179 



{V^-v)dP='^t,d;i,^ (27) 



As Fi — V and t^ are both positive, it is apparent that tiie pressure 

 is a minimum. In accordance wiih our previous considerations (see 

 fig. 2) we find therefore: on tlie satnrationcurve under its own 

 vapourpi-essure of the solid substance F the pressure is a minimum 

 in a point m, when tlie vapour cori-espondiuft- witli this liquid has 

 the composition F. 



In order to examine the change of pressure along the satnration- 

 curve in the vicinity of its extreme ends h and ii (fig. 2, 3, and 4) 

 we equate x = ; from (22) and (25) we then obtain : 



[(3/ — /^) -^ + RT] dv + {y -/?) t dy -= [ V-{y-(^) -^ -r] dP . . (28) 



d;/ 



{y-fl)sdx + iy-^)tdy = [V,-{y,~-:'J)-^-v] dP. . (29) 



From this follows : 



{y, -ft) RTdx = [{y-ft) V + {^-y) V, + {y-y^) v] dP . (30) 



When A V^ is the change of \olume, which occurs when between 

 the three phases of the binary equilibrium F -\- L -\- G a reaction 

 occurs, in which one quantity of vapour arises, then we may write 

 for (30): 



dP=:—Z—ll, , da; (31) 



^-y hV, ^ ^ 



Now A Fi is always positive in the binary system F -\- L -{- G> 

 except between the minimum-melting point Tf and the point of 

 maximumtemperature Tu, wheie AT" is negative. In fig. 4 A Fj 

 is consequently negative for liquids between F and H, positive for 

 all other liquids on the side BC. 



ft — y is positive, when the liquid is situated between F and C, 

 negative when the liquid is situated between F and B (figs. 2 — 4). 



ft — ?/i is positive, when the vapour is situated between F and C, 

 negative when the vapour is situated between F" and B (figs 2 — 4). 



In the points h of tigs. 2—4 is A ri>0, /•'— ?/>Oand /^— yi>0; 

 from (31) follows therefore dP<^0. From each of tlie points h 

 the pressure must, therefore, decrease along the saturationcurves, we 

 see that this is in accordance with the direction of the arrows in 

 the vicinity of the points h (figs. 2 -4). 



In the point n of fig. 2 is A T^i > 0. /■///< and />—. Vi<0; 

 from (31) follows, therefore (IP<^0, Consequently we find that 



