Although the vapour pressure of the substance in the solid, or liquid state, has not been determined, it will be understood from what we have already learned, that the curves AO, OB, BC, representing the vapour pressure of solid crystals, liquid crystals, isotropic liquid, must have the relative positions shown in the diagram. Point O, the transition point of the solid into the liquid crystals, lies at 118.27°, and the change of the transition point with the pressure is +0.032° pro 1 atm. The transition curve OE slopes, therefore, slightly to the right. The point B, the melting point of the liquid crystals, lies at 135.85°, and the melting point is raised 0.0485° pro 1 atm. The curve BD, therefore, also slopes to the right, and more so than the transition curve. In this respect azoxyanisole is different from sulphur. The areas bounded by the curves represent the conditions for the stable existence of the four single phases, solid crystals, liquid crystals, isotropic liquid and vapour. The most important substances hitherto found to form liquid crystals are[98]:-- ----------------------------------+------------+-------- | | Substance. | Transition | Melting | point. | point. ----------------------------------+------------+-------- | | Cholesteryl benzoate | 145.5° | 178.5° Azoxyanisole | 118.3° | 135.9° Azoxyphenetole | 134.5° | 168.1° Condensation product from | | benzaldehyde and benzidine | 234° | 260° Azine of _p_-oxyethylbenzaldehyde | 172° | 196° Condensation product from | | _p_-tolylaldehyde and benzidine | 231° | -- _p_-Methoxycinnamic acid | 169° | 185° ----------------------------------+------------+-------- * * * * * {55} CHAPTER IV GENERAL SUMMARY In the preceding pages we have learned how the principles of the Phase Rule can be applied to the elucidation of various systems consisting of one component. In the present chapter it is proposed to give a short summary of the relationships we have met with, and also to discuss more generally how the Phase Rule applies to other one-component systems. On account of the fact that beginners are sometimes inclined to expect too much of the Phase Rule; to expect, for example, that it will inform them as to the exact behaviour of a substance, it may here be emphasized that the Phase Rule is a general rule; it informs us only as to the general conditions of equilibrium, and leaves the determination of the definite, numerical data to experiment.