778 



vapoLirpressiire of Na.^SO^ . 10 H^O [F], sa tlie saturatioiicurve uf the 

 anhjdric Na2S04 {B). Tlien there exists a series of solutions, saturated 

 under their own vapourpressure with Na^SO^ . 10 HjG (curve cs) 

 and one seiies saturated with Na.^SO^ (curve sa) ; the equilibrium 

 Na^SO^ . 10 H^O -|- NajjSO^ -J- Lg + ^s occurs only under a definite 

 pressure Pg. The solution L, has then a delinite composition s and 

 the vapour, which consists only of water and alcohol has a definite 

 composition s^ . 



Ifi (he binary system Na^SO^ -[~ ^"^'^^^i'' the equilibrium Na^SO^ . JO 

 HjO -f- Na^SO^ -j- vapour exists only under one definite pressure; we 

 shall call this pressure P„ . In the ternary system NajSO^ -f" water -\- 

 alcohol the equilibrium Na.SO^ . 10 H,0+Na,SO,-|-L,-l-G, exists also 

 only under a definite vapourpressure Pg. This pressure Ps is infiuenced 



Fig. 6. 

 by the watervapour and the alcohol-vapour together; now we may 

 show that the partialpressure of the watervapour herein is also equal 

 to Po and that the pressure of the alcohol vapour is consequently 



In order to show this, we consider the general case that in the 

 system .4 + ^ + ^- (^igs- ^ — ^) the substances A and C are volatile 

 and that a compound F of B and C occurs. 



The binary equilibrium B -\- F -{- G^, wherein the vapour consists 

 of C only, occurs under a single pressure P^ only. 



The ternary equilibrium B -\- F -\- G, wherein consequently the 

 vapour consists of A and C, can occur at a whole series of vapour- 

 pressures. 



When we represent the $ of B and F by ^ and ^^ , then the 

 condition of equilibrium is true : 



?x -/?? = (! 



'K^^~^'^ë) 



(21) 



