This behaviour will perhaps be more clearly understood when one reflects that since the composition of the two phases is the same, the system may be regarded as being formed of _one component_, just as the system NH_{4}Cl <--> NH_{3} + HCl was regarded as being composed of one component when the vapour had the same total composition as the solid (p. 13). One component in two phases, however, constitutes a univariant system, and we can therefore see that calcium chloride hexahydrate in contact with solution of the same composition will constitute a univariant system. The temperature of equilibrium will, however, vary with the pressure;[228] if the latter is constant, the temperature will also be constant. A point such as has just been referred to, which represents the special behaviour of a system of two (or more) components, in which the composition of two phases becomes identical, is known as an _indifferent point_,[229] and it has been shown[230] that at a given pressure the temperature in the indifferent point is the _maximum_ or _minimum_ temperature possible at the particular pressure[231] (cf. critical solution temperature). At such a point a system loses one degree of freedom, or behaves like a system of the next lower order. The Hydrates of Ferric Chloride.--A better illustration of the formation of compounds possessing a definite melting point, and of the existence of retroflex solubility curves, is afforded by the hydrates of ferric chloride, which not only possess definite points of fusion, but these melting points are stable. A very brief description of the relations met with will suffice.[232] {152} Ferric chloride can form no less than four stable hydrates, viz. Fe_{2}Cl_{6},12H_{2}O, Fe_{2}Cl_{6},7H_{2}O, Fe_{2}Cl_{6},5H_{2}O, and Fe_{2}Cl_{6},4H_{2}O, and each of these hydrates possesses a definite, stable melting point. On analogy with the behaviour of calcium chloride, therefore, we shall expect that the solubility curves of these different hydrates will exhibit a series of _temperature maxima_; the points of maximum temperature representing systems in which the composition of the solid and liquid phases is the same. A graphical representation of the solubility relations is given in Fig. 39, and the composition of the different saturated solutions which can be formed is given in the following tables, the composition being expressed in molecules of Fe_{2}Cl_{6} to 100 molecules of water. The figures printed in thick type refer to transition and melting points. [Illustration: FIG. 39.] {153} COMPOSITION OF THE SATURATED SOLUTIONS OF FERRIC CHLORIDE AND ITS HYDRATES. (_The name placed at the head of each table is the solid phase._) ICE. --------------------------- Temperature. | Composition. --------------------------- ±-55° | ±2.75 -40° | 2.37 -27.5° | 1.90 -20.5° | 1.64 -10° | 1.00 0° | 0 ---------------------------