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(1) INTRODUCTION.THE refractive index of a protein solution is greater than that of pure water, and, according to the observations of Reiss [1904] and Robertson [1912], the difference between the refractions of the protein solution and the solvent is equal to the protein concentration in g. per 100 cc. of solution, multiplied by a constant. This constant is termed the specific refraction increment or the specific refraction of the protein.A critical account of the work of previous investigators on the refractions of the proteins in normal and in pathological sera has been published by Schretter [1926] who has drawn attention to the question as to whether the specific refraction increments show physiological or pathological variations. The results obtained by different observers show a wide range of variation in normal as well as in pathological sera, but further investigation is required in view of the risks of error due to the relatively low stability of the proteins and their association with lipins and other impurities.The first problem investigated in this work is the preparation of a purified solution of crystalline horse serum-albumin under conditions which minimise the risk of error due to alterations in the protein. The purity and stability of the preparations have beef tested by comparing their specific refraction increments after successive recrystallisations and after different periods of dialysis in accordance with the procedure adopted by Hopkins [1900] in establishing the individuality of egg-albumin by measurements of the optical rotation of the protein after successive recrystallisations.The second problem is the application of Donnan's theory of membrane equilibrium to measurements of the refractive indices of serum-albumin andserum-globulin solutions which have been enclosed in collodion membranes and dialysed against phosphate mixtures of well-defined hydrogen ion concentration. An experimental investigation of the effects of the unequal distribution of salts on refraction is of value in determinations of the specific refraction increment of globulin, which cannot be dissolved in the absence of electrolytes.

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