412 K T. Allen, F. E. Wright and J. K. Clement— 



Enantiotropy . — In fig. 7, 



OT = axis of temperatures. 



OP = axis of vapor pressures. 



abc = vapor tension curve of liquid. 



ed — vapor tension of solid stable below T 2 . 



db = vapor tension of solid stable above T 2 . 



T 2 = inversion point. 



T 1 = melting point of form stable above T 2 . 



Fig. 7. Vapor pressure curves in a case of enantiotropy ; ed is curve of 

 solid stable below T 2 ; db is curve of solid stable above T 2 . The full lines 

 represent a condition of stability, the dotted lines a condition of instability. 



The two curves ed and fb intersect at d below the melting 

 point curve. At this point the two forms have the same 

 vapor pressure, and are therefore in equilibrium, and the 

 temperature, T a , is the inversion point. Likewise, the point 

 T 1 is common to two curves, fid of the solid stable at higher 

 temperatures, and ac of the liquid ; at that point, therefore, 

 this solid and the liquid are in equilibrium, and T 1 is the melt- 

 ing point. Since in general the curve ed cannot be prolonged 

 so as to intersect with the curve ac, the solid stable at lower 

 temperature usually has no melting point, bd can usually be 

 prolonged to low temperatures by sudden cooling or other- 

 wise, a fortunate circumstance without which nearly all physi- 

 cal and chemical data upon the solids stable only at high tem- 

 peratures would be unattainable. 



