14 PROPERTIES OF STEAM AND OTHl K VAPORS. 



The heat required to do the disgregation work, or the internal latent 

 heat, is 



p - r - Apu. 



Specific Volume and Density of Steam. On account of the great 

 difficulty of direct determination of the weight of saturated steam, it is 

 customary to calculate the specific volume of steam by aid of the following 

 equation, derived by the application of the principles of thermodynamics 

 to the general equation representing the properties of saturated vapor: - 



dt 



in which A is the reciprocal of the mechanical equivalent of heat, T is the 

 temperature from the absolute zero, and <r is the volume of one unit of 

 weight of the liquid from which the vapor is formed. The ditTerentiai 



coefficient -- can be calculated by aid of the equations on page 8. 



The absolute temperature is obtained by adding 273 to the temperature 

 in degrees Centigrade, or 459. 5 to the temperature in degrees Fahrenheit. 



The volumes and densities of saturated steam given in Tables I and 

 III were calculated by this method. 



It is of interest to consider the degree of accuracy that may be expected 

 from this method of calculating the density of saturated vapor. The 

 value of r depends on H and q, the total heat and the heat of the liquid; 

 the latter is now well known, but the total heat is probably in doubt to 

 the extent of 5 J<j and may be more. The absolute temperature / appears 

 to be better known and may be subject to an error of no more than yuW or 



; and the mechanical equivalent of heat is perhaps as well determined 



A 



as the absolute temperature. The least satisfactory factor in th 



sion is the differential coefficient -j-, which is derived by differentiating one 



dt 



of the empirical equations on pages 5 and 6. It is true that the resulting 

 equations on page 8 afford a ready means of computing values of the 

 coefficient with great apparent accuracy, but some idea of the essential 

 vagueness of the method may be obtained by comparing computations 

 of the specific volume of saturated steam at 212 C., a point for which 



