MOLECULAR MIXTURES 



241 



knowledge of this pressure. He also developed, on 

 the basis of a perfect gas relation for the dissolved 

 molecules of a solution, an expression for osmotic pres- 

 sure in terms of the lowering of the freezing point. 

 This relation need not be derived, but the physical 

 phenomenon illustrates some interesting principles. 



Solid substances, like liquids, lose molecules as 

 vapor although in most cases the vapor pressure x is 

 too small to be meas- 

 ured. As the tempera- 

 ture increases the vapor 

 pressure rises. We may 

 plot pressure and tem- 

 perature for a solid in 

 equilibrium with its 

 vapor, just as in Fig. 29 

 of page 239 we plotted 

 the relation for the 



liquid in equilibrium with its vapor. These two curves 

 will intersect as at in Fig. 30. Curve A is for solid- 

 vapor equilibrium and B is for liquid- vapor equilibrium. 

 At is represented the condition at which all three 

 states may coexist in equilibrium. is therefore called 

 a " triple point." 



This point is the freezing point (or the melting point, 

 depending upon our point of view). Under the con- 

 ditions it represents, the vapor arising from the solid 

 is in equilibrium with that rising from the liquid. But 

 suppose that the vapor pressure of the solid was not 

 just equal to that of the liquid. Either the solid or 



1 For solid benzine at 5.5 C. the pressure is 3.55 cm. of mercury, 

 and for ice at about C. it is 0.46 cm. 



