310 SCHREINEMAKERS art. h 



(i.e., f = ee'). The f of the three-phase system (14) remains 

 unchanged, therefore, whether the reaction (15) occurs in the 

 one or the other direction. When this reaction proceeds from 

 left to right, heat is absorbed; when it proceeds from right to 

 left, heat is produced. Given a unit system of composition e 

 at temperature T(e) (and the given pressure) we cannot predict 

 its phase structure without further information (e.g., concerning 

 its past history, or its behavior on adding or abstracting heat 

 energy, etc.). 



The hquid e is represented in Fig. 10 by the point e, and the 

 systems discussed by points on the line 4 • e • 4'. 



(v) T{X) > TiW) > T{e) > T (case of Fig. 9). Since the 

 line {W){X) now lies wholly below the f -curve (the free energy 

 liquidus curve), all the liquids are metastable and tend to pass 

 into the mixture, solid W + solid X. From this discussion it 

 follows that T{e) is the lowest temperature for the existence 

 of a stable liquid phase. T{e) is therefore the eutectic tem- 

 perature and L{e) the eutectic liquid of the (W, X) system. If 

 we take W = water, so that the three-phase system (14) be- 

 comes L(e) -\r ice + solid X, then we call T{e) also the cryo- 

 hydrate temperature. 



13. From the preceding considerations we can now make 

 the following statements about Fig. 10. The liquids saturated 

 with solid W are represented by the points of a curve eT{W), 

 the saturation curve of W, whilst the liquids saturated with 

 solid X are represented by the points of a curve eT(X), the 

 saturation curve of X. These two curves and the line 4-e-4' 

 divide Fig. 10 into four fields. Each point of field I represents 

 an unsaturated liquid. Each point of field II represents a 

 system L(z) + solid X, or alternatively a liquid which is super- 

 saturated with respect to solid A^. Similarly each point of 

 field III represents a system L(u) + solid W, or a liquid super- 

 saturated with respect to solid W, whilst finally each point of 

 field IV represents a mixture of solid W and solid X. 



The two saturation curves do not terminate in e but are 

 prolonged into field IV, in which they represent metastable 

 states. We find the points of these prolongations, and we see 

 also that they represent metastable states, when we imagine 



