{ 457 ) 



This invesligation is capable of further extension and so we can 

 examine tiie e(|niiihria of phases in I)inary mixtures in which the 

 two comi)onenls form one or more compounds. 



Let us limit oui'selves to the ilrst case. If an eciuilihrium exists 

 the (|uantity of the compound in the licjuid or vapour will be depen- 

 dent on the proportion of the two mixed components and on the 

 temperature and pressure. 



We now consider onl)' the equilibria between liijuid and solid 

 and this at constant pressure. If the compound is wholly- undissociated, 

 the phenomena of melting and solidifying may be represented in space 

 by means of an equilateral triangular prism in which the height 

 represeiits the temperature and points in the equilateral triangle 

 represent the i-elative proportions of the components a., and />., and 

 of the compound. 



For convenience we suppose the latter to be ah. It now behaves 

 as an independent component, as it is supposed that there is no 

 equilibrium between ah, a^ and 6,. We then obtain, in space, for 

 each of the three solid substances a melting-surface which takes a 

 downward course from the melting point. 



Should, however, the compound be in equilibrium with its com- 

 ponents, it ceases to be an independent component and at each tem- 

 perature only those relative proportions can exist in a liquid condition, 

 which are in internal equilibrium. 



The curved line «._, r h^ in the 

 adjoining figure represents such 

 an equilibrium line, which there- 

 fore indicates the only proportions 

 capable of existing at a given 

 temperature. We call this line 

 the dissociation-isotherm. 



If the compound did not form 

 ail equilibrium with its compo- 

 nents and if the chosen tempera- 

 ture was situated below the melting 

 point of a.^ a line p q would then 

 be the solubility-isotherm for the solid substance a.^ and in the case 

 of an ideal course of the melting-surface of this component a^ p 

 would be equal to a^q and the \\nQ p q would be straight. The 

 points of the line pq then indicate the solutions which can be in 

 equilibrium with solid a.^ at the temperature in question. 



If, hcTAvever, a., r />., is the equilibrium line of the liquid phase, 

 the point .v will be the only point of the line p q which can exist 



fiR'. 1. 



