Summary: TO a reader who has not followed recent American work on solubility, the contents of Prof. Hilde-brand's book provide a pleasant surprise. Instead of a mere compendium of data, there is here a real attempt to develop a general theory of solubility, and of the even wider problems of miscibility in liquids, and of the properties of the resulting mixtures. The starting-point of this general theory is not the familiar Law of Osmotic Pressure of van't Hoff, but the more fundamental Law of Vapour Pressures of Raoult. Thus, in the case of certain mixtures, Raoult's law is valid over the whole range of concentrations from? to 100 per cent., whereas van't HofFs law is limited to dilute solutions and, even when modified, cannot be.extended in a similar way. An ideal solution is therefore defined as one which obeys Raoult's law at all temperatures and all pressures. This depends on the internal forces remaining undisturbed on substituting molecules of solute for molecules of solvent. The author discusses what are the properties of the individual liquids, the equality of which is likely to lead to this result, and concludes that equality of “internal pressure “is the most important factor. Moreover, if this condition is realised at one temperature and pressure, it is likely to be realised over the whole range.