THE Comptes-rendus des travaux du Laboratoire Carlsberg, vol. 18, No. 5, 1930 (Copenhagen: H. Hagerup, 1930. 6·25 Kr.), contains a long paper (in English) by Prof. S¸rensen on recent work on the soluble proteins considered in the light of his theory of reversibly dissociable component systems. According to this, soluble proteins consist of a series of complexes or components, reversibly combined, Ax, By, Cz . . ., these components being, for example, mainly polypeptides, but in other cases containing phosphorus, in each of which the atoms are linked by principal valencies, whereas the complexes themselves are held together comparatively loosely and reversibly by residual valencies. The strength and nature of these residual valencies depend on the chemical composition of the component in question as well as on its physical properties-above all on its dimensions and the resulting shape and surface. Alterations in the composition of the solution (salt content, pH, alcohol content, temperature) may give rise to reversible dissociation of the component systems and interchange of components between them. When these alterations in composition are so suited as to render possible the formation in sufficient quantities of a component system insoluble or sparingly soluble under the new conditions, such a system will be formed and precipitated.