74 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
and 4' 6 mm. pressure) there are three phases — one solid (ice), one Kquid (water), and one gaseous (water vapour). Qibbs has deduced a law for the number of these phases which is known as " Gibbs's phase rule." This law may be stated as follows: n bodies (different chemical substances, simple or compound) can form n + 2 phases, and these co-exist only at a single point (i,e, all the external conditions of the system, pressure, temperature, and composition of each phase, are given). Let us consider the substance water ; here w = 1, and therefore three phases of the substance, the solid, the liquid', and the gaseous, may co-exist, but only at one point, namely, at 0° and 4*6 mm. pressure. The composition must be the same throughout, since only one kind of molecule is present. If the system consists of two bodies, e.g, common salt and water, then n = 2 and the number of phases is 71 + 2 = 4. These phases can co-exist at about — 21°, at which temperature by loss of heat a so-called cryohydrate (constant mixture of ice and salt crystals) separates from the saturated solution.
At this temperature ( — 21°, the corresponding pressure being 0*73 mm.) there are present two solid phases (ice and grams of salt to 100 grams of water), and a gaseous phase (water vapour at 0*73 mm. pressure).
When the number of phases is only n + 1, one of the external conditions can (within certain limits) be fixed at pleasure; thereby, however, the other conditions are also fixed. Thus if we take water (n = 1) in the liquid and gaseous states, the number of phases is ti -h 1 = 2. At any particular temperature we may happen to choose, the pressure at which the two phases can exist in presence of each other can only have one value. (Saturated water vapour at 20° has a pressure of 17*4 mm.)
Two bodies, such as salt and water (n = 2), can co-exist at a temperature of, say, 20° in n + 1 = 3 phases ; these are (1) salt crystals ; (2) saturated solution, containing 36 grams of salt to 100 grams of water; and (3) water vapour of pressure
�� �
