238 MOREY art. g 



It will be of interest to consider the detailed application of the 

 equation 



d'p r}^ — r/^ 

 dt v" — y' 



to the pressure-temperature curve of water. 



* The thermodynamic properties of water are known to a 

 considerable degree of precision, and tables giving the specific 

 entropy and specific volume of water and steam are in common 

 use by engineers. In such tables it is customary to take the 

 specific entropy of liquid water at zero degrees centigrade as 

 zero, but since we are always dealing with differences in entropy 

 this is immaterial. Absolute values of entropy are not deter- 

 minable; to determine absolute values of entropy we would 

 have to know the value of the entropy at absolute zero,t and its 

 variation with temperature from the absolute zero up, and we 

 do not possess the necessary data for this. Herein Hes one of 

 the reasons for the entropy concept being a difficult one to 

 grasp; we are not able to measure entropy directly as we are 

 able to measure the other quantity factors, volume and mass. 

 For practical purposes, however, this is not material, since we 

 are always dealing with entropy differences. In Fig. 1 are shown 

 plotted the specific entropy of Uquid water and the specific 

 entropy of saturated water vapor from zero to 200°C., the 

 specific volume of water vapor at the saturation pressure in 

 the same temperature range, and the pressure-temperature 

 curve of the equilibrium, liquid -(- vapor. Since the slope of the 

 p-t curve is determined by the difference in entropy between 

 vapor and liquid, it is immaterial whether the entropy of the 



* From this point to the end of section (11), p. 251, the text is taken, 

 with some omissions, alterations and additions, from the author's article, 

 Jour. Franklin Inst., 194, 439-450 (1922) ; sections (16) to (23) inclusive 

 (except (18) and (22)) are taken in like manner from the same article, 

 pp. 450-460. 



t Absolute values of entropy may be calculated for many substances 

 by the use of the so-called Third Law of Thermodynamics, a principle 

 whose validity has not been completely demonstrated. 



