much more difficult to achieve than with solutions. The internal-standard methodl ,2 has been used with success, and a good accuracy has been attained in the analysis of multicomponent mixtures by combining the internal-standard method with the method of compensated extinctions ; however, the method runs rapidly into some difficulties owing to Beer's law not being applicable.The potassium bromide technique, in which the infra-red absorbing solid, finely ground and dispersed in a powdered halide, is pressed into a clear disc, proposed in 1952 by Stimson3 and Schiedt,* was received with much enthusiasm; the absence of adsorption bands in the dispersing medium and the possibility of controlling the sample weight accurately are certainly two important advantages of the disc technique over the mull method. However, I would like to draw attention to the difficulties that are encountered, both in general in quantitative measurements of absorption spectra of solids on the one hand (physical factors), and on the other hand, to the difficulties that arise through the use of alkali halide discs (the chemical and physico-chemical factors).

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We have measured the standard potentials of the system U(III)/U(0), in the molten LiCl‐KCl eutectic, within the temperature range from 400 to 550°C. The cell used has been the following: The work has been carried out in glove‐boxes, under inert atmosphere, and special care has been taken in preventing even the slightest oxidation of U(III). A comparison has been made with previous results, obtained by different Authors.