Further, the behaviour of sodium sulphate and water furnishes a very good example of the fact that a "break" in the solubility curve occurs when, and only when, the solid phase undergoes change. So long as the decahydrate, for example, remained unaltered in contact with the solution, the solubility curve was continuous; but when the anhydrous salt appeared in the solid phase, a distinct change in the direction of the solubility curve was observed. Dehydration by Means of Anhydrous Sodium Sulphate.--The change in the relative stability of sodium sulphate decahydrate and anhydrous salt in presence of water at a temperature of 32.5° explains why the latter salt cannot be employed for dehydration purposes at temperatures above the transition point. The dehydrating action of the anhydrous salt depends on the formation of the decahydrate; but since at temperatures above 33° the latter is unstable, and cannot be formed in presence of the anhydrous salt, this salt cannot, of course, effect a dehydration above that temperature. Pressure-Temperature Diagram.--The consideration of the pressure-temperature relations of the two components, sodium sulphate and water, must include not only the vapour pressure of the saturated solutions, but also that of the crystalline hydrates. The vapour pressures of salt hydrates have already been treated in a general manner (Chap. V.), so that it is only necessary here to point out the connection between the two classes of systems. {139} In most cases the vapour pressure of a salt hydrate, _i.e._ the vapour pressure of the system hydrate--anhydrous salt (or lower hydrate)--vapour, is at all temperatures lower than that of the system anhydrous salt (or lower hydrate)--solution--vapour. This, however, is not a necessity; and cases are known where the vapour pressure of the former system is, under certain circumstances, equal to or higher than that of the latter. An example of this is found in sodium sulphate decahydrate. On heating Na_{2}SO_{4},10H_{2}O, a point is reached at which the dissociation pressure into anhydrous salt and water vapour becomes equal to the vapour pressure of the saturated solution of the anhydrous salt, as is apparent from the following measurements;[212] the differences in pressure being expressed in millimetres of a particular oil. Temperature: 29.0° 30.83° 31.79° 32.09° 32.35° 32.6° Difference of pressure: 23.8 10.8 5.6 3.6 1.6 0 At 32.6°, therefore, the vapour pressures of the two systems Na_{2}SO_{4},10H_{2}O--Na_{2}SO_{4}--vapour Na_{2}SO_{4}--solution--vapour