In the year 1847, as one of the results of an investigation to determine the part played by the mass action of water in chemical reactions, H. Rose (Pogg. Ann., lxxxii. 545) showed that an acid sulphate in aqueous solution is progressively decomposed into free acid and neutral sulphate by increasing quantities of water. His observations were confirmed and extended by the thermo-chemical researches of Thomsen (Pogg. Ann. 1869, cxxxviii. 72), and Berthelot (Ann. Chim. Phys., 1873, xxix. 433), who indicated more exactly the course and extent of this decomposition, and from whose work it is known that in the solution of an acid sulphate there exist free sulphuric acid, neutral sulphate and acid sulphate. Finally Ostwald, in his first memoir on chemical affinity (Jour, prakt. Cliem. 1879, xix. 483), showed how to determine the magnitude of this decomposition, and was able to approximately measure the quantity of free acid in solutions of the acid sulphates at different dilutions. In a later paper (ibid., 1880, xxii. 305), Ostwald investigated the question of the influence of water on the action between sulphuric acid and a neutral sulphate. He measured the changes of volume which occurred when solutions of sulphuric acid and sodium sulphate were mixed in varying proportions, and at different dilutions, and obtained results in agreement with his previous work. He also pointed out, that if the mutual action between the acid and neutral sulphate obeyed the general law of mass action, it should follow that, for example, one molecule sodium sulphate plus three molecules sulphuric acid, give the same quantity of acid sulphate as one molecule sulphuric acid, and three molecules sodium sulphate, the volume of the mixture being the same in each case.