It is well known that salts such as the chlorides of the alkali metals are less soluble in water containing the corresponding acid than in water alone; and for sparingly soluble salts, in weakly acid solution, the decrease of solubility has been shown to follow quantitatively the course predicted from simple ionic notions. With concentrated solutions of strong electrolytes, however, the causes which operate are still obscure; and, indeed, much has yet to be discovered as to the facts in such cases. There may be said to be two methods of studying the isothermal phenomena in question. The first, which has received attention from numerous observers, consists in determining the solubility of a salt in water containing the acid in varying concentrations. This may be termed the “solubility“ method. The second method seeks to ascertain the minimum concentration of aqueous acid which must be used in order to cause deposition of salt, when the acid is added in small quantity to the saturated aqueous salt solution. This we shall refer to as the “precipitation“ method; it was adopted by Gibson and Denison, and formed the subject of a communication by them to this Society (Proc., vol. xxx., 1910, p. 562).