( 232 ) 



For a contact-curve which passes tliroiigh the plait of the i|--SLir- 

 face, the property holds of course ^iood that the pressure is the 

 same for the two points, in which it meets tiie coiniodal curve of 

 the transverse plait. If namely a bi-tangent plane is made to roll 

 simultaneously over the t|'-curve (or the if--surface) of the solid 

 substance, and over the gas part of the i|'-surface of the binary 

 mixture, then if this tangent plane meets a point of tlie binodal cui-ve 

 of the transverse plait, this tangent plane will also touch the if'-sui*face 

 in a point of the other l)ranch of the binodal curve, and this point 

 will represent a liquid phase. Three phases are then in equilil)rium. 

 The pressure that then i)revails, is therefore liie three-piiase-pressure 

 at given temperature. If the temperature should be suci» that the 

 contact-curve no longer i)asses through the plait, then no three- 

 phase pressure exists an}' longer for that \abie of T. For tiie 

 intermediate case the solid l)ody is in equiUbriuui \\ilh Um) phases, 

 whicli liave become equal and the Iwo points of the comiodal curve 

 which the contact-curve has in common with if, haxe coincided in 

 the plaitpoint. 



Particulars as to the course of the conlact cur\e are found tVom 

 the differential equation of /^ when .v and T varies. If we i-epresent 

 the concentration and tjie molecular volume of the solid body by 

 .^■., and I's and that of the coexisting phase, whether it I )e a gas phase 

 or a liquid phase, l)y ,/ /■ and ry, tliis equation may be brought under 

 the following form, ^vhicll is i)erfeclly analogous to lliat which holds 

 for the coexisting phases of a biliary mixture : 



V f dp — (.I's — .(•ƒ) -— dxf -\- ^- dl 

 \d.v/J,,T d 



For the signiticalion of r.y and ll's/I refer to ('out. 11, |t. 107 etc. 



If T is kept constant, we have for the course of y> the ditferential 



equation : 



dp { d'% 



Vsf-— = (.*-.s— .9) 



As long as the contact-curve does not pass through the plait, 



d% 



is alwavs positive. 



dx\f • ^ 



If in the solid state only the pure first substance (in the case under 

 consideration antliraquinone) should occur, then ,Vs ^ 0. 



But the same differential equation holds also, if .v^ should be 

 variable. For the case of antlira(|uiiioiie and ether the value of .i' 

 in the gas phase is higher than that of the li(|ui(l phase for coexisting 

 liquid and gas phases, or ,v.^ ^ x^. It is therefore to be expected, 



