THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



V ^SX&TH SERIES.] 



JUNE 1920. 



LXII. The Specific Heat of Saturated Vapour and the 

 Entropy-Temperature Diagrams of certain Fluids. By 

 Sir j; A. Ewing, K.C.B., F.R.S. 4 



THE " specific heat o£ saturated vapour," is a thermo- 

 dynamic quantity whose interest is mainly historical, 

 for the phenomena associated with it are now usually stated 

 in other ways. But it lias given rise to a misconception 

 which is still reflected in authoritative text-books. This is 

 readily avoided if the subject be considered with reference 

 to a type of diagram, familiar to engineers, in which the 

 entropy of the saturated vapour is exhibited in relation to 

 the temperature. 



Following Rankine, by whom the phrase was first used f, 

 we shall represent the specific heat of a saturated vapour by 

 K s . It means the quantity of heat required, per unit of 

 mass, to increase the temperature by one degree, while the 

 pressure and the volume are so altered that a state of 

 saturation is maintained. In steam, as is now well known, 

 K s is a negative quantity. It is also well known that when 

 K 8 is negative a vapour becomes supersaturated or partially 

 condensed when it suffers adiabatic expansion, and becomes 

 superheated when it suffers adiabatic compression ; con- 

 versely, when K 6 . is positive a vapour is superheated by 



* Communicated by the Author. 



t In a paper read before the Royal Society of Edinburgh, Feb. '4 r 

 1850, Trans. R. S. E. vol. xx., or Miscellaneous. Scientific Papers, p. 259. 



Phil. Mag. S. 6. Vol. 39, No. 231. June 1920. 2 T 



