Since three components existing in three phases (two liquid and a vapour phase) constitute a bivariant system, the final result, _i.e._ the composition of the two layers and the total vapour pressure, will not depend merely on the temperature, as in the case of two-component systems (p. 102), but also on the composition of the mixture with which we start. At constant temperature, however, all mixtures, the composition of which is represented by a point on one and the same tie-line, will separate into the same two liquid phases, although the relative _amounts_ of the two phases will vary. If we omit the vapour phase, the condition of the system will depend on the pressure as well as on the temperature and composition of the initial mixture. By keeping the pressure constant, _e.g._ at atmospheric pressure (by working with open vessels), the system again becomes bivariant. We see, therefore, that the position of the curve _a_K_b_, or, in other words, the composition of the different conjugate ternary solutions, will vary with the temperature, and only with the temperature, if we assume either constancy of pressure or the presence of the vapour phase. Since at the critical point the condition is imposed that the two liquid phases become identical, one degree of freedom is thereby {244} lost, and therefore only one degree of freedom remains. The critical point, therefore, depends on the temperature, and only on the temperature; always on the assumption, of course, that the pressure is constant, or that a vapour phase is present. Fig. 84, therefore, represents an isothermal (p. 239).