368 KEYES ART. J 



vapor pressure when the emitting soHd or Hquid is compressed. 

 It will be recognized that the proof depends on the use of 

 [272] by which the change in vapor pressure with pressure 

 on the liquid or solid phases was computed. It may be well to 

 remark that the energy equation corresponding to this case may 

 be easily deduced from the general equations (73) applied to one 

 component. Thus, 



u' dp = n' dt + m/ d^ii', \ (84) 



v"dP = ■q"dt + mi"dni".j 



Here dp refers to the vapor pressure change of the pure substance 

 (single accent), but if the pressure P is maintained constant on 

 the liquid phase and equilibrium subsists we have 



or 



dp . , ^ 



\p = t-^ v'. (85) 



at 



The latent heat of evaporation under conditions of constant 

 pressure on the liquid phase accordingly differs from the normal 

 heat under saturation conditions. 



In a similar manner if a pressure P is applied to the solid 

 phase but not the liquid phase we find 



Xp = < ^ v", (86) 



dt 



dt 



where v" is the volume of the liquid. Evidently — , the change 



in melting point with pressure, will be large compared with the 

 ordinary change of melting point with pressure where the same 

 pressure is applied to both phases. The equation aids inci- 

 dentally in understanding the extruding of metals, made possible 

 no doubt because of actual instantaneous creation of liquid 

 phases under the enormous pressures applied to the solid. 



