Since the cryohydric point is a quadruple point in a two-component system, it represents an invariant system. The condition of the system is, therefore, completely defined; the four phases, ice, salt, solution, vapour, can co-exist only when the temperature, pressure, and concentration of the solution have constant and definite values. Addition or withdrawal of heat, therefore, can cause no alteration of the condition of the system except a variation of the relative amounts of the phases. Addition of heat at constant volume will ultimately lead to the system salt--solution--vapour or the system ice--solution--vapour, according as ice or salt disappears first. This is readily apparent from the diagram (Fig. 32), for the systems ice--salt--solution and ice--salt--vapour can exist only at temperatures below the cryohydric point (provided the curve for ice--salt--solution slopes towards the pressure axis). Bivariant Systems.--Besides the univariant systems already discussed, various bivariant systems are possible, the conditions for the existence of which are represented by the different areas of Fig. 32. They are as follows:-- _Area._ _System._ OAMF Salt--vapour. CBAMF Solution--vapour; salt--solution. EABD Salt--solution; ice--solution. EAO Ice--salt. {130} Deliquescence.--As is evident from Fig. 32, salt can exist in contact with water vapour at pressures under those represented by OAMF. If, however, the pressure of the vapour is increased until it reaches a value lying on this curve at temperatures above the cryohydric point, solution will be formed; for the curve AMF represents the equilibria between salt--solution--vapour. From this, therefore, it is clear that if the pressure of the aqueous vapour in the atmosphere is greater than that of the saturated solution of a salt, that salt will, on being placed in the air, form a solution; it will _deliquesce_. Separation of Salt on Evaporation.--With the help of Fig. 32 it is possible to state in a general manner whether or not salt will be deposited when a solution is evaporated under a constant pressure.[208]