From the above figures, therefore, it will be seen that at a temperature just above the transition point a solution in contact with the two solid phases, astracanite and Glauber's salt, contains a relatively smaller amount of sodium sulphate than a pure solution of astracanite would; for in this case there would be equal molecular amounts of Na_{2}SO_{4} and MgSO_{4}. A solution which is saturated with respect to astracanite alone, will contain more sodium sulphate than the solution saturated with respect to astracanite plus Glauber's salt, and the latter will therefore be deposited. From this, therefore, it is clear that if astracanite is brought in contact with water at about the transition point, it will undergo decomposition with separation of Glauber's salt (supersaturation being excluded). [Illustration: FIG. 101.] This will perhaps be made clearer by considering Fig. 101. In this diagram the ordinates represent the ratio of sodium sulphate to magnesium sulphate in the solutions, and the abscissæ represent the temperatures. The line AB represents solutions saturated with respect to a mixture of the single salts (p. 268); BC refers to solutions in equilibrium with astracanite and magnesium sulphate; while BX represents the composition of solutions in contact with the solid phases astracanite and Glauber's salt. The values of the solubility are contained in the following table, and in that on p. 268, and are, as before, expressed in gm.-molecules of salt in 100 gm.-molecules of water.[352] {270} ------------------------------------------------------------------------- | Astracanite | Astracanite Temperature. | + sodium sulphate. | + magnesium sulphate. |----------------------------|------------------------------ | Na_{2}SO_{4}. | MgSO_{4}. | Na_{2}SO_{4}. | MgSO_{4}. ------------------------------------------------------------------------- 18.5° | -- | -- | 3.41 | 4.27 22° | 2.95 | 4.70 | 2.85 | 4.63 24.5° | 3.45 | 3.62 | 2.68 | 4.76 30° | 4.58 | 2.91 | 2.30 | 5.31 35° | 4.30 | 2.76 | 1.73 | 5.88 ------------------------------------------------------------------------- At the transition point the ratio of sodium sulphate to magnesium sulphate is approximately 1 : 1.6. In the case of solutions saturated with respect to both astracanite and Glauber's salt, the relative amount of sodium sulphate increases as the temperature rises, while in the solutions saturated for astracanite and magnesium sulphate, the ratio of sodium sulphate to magnesium sulphate decreases.