304 SCHREINEMAKERS art. h 



III. Binary Systems in Which Besides Liquids Only the Solid 

 Components W and X Can Occur 



11. In a system formed from the components W and X, 

 liquids, vapors and solid substances may occur, viz.: the pure 

 substances W and X and their compounds or mixed crystals. 

 It depends on the values of t and p, and on the nature of the com- 

 ponents, which of these phases are formed. At first we take a 

 system in which neither compounds nor mixed crystals occur. 

 If now we make the pressure so high that no vapor can be 

 formed, then the only types of phases possible will be liquids 

 and solids W and X. We have therefore only to deal with the 

 f-curve and the points (W) and (X). Furthermore, we shall 

 assume in the first place that the f-curve is wholly convex 

 towards the composition axis (Fig. 4, Figs. 6-9). 



If we lower the temperature for which Fig. 4 obtains, then, as 

 we have seen, the points (W) and (X) and the whole f-curve will 

 rise. Since X' rises more rapidly than (X), these points will 

 first become coincident, after which X' will rise above (X). 

 When this is the case, but W is still below (W), we get Fig. 6. 

 With further fall of temperature W also rises above (W) and 

 we get Fig. 7. Thus with continued decrease of temperature 

 we have the succession of diagrams: Fig. 4 — Fig. 6 — Fig. 7 — 

 Fig. 8— Fig. 9. 



We now represent the melting-points* of solids W and X (under 

 a definite pressure) by T{W) and T{X), and for the sake of 

 definiteness we take T{X) > T(W), e.g., X = a salt and W = 

 water. We call the T for which Fig. 8 holds good T{e). Later 

 on we shall see that this is the eutectic temperature of the 

 system. We can now distinguish the following cases for the 

 temperature T: 



(i) T > TiX) > T(W) > T(e). As T now is higher than 

 the melting-points of each of the components X and W, these 

 are stable only in the liquid state and hence W is lower than 

 (W), X' lower than (Z), (case of Fig. 4). 



(ii) T{X) > T > T(JV) > T{e). The stable state of X is 



*From this point onwards in the present article, and in the corre- 

 sponding figures, temperature is denoted by T. 



