Let us now follow the changes which take place on {86} increasing the pressure of the aqueous vapour in contact with anhydrous copper sulphate, the temperature being meanwhile maintained constant. If, starting from the point D, we slowly add water vapour to the system, the pressure will gradually rise, without formation of hydrate taking place; for at pressures below the curve OC only the anhydrous salt can exist. At E, however, the hydrate CuSO_{4},H_{2}O will be formed, and as there are now three phases present, viz. CuSO_{4}, CuSO_{4},H_{2}O, and vapour, the system becomes _univariant_; and since the temperature is constant, the pressure must also be constant. Continued addition of vapour will result merely in an increase in the amount of the hydrate, and a decrease in the amount of the anhydrous salt. When the latter has entirely disappeared, _i.e._ has passed into hydrated salt, the system again becomes _bivariant_, and passes along the line EF; the pressure gradually increases, therefore, until at F the hydrate 3H_{2}O is formed, and the system again becomes univariant; the three phases present are CuSO_{4},H_{2}O, CuSO_{4},3H_{2}O, vapour. The pressure will remain constant, therefore, until the hydrate 1H_{2}O has disappeared, when it will again increase till G is reached; here the hydrate 5H_{2}O is formed, and the pressure once more remains constant until the complete disappearance of the hydrate 3H_{2}O has taken place. Conversely, on dehydrating CuSO_{4},5H_{2}O at constant temperature, we should find that the pressure would maintain the value corresponding to the dissociation pressure of the system CuSO_{4},5H_{2}O--CuSO_{4},3H_{2}O--vapour, until all the hydrate 5H_{2}O had disappeared; further removal of water would then cause the pressure to fall _abruptly_ to the pressure of the system CuSO_{4},3H_{2}O--CuSO_{4},H_{2}O--vapour, at which value it would again remain constant until the tri-hydrate had passed into the monohydrate, when a further sudden diminution of the pressure would occur. This behaviour is represented diagrammatically in Fig. 20, the values of the pressure being those at 50°.