extensive hydration, probably increasing in extent with the magnitude of the charge. Hydration, by removing “free” solvent, increases the effective concentration of the solution and also the activity coefficient. Indeed, hydration would seem to be the only mechanism whereby we can explain the very high values of the activity coefficients of salts like calcium chloride in concentrated solution. The values for lanthanum chloride and thorium nitrate are not so high, but when we remember the very high charges on these cations which should result in considerable ion-pair formation and therefore in very low activity coefficients, like those observed for potassium ferricyanide and ferrocyanide, we can only conclude that some compensating mechanism must intervene to raise the activity coefficients to the observed values. Again hydration would appear to be the only force of sufficient magnitude. On the contrary, the very low activity coefficients observed when the high-valency ion has a negative charge, consistent with ion-pair formation between ions such as potassium and ferrocyanide, indicates that hydration of negative ions must be small in extent. For these reasons we put forward the view that ionic hydration is a phenomenon associated with positive rather than negative ions. Experimental. Potassium ferricyanide was crystallised from A.R. material. Thorium nitrate was prepared from commercial thoria and recrystallised three times from slightly acid solution. Isopiestic vapour pressure measurements were made by the method of Robinson and Sinclair (1934), using platinum dishes for both salts. Sodium chloride and sulphuric acid were used as reference electrolytes. Potassium ferricyanide proved to be a difficult salt to work with and, although we are convinced that the accuracy of the results is more than sufficient for our present purpose, high accuracy is not attributed to the data. It is possible that the measurements on thorium nitrate are in some error from hydrolysis; but we found no difficulty in obtaining reproducible and consistent results. The experimental results are given in Tables I and II and the osmotic and activity Table I.—Isopiestic Solutions of Potassium Ferricyanide and Sodium Chloride at 25°. K3Fe(CN)6 NaCl K3Fe(CN)6 NaCl K3Fe(CN)6 NaCl K3Fe(CN)6 NaCl 0.1357 0.2083 0.1976 0.2954 0.3864 0.5691 0.4813 0.7058 .6544 .9478 .7225 1.049 .8316 1.212 .9716 1.432 1.102 1.638 1.209 1.812 1.431 2.170 Table II.—Isopiestic Solutions of Thorium Nitrate and Sodium Chloride or Sulphuric Acid. Th(NO3)4 NaCl Th(NO3)4 NaCl Th(NO3)4 NaCl Th(NO3)4 NaCl 0.0502 0.0916 0.0820 0.1489 0.1958 0.3633 0.3630 0.7090 4625 .9348 .6054 1.294 .8332 1.897 1.068 2.583 1.147 2.849 1.332 3.422 1.477 3.840 1.635 4.306 1.700 1.926 1.926 5.175 2.038 5.534 2.244 6.148 Th(NO3)4 H2SO4 Th(NO3)4 H2SO4 Th(NO3)4 H2SO4 Th(NO3)4 H2SO4 2.564 4.880 2.634 4.992 3.095 5.730 3.151 5.820 3.674 6.576 4.247 7.328 5.151 8.447 5.201 8.498 The above tables express the experimental results as molalities of solutions of equal vapour pressure.