Properties of Gases and Vapours, &c. 



283 



given in the table for comparison with those of d(f>. They are calcu- 

 lated from the same table of the values of the specific heat, and the 

 difference d<f> - dh/0 is seen to be small. This difference occurs as a 

 small correction in the equation for the saturation-pressure, to be pre- 

 sently given. The values of the entropy of water, corrected for the 

 variation of the specific heat are given in the column headed < w . The 

 values of the entropy of steam are obtained by adding the values of 

 L/0 given in the next column, which are found from the values of the 

 latent heat already given in Table III. Values of <J> g , calculated from 

 Regnault's formulae, are given in the last column for comparison. 



Table IV. Entropy of Water and Steam. 



The Entropy Equation. 



By the application of the second law of thermodynamics we may 

 obtain a relation between the latent heat and the saturation-pressure. 

 Expressing the fact that the entropy of steam evaporated at 0, and 

 heated at constant pressure p up to any temperature 0, is the same as 

 the entropy of steam obtained by heating water up to the same 

 temperature, 0, evaporating it at saturation-pressure, p, and then 

 expanding it at constant temperature, 6, down to the pressure, p, 

 of saturation at 0, we immediately obtain the entropy equation 



(i - s) io ge e/e + L/e - L/e + K iog e p/ P 



= (34). 



