(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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Summary.Prussic acid, identified during the early part of the nineteenth century as a constituent of many members of the Rosaceae, is now known to be present in plants from about fifty natural orders. It is generally considered to exist in the living plant exclusively in glucosidal combination, but certain workers, including Treub and his colleagues, have postulated the existence of free cyanide in plant tissues.Ten cyanophoric glucosides have been isolated in crystalline form. Of these, seven are derivatives of benzaldehyde cyanhydrin; linamarin and gynocardin contain ketone groupings, and lotusin, the glucoside of Lotus arabicus is a derivative of γ‐pyrone combined with a sugar cyanhydrin. The glucosides prunasin, sambunigrin, prulaurasin, linamarin and amygdalin have been prepared synthetically.Amygdalin, prunasin and prulaurasin seem to be restricted to the order Rosaceae, sambunigrin to the Caprifoliaceae, and dhurrin to the Graminaceae; while vicianin has so far been found only in a few species of the genus Vicia. Linamarin, on the other hand, has been found in several natural orders, widely separated in morphological classification.In the plant, cyanophoric glucosides are, with few exceptions, accompanied by active, cyanide‐liberating, hydrolytic enzymes. The conception as to the specificity of such enzymes, as postulated by many authors, may need some modification in the light of the recent work of Willstätter and his pupils.The concentration and seasonal variation of prussic acid in the plant show considerable differences in the several cases known. In general, the concentration is greatest in young, growing organs; Sambucus nigra, on the other hand, shows little seasonal variation in cyanide content. In other cases, cyanide has been shown to exist in quantity in such tissues as the bark. In certain of the Rosaceae, cyanophoric glucosides appear to be constant constituents of the plant throughout its life cycle, while in the Graminaceae and in Lotus arabicus, they disappear at maturity and are absent in the seed.In some cases, the concentration of prussic acid in the plant has been shown to be diminished by cultivation (Amydalis communis, Vicia angustifolia, Phaseolus lunatus). Climatic conditions, especially drought, have furthermore been stated to cause variations in the content of dhurrin in Sorghum vulgare and of the cyanophoric glucoside of Lotus corniculatus. Temperatures below freezing point have been observed to cause a rapid and conspicuous increase in the glucoside content of Sorghum vulgare and in Prunus laurocerasus.The function of prussic acid in the plant is uncertain. Since the researches of Treub on the localisation of cyanophoric substances on Pangium edule, many workers have considered that cyanides may represent the first stage in the synthesis of organic nitrogen compounds by the plant. The rapid disappearance of cyanide under conditions of starvation, as observed in Prunus laurocerasus, supports the view that such nitrogen is readily utilizable by the plant. Other workers regard cyanophoric glucosides as excretory products, or, in virtue of their poisonous properties, as protective agencies.Quantitative investigations as to nitrogen partition in cyanophoric plants at various stages of growth might help to elucidate the important question as to the participation of prussic acid in protein synthesis by the plant. At the present time, adequate data on this subject are not available.

Heat pretreatment is recommended for detecting FIX inhibitors in samples with residual FIX:C. The heat/cold modification improved the sensitivity of the Nijmegen-Bethesda assay, resulting in higher tolerance for residual FIX:C.

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